When it comes to routing protocols and routers, administrative distance and metrics are two important sets of parameters. What do these two sets of parameters really mean? David Davis will introduce you to these two sets of parameters and explain why using Cisco routers requires understanding of the importance of administrative parameters.
When it comes to the routing protocol and which channel the router uses, management spacing and measurement are two important sets of parameters. Fully familiar with these two sets of parameters plays a very important role in understanding various parts such as network performance, reliability, and loop selection.
If you are not very familiar with managing spacing and measurements, you won't pay attention to these parameters even if you see them. If you enter a show ip route command, you will notice these two parameters appearing in the brackets behind the router. Here is an example:
O 10.1.103.0/24 [110/791] via 10.1.100.2, 00:39:44, Serial1/0:0.21
In this example, 110 represents the management spacing and 791 represents the measurement. You can see more detailed information by entering the same show ip route command and specifying the way the router is. Here is an example:
Router# show ip route 10.1.103.0
Routing entry for 10.1.103.0/24
Known via "ospf 100", distance 110, metric 791, type intra area
Last update from 10.1.100.2 on Serial1/0:0.21, 01:09:25 ago
Routing Descriptor Blocks:
* 10.1.100.2, from 172.16.1.1, 01:09:25 ago, via Serial1/0:0.21
Route metric is 791, traffic share count is 1
But what do these numbers really mean? Let's learn more about the meaning of each parameter.
Manage spacing
Administrative spacing (AD for short) means the choice of which channel the router decides when facing two identical channels from different sources. That is, if the router receives information from different sources but with the same content, which channel the router trusts. A better way is that since you have to try your best to select local information, and the global information has some duplicate events, local information is more trustworthy.
If your router only has one routing protocol and one WAN loop, or if you only use static routes, the management spacing will not affect you. But this does not mean you don't need to understand the role of managing spacing.
But if you have a more complex network system, such as two WAN loops, or you use two routing protocols (even if one of them is a static route), you should understand the importance of managing spacing.
Routing resources are not just likeRIP, OSPF or BGP, and there may be some resources connected to the router (such as the router interface) and static routes (the router you use as the management server).
The router chooses which resource to trust based on the management distance. The smaller the management spacing, the more trustworthy its routing resources are