Chapter 1: Router Choice Principle
1.1 Basic knowledge of routing
Routing is a relay process that transfers objects from one place to another
The mechanism for learning and maintaining knowledge of network topology is considered a routing function. Transfer data flows through the router and enters the interface
The process of being transferred through the router to the outbound interface is another separate function, considered a switch/forward function. The routing device must have both routing and switching functions to be an effective relay device.
In order to perform routing, the router must know the following three things:
lThe router must determine whether it activates support for the protocol group;
lThe router must know the destination network;
l The router must know which outgoing interface is the best way to reach the destination.
The routing protocol determines the best path to the destination by metrics. Small metrics represent preferred paths; if two or more paths have the same small metric, then all of them will be shared equally. Splitting data traffic through multiple paths is called load balancing to the destination.
The information required to perform routing operations is contained in the router's routing table, and they are generated by one or more routing protocol processes. The routing table consists of multiple routing entries, each of which indicates the following:
lLearn the mechanism used by this route (dynamic or manual)
Logical destination
Management distance
l Measurement value (it is a measure that measures the total "total overhead" of a path)
lThe address of the relay device (router) to the destination lower HOP;
lThe old and new route information
l Interface associated with the network to go to the destination
Use the command SHOW IP ROUTE to see the above content
The pre-allocation principle of default management distance is: manually set the priority of routing entries higher than dynamically learned routing entries, and the routing protocol with complex routing algorithms with higher priority of routing protocols with complex routing algorithms with simple routing protocols.
Routers generally choose paths with the smallest metric value; if multiple paths with the lowest and same metric value appear at the same time in the IP environment of the CISCO router, load balancing will be enabled on these multiple paths. C ISCO supports 4 paths with the same metric value by default. By using the "maximum-paths" command, it can be recognized that the CISCO router supports up to 6 paths with the same metric value.
RIP is a routing protocol used in small to medium-sized TCP/IP networks. It uses the number of hops as the measure value. Its load balancing function is enabled by default. RIP determines the optimal path without considering bandwidth! ! !
IGRP is a routing protocol used in medium to large TCP/IP networks. It uses composite metrics that take into account bandwidth, latency, reliability, load and maximum transmission unit (M TU), but uses bandwidth and delay values by default. IGRP can also perform load balancing
After the router starts, it immediately tries to establish a routing relationship with its adjacent routing devices. The purpose of this initial communication is to identify adjacent devices and start communication and learn the network phase structure. The method of establishing neighbor relationships and the initial learning of topology vary with the routing protocol.
The routing protocol exchanges periodic HELLO messages or periodic routing update packets to maintain communication between adjacent devices.
After understanding the network topology structure and the routing table already contains the best path to the known network, forwarding data to these destinations can begin;)
1.2 Routing Protocol
Classful routing overview
The routing protocol that does not send subnet mask information with each network address is called a class selection protocol (RIPv1, IGRP)
When using a class routing protocol, all subnets belonging to the same main class network (Class A, B and C) must use the same subnet mask. A router running a routing protocol with a category routing protocol will perform one of the following tasks to determine the routing network part:
lIf the routing update information is about the same main network configured on the receiving interface, the router will use the subnet mask configured on the interface;
lIf the routing update is about a network of different main classes that are assigned on the receiving interface, the router will use the default subnet mask according to its address category.
The generation of category inductive routes is automatically processed by category routing protocols.
Classless routing overview
Classless routing protocols include Open Shortest Path First (OSPF), EIGRP, RIPV2, Intermediate System-to-Intermediate System (IS-IS), and Border Gateway Protocol Version 4 (BGP4).
Using different mask lengths in the same main-class network is called a variable-length subnet mask (VLSM). The classless routing protocol supports VLSM, so the subnet mask can be set more effectively to meet the needs of different subnets for different hosts, and can make full use of the host address.
Most distance vector routing protocols produce regular, routine routing updates that are transmitted only to directly connected routing devices.
In a purely distance vector routing environment, routing updates include a complete routing table. By receiving the full routing table of adjacent devices, the routing can check all known routes and then modify the local routing table based on the received update information. The distance vector method that solves routing problems is sometimes called "routing by rumor"
CISCO IOS supports several distance vector routing protocols, RIPv1, RIPv2 and IGRP. CISCO also directly holds EIGRP, which is an advanced distance vector routing protocol.
The routing protocol is usually associated with the network layer of the protocol group.
Most distance vector routing protocols use Bellman-Ford algorithm to calculate routing. EIGRP is an advanced distance vector routing protocol that uses diffusion correction algorithm (D UAL)
Comparison of Cisco's IP Distance Vector Routing Protocol
Features RIPv1RIPv2IGRPEIGRP
Count to infinite XXX
Cross-side distance XXXX
Suppress timer XXX
Triggered update, routing reverse XXXX
Load balancing-equal cost path XXXX
Load balancing-non-equal cost path XX
VLSM supports XX
Routing algorithm Bellman-Ford Bellman-Ford Belman-Ford DUAL
Measurement value hop number hop number compound
Jump limit 1515100100
Easy to expand small and medium large
Note: The default hop limit for IGRP and EIGRP is 100, but it can be configured to a maximum of 255.
The link-state routing protocol generates routing update packets only when the network topology changes. When the link state changes, the device that detects the change generates a link state announcement (L SA) about the link (routing). The LSA is then propagated to all neighboring devices through a special multicast address. Each routing device retains an LSA copy and forwards the LSA to its neighbors (this process becomes called diffusion f loading) and then updates its topology database (this is a table containing all link status information for the network). LSA diffusion is used to ensure that all routing devices are aware of this change so that they can update their data and generate an updated routing table that reflects the new network topology.
Comparison of Cisco's Link-state Routing Protocol
Features OSPFIS-ISEIGRP
Requires systematic topological structure XX
Keep an understanding of all possible routes XXX
Routing Induction-Artificial XXX
Routing Induction-Automatic X
Event trigger notification XXX
Load balancing-equal cost path XXX
Load balancing-non-equal cost path X
VLSM supports XXX
Routing algorithm DijkstraIS-ISDUAL
Metric link cost (bandwidth) link cost (bandwidth) composite
No hop limit is 1024100
Very easy to expand
The routing processes in each router must leave a single loop-free path to each possible destination logical network. When all routing tables are synchronized and each routing table contains an available route to the destination network, the network reaches a convergence state. Convergence is the activity associated with the routing table synchronization after the network topology changes, such as the state of adding new routes or the existing route changes.
Convergence time is the time required for all routes in the network to achieve consistency in their perception of the current topology structure. The size of the network, the routing protocol used, and numerous configurable timers can all affect the convergence time.
There are two methods of detection:
l When the physical layer or data link layer fails to receive a certain number (usually 3) of continuous keepalive messages, the link is considered invalid.
l When the routing protocol fails to receive a certain number (usually 3) of continuous Hello messages or route updates or similar messages, the link is considered to be invalid.
Most routing protocols have timers to prevent topological loops from occurring during link state transitions.
Chapter 2 Extended IP Address
The Internet is developing incredibly fast. This rapid development has led to two major challenges in address:
lIP address exhaustion
lGrowth and manageability of routing tables
IP addressing solution:
Slow down IP address consumption and reduce Internet routing table entries by enabling more hierarchical layers in IP addresses
quantity. These solutions include:
l Subnet mask
lAddress allocation for private network
lNetwork Address Translation (NAT)
l Systematic addressing
lVariable length subnet mask (VLSM)
lRoute induction
lCategory Inter-Domain Routing (CIDR)
Category of IP address:
The first byte (decimal) address category of the address
Class 1~126A
Class 128~191B
Class 192~223C
Class 224~239D
Class 240~255E
Class D address has not been widely used yet, it is a multi-came multicast address; some routing protocols use the following:
OSPF-----224.0.0.5 and 224.0.0.6
RIPv2-----224.0.0.9
EIGRP----224.0.0.10
Systematic addressing:
Systematic addressing is like when we make a phone call. Each telephone bureau does not need to know the national phone number. If the first one is not 0, the switchboard will find the link in its own phone entry and then take it. If it is 0, then it depends on the area code. For example, it is 0791-6221155. It will transmit this information to the Nanchang Telephone Bureau (0791) and find the link 6221155 and connect it to the Nanchang Telephone Bureau. In this way, your switchboard does not need to have entries from other places. Let others have a taste of it. J. The principle can also be used in the router.
Advantages of systematic addressing:
lReduce the number of routing entries
Routing induction is a method when we adopt a systematic addressing planning method that uses an IP address to represent a set of IP addresses. By summarizing the route, we can keep the routing table entries as manageable, and it can bring the following benefits:
------Improve routing (forwarding) efficiency;
------ When recalculating the routing table or retrieving a match through the routing table entry, the number of CPU cycles required is reduced;
------Reduced memory requirements for routers
------Convergence faster when the network changes
-------Easy to make mistakes
l Valid address assignment
Systematic addressing allows us to take advantage of all possible addresses, because our address packets are continuous;
Variable length subnet mask (VLSM)
VLSM proposes the ability to include multiple subnet masks in a main class (A, B, C) network, as well as the ability to subnet the subnet again. Its advantages are as follows:
l More efficient use of IP addresses - if VLSM is not used, the company will be restricted to only use one subnet mask within a Class A, B, and C network number;
lThe ability to use routing induction is stronger - VLSM allows more system hierarchies in the addressing plan, so better routing induction can be performed in the routing table.
Routing Induction
In large interconnected networks, there are hundreds of networks. In this environment, it is generally not intended that the router saves all these routes in its routing table. Routing induction (also called routing aggregation or hypernetting) can reduce the number of routing entries a router must save because it is a way to represent a series of network numbers in an inductive address.
Another advantage of using routing induction in large, complex networks is that it can protect other routers from changes in network topology.
Only when a correct address planning is used can routing induction be feasible and most effective. In a subnet environment, routing induction is the most effective when the network address is a continuous block in the form of an exponential form of 2.
The routing protocol summarizes or aggregates routes based on the shared network address part. Classless routing protocols--OSPF and EIGRP-support routing induction based on subnet addresses, including VLSM editor address. Category routing protocols - RIPv1 and IGRP - automatically summarize routing on the boundaries of a Category network. The Category Routing Protocol does not support routing induction on any other bit boundary, while the Category Routing Protocol does not support routing induction on any bit boundary.
Because there are fewer entries in the routing table, routing induction can reduce the use of router memory and reduce network traffic caused by routing protocols. To enable routing induction in the network to work correctly, the following requirements must be met:
l Multiple IP addresses must share the same high bit;
lRouting protocol must make routing forwarding decisions based on the 32-bit IP address and the prefix length up to 32-bit
lRouting update must transmit the prefix length (subnet mask) with the 32-bit IP address.
Operation of routing induction in Cisco routers
CISCO manages routing induction through the following two methods:
lSend route induction
lSelect route from route summary
Subnets with discontinuous addresses refer to some subnets in the same main network separated by other different main networks.
Support of routing protocols for routing induction
Is the protocol automatically summarized in the boundaries of categories? Can automatic induction be turned off? Can automatic induction be performed outside the boundaries of the category network
Is RIPv1 no
Is RIPv2?
Is IGRP
EIGRP Yes Yes Yes
OSPF No-Yes
Classless inter-domain routing (CIDR)
CIDR is a technology developed to help slow down IP addresses and routing table enlargement problems. The concept of CIDR is that multiple C-class address blocks can be combined or aggregated together to generate a larger set of classless IP addresses (that is, allow more hosts). The C-class address of the block is assigned to each ISP
Using an unnumbered IP address on a serial interface
To enable IP processing on a serial interface without assigning an explicit IP address to the interface, you can use the "ip unnumbered type number" interface configuration command. In this command "type number" is the type and number of another interface on the router with an assigned IP address (this interface is called the specified interface or reference interface, that is, the interface from which the unnumbered interface borrows the IP address). It cannot be another numberless interface. If you want to turn off the IP processing function in the serial interface, you can use the NO form of this command.
Limitations of no numbered interface:
lSerial interfaces using HDLC, PPP, LAPB, SLIP protocols, and tunnel interfaces can be used without numbers. Can't be in X. 25 or switched multi-megabit data service SMDS interface use the numberless interface configuration command.
l We cannot use the PING command to determine whether the unnumbered interface has been UP because the interface has no address. SNMP can remotely monitor the status of the interface.
example:
Interface Ethernet0 Ip address 10.1.1.1 255.255.255.0!interface Serial0 ip unnumbered Ethernet0
Use Helper Address
The router does not forward broadcasts, and the help address helps the client and the server establish a connection by forwarding these broadcast packets directly to the target server.
The Help Address command changes the broadcast destination address to a single point communication room address (or a directed broadcast - this broadcast within a subnet), so that the broadcast message can be routed to a specific destination instead of all places
Use the "ip helper-address address" interface configuration command to configure an interface that may receive broadcasts. In this command "ADDRESS" refers to the destination address used when forwarding User Datagram Protocol (UDP) broadcasts. The specified address can be a single-pass address or a directed broadcast address of the remote server.
If the "ip helper-address address" command is defined, the function of forwarding for the 8 default UDP ports is automatically enabled, namely: TFTP (69), DNS (53), Time (37), NETBIOS service (137), N ETBIOS datagram service (138), BOOTP server (67), BOOTP client (68), and terminal access controller access control system TACACS (49).
If the "ip helper-address address" command is defined and the "ip forward-protocol udp" command that specifies these 8 UDP ports, then the broadcast packets addressing these 8 UDP ports will be automatically forwarded.
"ip forward-protocol" description:
"ip forward-protocol" command description
udpUDP-Transport Layer Protocol
port (optional) When the "udp" keyword is specified, the UDP destination port number or port name can be defined.
nd network disk; an old protocol used by diskless Sun workstations
sdns network security protocol
Example:
Interface Ethernet 0 Ip address 172.16.1.100 255.255.255.0 Ip helper-address 172.16.2.2!ip forward-protocol udp 3000no ip forward-protocol udp tftp
The "ip helper-address" command must be configured on the router interface that received the initial client broadcast packet.
Chapter 3: Configuring OSPF in a single area
OSPF is a link state technology, such as the Routing Information Protocol (RIP) distance vector technology. The OSPF protocol completes two major functions of each routing protocol algorithm: path selection and path exchange.
OSPF is an internal gateway protocol (IGP), which means it publishes routing information between routers belonging to the same autonomous system.
OSPF is written to solve large, scalable network requirements that RIP cannot solve. It solves the following problems:
l Convergence rate
l Support for variable length mask (VLSM)
OSPF and RIPV2 support VLSM, RIP only supports fixed-length subnet mask (FLSM)
lNetwork accessibility
When the RIP span reaches 16 jumps, it is considered unreachable, and OSPF has no accessibility limit in theory.
Bandwidth usage
RIP broadcasts the complete route every 30 seconds, and OSPF will be updated only if the link changes.
lPath selection method
RIP selects the best path based on the number of hops, and OSPF uses a path cost value (for Cisco routers it is based on the connection rate) as the basis for path selection. OSPF is the same as RI P and IGRP and other overhead paths.
OSPF information is in the IP packet, using protocol number 89
OSPF can run on broadcast or non-broadcast networks
OSPF operation in broadcast multi-access topology structure
Hello Agreement is responsible for establishing and maintaining neighbor relationships
Through IP multi-member broadcast 224.0.0.5, also known as ALLSPFROUTER (all SPF routers) address, Hello packets are regularly sent from various interfaces participating in OSPF.
The information contained in the Hello packet is as follows:
lRouter ID
This 32-bit number uniquely identifies a router within an autonomous system. It defaults to select the highest IP address on the active interface. This identification is important when establishing neighbor relationships and running messages copied by municipalities in the network by S PF algorithm.
lHELLO interval and DOWN machine judgment interval (dead interval)
The HELLO interval specifies the time interval (seconds) for routing the HELLO to be sent. The DOWN machine determines that the interval is the time when the router waits for receiving messages from neighbors before deeming the adjacent router invalid, and the unit is in seconds, and the default is 4 times the H ELLO interval.
Neighbor
These are adjacent routers that have established bidirectional communication relationships
l Region ID
To be able to communicate, both routers must share a common network segment
lRouter priority
These 8 bit numbers indicate the priority of this router when selecting DR and BDR.
lDR and BDR IP addresses
lCertification Password
lUnsection (stb) area flag
Each domain in the OSPF packet header:
l Version number 1 (byte number)
l Type 1
HELLO
Link status request
Link status update
Link status confirmation
l Packet length 2
lRouter ID 4
l Region ID 4
l Checksum 2
lCertification Type 2
lCertification 8
l Data variable
Specify router DR and alternate specified router BDR
A router in a multi-access environment such as Ethernet segmentation must elect a DR and a BDR to represent the network. BDR does not perform any DR functions while DR is running. But it will receive all information, but it just does not process it, and D R completes the forwarding and synchronization tasks. BDR only takes on DR work when it fails.
The value of DR and BDR:
l Reduce routing update data flow
DR and BDR play a central point for the exchange of link state information on a given multiple access network. Each router must establish an adjacency relationship with DR and BDR, and DR sends link status information for each route to all other routers in the multi-access network. This diffusion process greatly reduces the data flow associated with the router on the network segment.
lManage link status synchronization
DR and BDR can ensure that other routers on the network have the same link status information about the network.
The adjacency relationship is the relationship that exists between the router and its DR and BDR. Adjacent routers will have a synchronized link state database
When DR and BDR are elected, the router will view the priority values between each other during the HELLO packet exchange.
Determine DR and BDR according to the following conditions
lThe router with the highest priority value becomes DR
l The router with the second highest priority is called BDR
l Routers with priority 0 cannot be cocooned to DR or BDR, and are called Drother (non-DR)
lIf a router with higher priority is added to the network, the original DR and BDR remain unchanged, and will only change if DR or BDR fails.
The process of OSPF startup:
1. Exchange process
When a router A is started, it is in the DOWN state. It sends HELLO packets from its various interfaces through 224.0.0.5 to other routers running OSPF. After receiving this HELLO packet, other routers will add it to their neighbor list. This is called the "init" state, and then send a single-point reply HELLO packet, which contains information from its own and other neighbor routers. After receiving this HELLO, router A will add a neighboring relational database to its own library. This is called the "two-way" state, and two-way communication is established.
2. Discover the route
After DR and BDR are selected, the router is considered to be in the "exstart state" and has
Be prepared to discover link status information about the network and generate their link status database. This process used to discover network routing is called the switching protocol, which is executed to use the right router to achieve the "FULL" state of communication. The first step in this protocol is to enable DR and BDR to establish a neighbor relationship with other routers. When adjacent routers are in "full" state, they do not execute the switching protocol repeatedly unless the "full" state changes.
3. Select a route
When the router has a complete link state database, it is ready to create its routing table so that it can
Forward the data flow. The default overhead metric on a CISCO router is based on the bandwidth of the network media. To calculate the minimum overhead to reach the destination, the link state routing protocol (such as OSP F) uses the Dijkstra algorithm. Up to 6 overhead routing entries are saved in the OSPF routing table for load balancing, which can be configured through "maximum-paths".
If fapping flips occur on the link, the router will constantly calculate a new routing table, which may cause the router to fail to converge. The router needs to recalculate the routing table objectively stored in it, and the default value is 5 seconds. In the CISCO configuration command "times spf spf-delay spy-holdtime" can configure the shortest time (default value 10 seconds) between two consecutive SPF calculations.
4. Maintain routing information
In a link-state routing environment, it is important that the topological databases of all routers must remain synchronized. When the link state changes, the router notifies other routers in the network through the diffusion process. The link state update packet provides the technology of diffusion L SA
Each LSA has its own aging timer, which is carried within the LS life domain. The default value is 30 minutes
OSPF operation in point-to-point topology structure
On a point-to-point network, a router detects its neighbors by multicasting to a multi-directional address. There is no need to perform selection, because there is no concept of DR and BDR on point-to-point. The default O SPF hello interval and down machine interval are 10 seconds and 40 seconds on NBMA topology structure.
OSPF operation in non-broadcast multi-access (NBMA) topology
NBMA networks are networks that can support multiple (more than two) routers but do not have broadcast capabilities.
Frame relay, ATM, and X.25 are all examples of NBMA networks
The default OSPF hello interval and down machine interval are 30 seconds and 120 seconds on NBMA topology
The following table shows the default OSPF hello interval and down machine interval on various topology structures
OSPF environment Hello interval Down machine determination interval
Broadcast 10 seconds 40 seconds
Point-to-point 10 seconds 40 seconds
NBMA 30 seconds 120 seconds
OSPF operates in one of two formal modes in NBMA topology:
lNon-broadcast multiple access
l point to multi point
When configuring a router in an NBMA topology, sub-interfaces are usually used
The following commands can be used to create a subinterface:
iterface serial -number {multpiont | point-to-point}
In large networks, point-to-multipoint mode can reduce the number of PVCs required for full connectivity
Point-to-multipoint has the following properties
l No need for fully interconnected network
lNo static neighbor configuration is required
lUse an IP subnet
lCopy the LSA packet
OSPF summary on NBMA topology
The pattern expects topological structure subnet address adjacent relationship RFC or Cisco definition
NBMA fully interconnected neighbors must belong to the same subnet number. Manually configured election DR/BDRRFC
Broadcast fully interconnected neighbors must belong to the same subnet number to automatically elect DR/BDRCisco
Point-to-multipoint mutual edges or star-shaped neighbors must belong to the same subnet number automatically, without DR/BDRRFC
Point-to-multipoint non-broadcasting part mutual edges or star-shaped neighbors must belong to the same subnet number manually configured without DR/BDRCisco
Point-to-point interconnection through subinterfaces or star-type subinterfaces belong to different subnets automatically, without DR/BDRCisco
Configure OSPF in a single area
To configure OSPF, we must perform the following steps:
l Start the OSPF process on the route through the "router ospf process-id" global configuration command
process-id is an internal number
lUse the "network area" router configuration command to identify which IP network numbers on the router are part of the OSPF network.
network address wildcard area area-id
To confirm the router's ID, you can enter the show ip ospf interface command
Modify the priority of the router: router(config)#ip ospf priority number
number is a number between 1 and 255, the default is `1, 0 means that it cannot be elected as DR or BDR
Modifying the link overhead must overwrite the default overhead value assigned to an OSPF interface through the "ip ospf cost cost" command.
To control how OSPF calculates the interface default metric (overhead) you can use "auto-cost reference-bandwidth"
In the interface configuration mode, enter the "ip ospf network" command to specify the OSPF network mode configuration
Chapter 4: Connecting multiple OSPF areas
To solve the frequent calculations of the shortest path-first (SPF) algorithm, large routing tables, and large link state tables, OSPF is designed to divide a large network into multiple regions, also known as systematic routing. Systematic routing allows us to divide large networks (autonomous systems) into small networks called regions
The systematic topological structure of OSPF has the following advantages:
lSPF calculation frequency decreases
lSmaller routing table
l Link status update (LSU) load reduction
The OSPF router types are as follows:
lInternal router
lBronze router
l Regional Boundary Router (ABR)
lAutomatic System Boundary Router (ASBR)
Type of area
l Standard area
Main area
lUnsection Area
lA totally unblocked area
1 times unchained areas
How packets pass through multiple areas:
lIf the destination of the packet is a network outside the local area, it will be forwarded to the destination internal router by the internal router;
lIf the destination of the packet is a network outside the region, it must pass the following path
-------The packet from the source network to an ABR
------- ABR sends data packets to the destination network ABR through outside the backbone area
-------Destination ABR forwards data packets to the destination network within the domain
There are two conditions for virtual links:
l It must be established between two ABRs next to one common area
lOne of these two ABRs must be connected to the main area
There are no special commands on the router to activate the functions of ABR or ASBR. The router assumes this role through the situation of the area it connects to. The basic configuration steps of OSPF are as follows:
lEnable OSPF on the router
router(config)#router ospf process-id
lIndicate which IP networks on the router are used as part of the OSPF
router(config-router)#network address wildcard-mask area area-id
l (optional) If the router has an interface connected to a non-OSPF network, then the corresponding configuration steps must be performed.
To further reduce the number of routing tables, we can create a completely unsectioned area, a proprietary feature of CISCO.
Router ospf 200
Enable OSPF with process ID 200
network 0.0.0.0 area 0
Specify the interfaces running OSPF and their regions
area x range 192..0 255.255.255.0
Inductive address
area X stub [no-summary]
Configure a zone as an unchained or completely unchained zone
area x virtual-link 192..49
Create an OSPF virtual link
area x nssa
Configure a zone as a sub-unsegmented zone (NSSA)
summary-address 172.16.0.0 255.255.0.0
Summary and publish external addresses to OSPF
show ip ospf
Show general information about OSPF routing process
show ip ospf neighbor
Show information about OSPF neighbors
show ip ospf database
Display entries in the OSPF link status database
show ip ospf interface
Display specific OSPF information about an interface
show ip ospf virtual-links
Display the status of the OSPF virtual link
debug ip ospf adj
Shows events involving the establishment or demolition of an OSPF adjacency relationship
1.1 Basic knowledge of routing
Routing is a relay process that transfers objects from one place to another
The mechanism for learning and maintaining knowledge of network topology is considered a routing function. Transfer data flows through the router and enters the interface
The process of being transferred through the router to the outbound interface is another separate function, considered a switch/forward function. The routing device must have both routing and switching functions to be an effective relay device.
In order to perform routing, the router must know the following three things:
lThe router must determine whether it activates support for the protocol group;
lThe router must know the destination network;
l The router must know which outgoing interface is the best way to reach the destination.
The routing protocol determines the best path to the destination by metrics. Small metrics represent preferred paths; if two or more paths have the same small metric, then all of them will be shared equally. Splitting data traffic through multiple paths is called load balancing to the destination.
The information required to perform routing operations is contained in the router's routing table, and they are generated by one or more routing protocol processes. The routing table consists of multiple routing entries, each of which indicates the following:
lLearn the mechanism used by this route (dynamic or manual)
Logical destination
Management distance
l Measurement value (it is a measure that measures the total "total overhead" of a path)
lThe address of the relay device (router) to the destination lower HOP;
lThe old and new route information
l Interface associated with the network to go to the destination
Use the command SHOW IP ROUTE to see the above content
The pre-allocation principle of default management distance is: manually set the priority of routing entries higher than dynamically learned routing entries, and the routing protocol with complex routing algorithms with higher priority of routing protocols with complex routing algorithms with simple routing protocols.
Routers generally choose paths with the smallest metric value; if multiple paths with the lowest and same metric value appear at the same time in the IP environment of the CISCO router, load balancing will be enabled on these multiple paths. C ISCO supports 4 paths with the same metric value by default. By using the "maximum-paths" command, it can be recognized that the CISCO router supports up to 6 paths with the same metric value.
RIP is a routing protocol used in small to medium-sized TCP/IP networks. It uses the number of hops as the measure value. Its load balancing function is enabled by default. RIP determines the optimal path without considering bandwidth! ! !
IGRP is a routing protocol used in medium to large TCP/IP networks. It uses composite metrics that take into account bandwidth, latency, reliability, load and maximum transmission unit (M TU), but uses bandwidth and delay values by default. IGRP can also perform load balancing
After the router starts, it immediately tries to establish a routing relationship with its adjacent routing devices. The purpose of this initial communication is to identify adjacent devices and start communication and learn the network phase structure. The method of establishing neighbor relationships and the initial learning of topology vary with the routing protocol.
The routing protocol exchanges periodic HELLO messages or periodic routing update packets to maintain communication between adjacent devices.
After understanding the network topology structure and the routing table already contains the best path to the known network, forwarding data to these destinations can begin;)
1.2 Routing Protocol
Classful routing overview
The routing protocol that does not send subnet mask information with each network address is called a class selection protocol (RIPv1, IGRP)
When using a class routing protocol, all subnets belonging to the same main class network (Class A, B and C) must use the same subnet mask. A router running a routing protocol with a category routing protocol will perform one of the following tasks to determine the routing network part:
lIf the routing update information is about the same main network configured on the receiving interface, the router will use the subnet mask configured on the interface;
lIf the routing update is about a network of different main classes that are assigned on the receiving interface, the router will use the default subnet mask according to its address category.
The generation of category inductive routes is automatically processed by category routing protocols.
Classless routing overview
Classless routing protocols include Open Shortest Path First (OSPF), EIGRP, RIPV2, Intermediate System-to-Intermediate System (IS-IS), and Border Gateway Protocol Version 4 (BGP4).
Using different mask lengths in the same main-class network is called a variable-length subnet mask (VLSM). The classless routing protocol supports VLSM, so the subnet mask can be set more effectively to meet the needs of different subnets for different hosts, and can make full use of the host address.
Most distance vector routing protocols produce regular, routine routing updates that are transmitted only to directly connected routing devices.
In a purely distance vector routing environment, routing updates include a complete routing table. By receiving the full routing table of adjacent devices, the routing can check all known routes and then modify the local routing table based on the received update information. The distance vector method that solves routing problems is sometimes called "routing by rumor"
CISCO IOS supports several distance vector routing protocols, RIPv1, RIPv2 and IGRP. CISCO also directly holds EIGRP, which is an advanced distance vector routing protocol.
The routing protocol is usually associated with the network layer of the protocol group.
Most distance vector routing protocols use Bellman-Ford algorithm to calculate routing. EIGRP is an advanced distance vector routing protocol that uses diffusion correction algorithm (D UAL)
Comparison of Cisco's IP Distance Vector Routing Protocol
Features RIPv1RIPv2IGRPEIGRP
Count to infinite XXX
Cross-side distance XXXX
Suppress timer XXX
Triggered update, routing reverse XXXX
Load balancing-equal cost path XXXX
Load balancing-non-equal cost path XX
VLSM supports XX
Routing algorithm Bellman-Ford Bellman-Ford Belman-Ford DUAL
Measurement value hop number hop number compound
Jump limit 1515100100
Easy to expand small and medium large
Note: The default hop limit for IGRP and EIGRP is 100, but it can be configured to a maximum of 255.
The link-state routing protocol generates routing update packets only when the network topology changes. When the link state changes, the device that detects the change generates a link state announcement (L SA) about the link (routing). The LSA is then propagated to all neighboring devices through a special multicast address. Each routing device retains an LSA copy and forwards the LSA to its neighbors (this process becomes called diffusion f loading) and then updates its topology database (this is a table containing all link status information for the network). LSA diffusion is used to ensure that all routing devices are aware of this change so that they can update their data and generate an updated routing table that reflects the new network topology.
Comparison of Cisco's Link-state Routing Protocol
Features OSPFIS-ISEIGRP
Requires systematic topological structure XX
Keep an understanding of all possible routes XXX
Routing Induction-Artificial XXX
Routing Induction-Automatic X
Event trigger notification XXX
Load balancing-equal cost path XXX
Load balancing-non-equal cost path X
VLSM supports XXX
Routing algorithm DijkstraIS-ISDUAL
Metric link cost (bandwidth) link cost (bandwidth) composite
No hop limit is 1024100
Very easy to expand
The routing processes in each router must leave a single loop-free path to each possible destination logical network. When all routing tables are synchronized and each routing table contains an available route to the destination network, the network reaches a convergence state. Convergence is the activity associated with the routing table synchronization after the network topology changes, such as the state of adding new routes or the existing route changes.
Convergence time is the time required for all routes in the network to achieve consistency in their perception of the current topology structure. The size of the network, the routing protocol used, and numerous configurable timers can all affect the convergence time.
There are two methods of detection:
l When the physical layer or data link layer fails to receive a certain number (usually 3) of continuous keepalive messages, the link is considered invalid.
l When the routing protocol fails to receive a certain number (usually 3) of continuous Hello messages or route updates or similar messages, the link is considered to be invalid.
Most routing protocols have timers to prevent topological loops from occurring during link state transitions.
Chapter 2 Extended IP Address
The Internet is developing incredibly fast. This rapid development has led to two major challenges in address:
lIP address exhaustion
lGrowth and manageability of routing tables
IP addressing solution:
Slow down IP address consumption and reduce Internet routing table entries by enabling more hierarchical layers in IP addresses
quantity. These solutions include:
l Subnet mask
lAddress allocation for private network
lNetwork Address Translation (NAT)
l Systematic addressing
lVariable length subnet mask (VLSM)
lRoute induction
lCategory Inter-Domain Routing (CIDR)
Category of IP address:
The first byte (decimal) address category of the address
Class 1~126A
Class 128~191B
Class 192~223C
Class 224~239D
Class 240~255E
Class D address has not been widely used yet, it is a multi-came multicast address; some routing protocols use the following:
OSPF-----224.0.0.5 and 224.0.0.6
RIPv2-----224.0.0.9
EIGRP----224.0.0.10
Systematic addressing:
Systematic addressing is like when we make a phone call. Each telephone bureau does not need to know the national phone number. If the first one is not 0, the switchboard will find the link in its own phone entry and then take it. If it is 0, then it depends on the area code. For example, it is 0791-6221155. It will transmit this information to the Nanchang Telephone Bureau (0791) and find the link 6221155 and connect it to the Nanchang Telephone Bureau. In this way, your switchboard does not need to have entries from other places. Let others have a taste of it. J. The principle can also be used in the router.
Advantages of systematic addressing:
lReduce the number of routing entries
Routing induction is a method when we adopt a systematic addressing planning method that uses an IP address to represent a set of IP addresses. By summarizing the route, we can keep the routing table entries as manageable, and it can bring the following benefits:
------Improve routing (forwarding) efficiency;
------ When recalculating the routing table or retrieving a match through the routing table entry, the number of CPU cycles required is reduced;
------Reduced memory requirements for routers
------Convergence faster when the network changes
-------Easy to make mistakes
l Valid address assignment
Systematic addressing allows us to take advantage of all possible addresses, because our address packets are continuous;
Variable length subnet mask (VLSM)
VLSM proposes the ability to include multiple subnet masks in a main class (A, B, C) network, as well as the ability to subnet the subnet again. Its advantages are as follows:
l More efficient use of IP addresses - if VLSM is not used, the company will be restricted to only use one subnet mask within a Class A, B, and C network number;
lThe ability to use routing induction is stronger - VLSM allows more system hierarchies in the addressing plan, so better routing induction can be performed in the routing table.
Routing Induction
In large interconnected networks, there are hundreds of networks. In this environment, it is generally not intended that the router saves all these routes in its routing table. Routing induction (also called routing aggregation or hypernetting) can reduce the number of routing entries a router must save because it is a way to represent a series of network numbers in an inductive address.
Another advantage of using routing induction in large, complex networks is that it can protect other routers from changes in network topology.
Only when a correct address planning is used can routing induction be feasible and most effective. In a subnet environment, routing induction is the most effective when the network address is a continuous block in the form of an exponential form of 2.
The routing protocol summarizes or aggregates routes based on the shared network address part. Classless routing protocols--OSPF and EIGRP-support routing induction based on subnet addresses, including VLSM editor address. Category routing protocols - RIPv1 and IGRP - automatically summarize routing on the boundaries of a Category network. The Category Routing Protocol does not support routing induction on any other bit boundary, while the Category Routing Protocol does not support routing induction on any bit boundary.
Because there are fewer entries in the routing table, routing induction can reduce the use of router memory and reduce network traffic caused by routing protocols. To enable routing induction in the network to work correctly, the following requirements must be met:
l Multiple IP addresses must share the same high bit;
lRouting protocol must make routing forwarding decisions based on the 32-bit IP address and the prefix length up to 32-bit
lRouting update must transmit the prefix length (subnet mask) with the 32-bit IP address.
Operation of routing induction in Cisco routers
CISCO manages routing induction through the following two methods:
lSend route induction
lSelect route from route summary
Subnets with discontinuous addresses refer to some subnets in the same main network separated by other different main networks.
Support of routing protocols for routing induction
Is the protocol automatically summarized in the boundaries of categories? Can automatic induction be turned off? Can automatic induction be performed outside the boundaries of the category network
Is RIPv1 no
Is RIPv2?
Is IGRP
EIGRP Yes Yes Yes
OSPF No-Yes
Classless inter-domain routing (CIDR)
CIDR is a technology developed to help slow down IP addresses and routing table enlargement problems. The concept of CIDR is that multiple C-class address blocks can be combined or aggregated together to generate a larger set of classless IP addresses (that is, allow more hosts). The C-class address of the block is assigned to each ISP
Using an unnumbered IP address on a serial interface
To enable IP processing on a serial interface without assigning an explicit IP address to the interface, you can use the "ip unnumbered type number" interface configuration command. In this command "type number" is the type and number of another interface on the router with an assigned IP address (this interface is called the specified interface or reference interface, that is, the interface from which the unnumbered interface borrows the IP address). It cannot be another numberless interface. If you want to turn off the IP processing function in the serial interface, you can use the NO form of this command.
Limitations of no numbered interface:
lSerial interfaces using HDLC, PPP, LAPB, SLIP protocols, and tunnel interfaces can be used without numbers. Can't be in X. 25 or switched multi-megabit data service SMDS interface use the numberless interface configuration command.
l We cannot use the PING command to determine whether the unnumbered interface has been UP because the interface has no address. SNMP can remotely monitor the status of the interface.
example:
Interface Ethernet0 Ip address 10.1.1.1 255.255.255.0!interface Serial0 ip unnumbered Ethernet0
Use Helper Address
The router does not forward broadcasts, and the help address helps the client and the server establish a connection by forwarding these broadcast packets directly to the target server.
The Help Address command changes the broadcast destination address to a single point communication room address (or a directed broadcast - this broadcast within a subnet), so that the broadcast message can be routed to a specific destination instead of all places
Use the "ip helper-address address" interface configuration command to configure an interface that may receive broadcasts. In this command "ADDRESS" refers to the destination address used when forwarding User Datagram Protocol (UDP) broadcasts. The specified address can be a single-pass address or a directed broadcast address of the remote server.
If the "ip helper-address address" command is defined, the function of forwarding for the 8 default UDP ports is automatically enabled, namely: TFTP (69), DNS (53), Time (37), NETBIOS service (137), N ETBIOS datagram service (138), BOOTP server (67), BOOTP client (68), and terminal access controller access control system TACACS (49).
If the "ip helper-address address" command is defined and the "ip forward-protocol udp" command that specifies these 8 UDP ports, then the broadcast packets addressing these 8 UDP ports will be automatically forwarded.
"ip forward-protocol" description:
"ip forward-protocol" command description
udpUDP-Transport Layer Protocol
port (optional) When the "udp" keyword is specified, the UDP destination port number or port name can be defined.
nd network disk; an old protocol used by diskless Sun workstations
sdns network security protocol
Example:
Interface Ethernet 0 Ip address 172.16.1.100 255.255.255.0 Ip helper-address 172.16.2.2!ip forward-protocol udp 3000no ip forward-protocol udp tftp
The "ip helper-address" command must be configured on the router interface that received the initial client broadcast packet.
Chapter 3: Configuring OSPF in a single area
OSPF is a link state technology, such as the Routing Information Protocol (RIP) distance vector technology. The OSPF protocol completes two major functions of each routing protocol algorithm: path selection and path exchange.
OSPF is an internal gateway protocol (IGP), which means it publishes routing information between routers belonging to the same autonomous system.
OSPF is written to solve large, scalable network requirements that RIP cannot solve. It solves the following problems:
l Convergence rate
l Support for variable length mask (VLSM)
OSPF and RIPV2 support VLSM, RIP only supports fixed-length subnet mask (FLSM)
lNetwork accessibility
When the RIP span reaches 16 jumps, it is considered unreachable, and OSPF has no accessibility limit in theory.
Bandwidth usage
RIP broadcasts the complete route every 30 seconds, and OSPF will be updated only if the link changes.
lPath selection method
RIP selects the best path based on the number of hops, and OSPF uses a path cost value (for Cisco routers it is based on the connection rate) as the basis for path selection. OSPF is the same as RI P and IGRP and other overhead paths.
OSPF information is in the IP packet, using protocol number 89
OSPF can run on broadcast or non-broadcast networks
OSPF operation in broadcast multi-access topology structure
Hello Agreement is responsible for establishing and maintaining neighbor relationships
Through IP multi-member broadcast 224.0.0.5, also known as ALLSPFROUTER (all SPF routers) address, Hello packets are regularly sent from various interfaces participating in OSPF.
The information contained in the Hello packet is as follows:
lRouter ID
This 32-bit number uniquely identifies a router within an autonomous system. It defaults to select the highest IP address on the active interface. This identification is important when establishing neighbor relationships and running messages copied by municipalities in the network by S PF algorithm.
lHELLO interval and DOWN machine judgment interval (dead interval)
The HELLO interval specifies the time interval (seconds) for routing the HELLO to be sent. The DOWN machine determines that the interval is the time when the router waits for receiving messages from neighbors before deeming the adjacent router invalid, and the unit is in seconds, and the default is 4 times the H ELLO interval.
Neighbor
These are adjacent routers that have established bidirectional communication relationships
l Region ID
To be able to communicate, both routers must share a common network segment
lRouter priority
These 8 bit numbers indicate the priority of this router when selecting DR and BDR.
lDR and BDR IP addresses
lCertification Password
lUnsection (stb) area flag
Each domain in the OSPF packet header:
l Version number 1 (byte number)
l Type 1
HELLO
Link status request
Link status update
Link status confirmation
l Packet length 2
lRouter ID 4
l Region ID 4
l Checksum 2
lCertification Type 2
lCertification 8
l Data variable
Specify router DR and alternate specified router BDR
A router in a multi-access environment such as Ethernet segmentation must elect a DR and a BDR to represent the network. BDR does not perform any DR functions while DR is running. But it will receive all information, but it just does not process it, and D R completes the forwarding and synchronization tasks. BDR only takes on DR work when it fails.
The value of DR and BDR:
l Reduce routing update data flow
DR and BDR play a central point for the exchange of link state information on a given multiple access network. Each router must establish an adjacency relationship with DR and BDR, and DR sends link status information for each route to all other routers in the multi-access network. This diffusion process greatly reduces the data flow associated with the router on the network segment.
lManage link status synchronization
DR and BDR can ensure that other routers on the network have the same link status information about the network.
The adjacency relationship is the relationship that exists between the router and its DR and BDR. Adjacent routers will have a synchronized link state database
When DR and BDR are elected, the router will view the priority values between each other during the HELLO packet exchange.
Determine DR and BDR according to the following conditions
lThe router with the highest priority value becomes DR
l The router with the second highest priority is called BDR
l Routers with priority 0 cannot be cocooned to DR or BDR, and are called Drother (non-DR)
lIf a router with higher priority is added to the network, the original DR and BDR remain unchanged, and will only change if DR or BDR fails.
The process of OSPF startup:
1. Exchange process
When a router A is started, it is in the DOWN state. It sends HELLO packets from its various interfaces through 224.0.0.5 to other routers running OSPF. After receiving this HELLO packet, other routers will add it to their neighbor list. This is called the "init" state, and then send a single-point reply HELLO packet, which contains information from its own and other neighbor routers. After receiving this HELLO, router A will add a neighboring relational database to its own library. This is called the "two-way" state, and two-way communication is established.
2. Discover the route
After DR and BDR are selected, the router is considered to be in the "exstart state" and has
Be prepared to discover link status information about the network and generate their link status database. This process used to discover network routing is called the switching protocol, which is executed to use the right router to achieve the "FULL" state of communication. The first step in this protocol is to enable DR and BDR to establish a neighbor relationship with other routers. When adjacent routers are in "full" state, they do not execute the switching protocol repeatedly unless the "full" state changes.
3. Select a route
When the router has a complete link state database, it is ready to create its routing table so that it can
Forward the data flow. The default overhead metric on a CISCO router is based on the bandwidth of the network media. To calculate the minimum overhead to reach the destination, the link state routing protocol (such as OSP F) uses the Dijkstra algorithm. Up to 6 overhead routing entries are saved in the OSPF routing table for load balancing, which can be configured through "maximum-paths".
If fapping flips occur on the link, the router will constantly calculate a new routing table, which may cause the router to fail to converge. The router needs to recalculate the routing table objectively stored in it, and the default value is 5 seconds. In the CISCO configuration command "times spf spf-delay spy-holdtime" can configure the shortest time (default value 10 seconds) between two consecutive SPF calculations.
4. Maintain routing information
In a link-state routing environment, it is important that the topological databases of all routers must remain synchronized. When the link state changes, the router notifies other routers in the network through the diffusion process. The link state update packet provides the technology of diffusion L SA
Each LSA has its own aging timer, which is carried within the LS life domain. The default value is 30 minutes
OSPF operation in point-to-point topology structure
On a point-to-point network, a router detects its neighbors by multicasting to a multi-directional address. There is no need to perform selection, because there is no concept of DR and BDR on point-to-point. The default O SPF hello interval and down machine interval are 10 seconds and 40 seconds on NBMA topology structure.
OSPF operation in non-broadcast multi-access (NBMA) topology
NBMA networks are networks that can support multiple (more than two) routers but do not have broadcast capabilities.
Frame relay, ATM, and X.25 are all examples of NBMA networks
The default OSPF hello interval and down machine interval are 30 seconds and 120 seconds on NBMA topology
The following table shows the default OSPF hello interval and down machine interval on various topology structures
OSPF environment Hello interval Down machine determination interval
Broadcast 10 seconds 40 seconds
Point-to-point 10 seconds 40 seconds
NBMA 30 seconds 120 seconds
OSPF operates in one of two formal modes in NBMA topology:
lNon-broadcast multiple access
l point to multi point
When configuring a router in an NBMA topology, sub-interfaces are usually used
The following commands can be used to create a subinterface:
iterface serial -number {multpiont | point-to-point}
In large networks, point-to-multipoint mode can reduce the number of PVCs required for full connectivity
Point-to-multipoint has the following properties
l No need for fully interconnected network
lNo static neighbor configuration is required
lUse an IP subnet
lCopy the LSA packet
OSPF summary on NBMA topology
The pattern expects topological structure subnet address adjacent relationship RFC or Cisco definition
NBMA fully interconnected neighbors must belong to the same subnet number. Manually configured election DR/BDRRFC
Broadcast fully interconnected neighbors must belong to the same subnet number to automatically elect DR/BDRCisco
Point-to-multipoint mutual edges or star-shaped neighbors must belong to the same subnet number automatically, without DR/BDRRFC
Point-to-multipoint non-broadcasting part mutual edges or star-shaped neighbors must belong to the same subnet number manually configured without DR/BDRCisco
Point-to-point interconnection through subinterfaces or star-type subinterfaces belong to different subnets automatically, without DR/BDRCisco
Configure OSPF in a single area
To configure OSPF, we must perform the following steps:
l Start the OSPF process on the route through the "router ospf process-id" global configuration command
process-id is an internal number
lUse the "network area" router configuration command to identify which IP network numbers on the router are part of the OSPF network.
network address wildcard area area-id
To confirm the router's ID, you can enter the show ip ospf interface command
Modify the priority of the router: router(config)#ip ospf priority number
number is a number between 1 and 255, the default is `1, 0 means that it cannot be elected as DR or BDR
Modifying the link overhead must overwrite the default overhead value assigned to an OSPF interface through the "ip ospf cost cost" command.
To control how OSPF calculates the interface default metric (overhead) you can use "auto-cost reference-bandwidth"
In the interface configuration mode, enter the "ip ospf network" command to specify the OSPF network mode configuration
Chapter 4: Connecting multiple OSPF areas
To solve the frequent calculations of the shortest path-first (SPF) algorithm, large routing tables, and large link state tables, OSPF is designed to divide a large network into multiple regions, also known as systematic routing. Systematic routing allows us to divide large networks (autonomous systems) into small networks called regions
The systematic topological structure of OSPF has the following advantages:
lSPF calculation frequency decreases
lSmaller routing table
l Link status update (LSU) load reduction
The OSPF router types are as follows:
lInternal router
lBronze router
l Regional Boundary Router (ABR)
lAutomatic System Boundary Router (ASBR)
Type of area
l Standard area
Main area
lUnsection Area
lA totally unblocked area
1 times unchained areas
How packets pass through multiple areas:
lIf the destination of the packet is a network outside the local area, it will be forwarded to the destination internal router by the internal router;
lIf the destination of the packet is a network outside the region, it must pass the following path
-------The packet from the source network to an ABR
------- ABR sends data packets to the destination network ABR through outside the backbone area
-------Destination ABR forwards data packets to the destination network within the domain
There are two conditions for virtual links:
l It must be established between two ABRs next to one common area
lOne of these two ABRs must be connected to the main area
There are no special commands on the router to activate the functions of ABR or ASBR. The router assumes this role through the situation of the area it connects to. The basic configuration steps of OSPF are as follows:
lEnable OSPF on the router
router(config)#router ospf process-id
lIndicate which IP networks on the router are used as part of the OSPF
router(config-router)#network address wildcard-mask area area-id
l (optional) If the router has an interface connected to a non-OSPF network, then the corresponding configuration steps must be performed.
To further reduce the number of routing tables, we can create a completely unsectioned area, a proprietary feature of CISCO.
Router ospf 200
Enable OSPF with process ID 200
network 0.0.0.0 area 0
Specify the interfaces running OSPF and their regions
area x range 192..0 255.255.255.0
Inductive address
area X stub [no-summary]
Configure a zone as an unchained or completely unchained zone
area x virtual-link 192..49
Create an OSPF virtual link
area x nssa
Configure a zone as a sub-unsegmented zone (NSSA)
summary-address 172.16.0.0 255.255.0.0
Summary and publish external addresses to OSPF
show ip ospf
Show general information about OSPF routing process
show ip ospf neighbor
Show information about OSPF neighbors
show ip ospf database
Display entries in the OSPF link status database
show ip ospf interface
Display specific OSPF information about an interface
show ip ospf virtual-links
Display the status of the OSPF virtual link
debug ip ospf adj
Shows events involving the establishment or demolition of an OSPF adjacency relationship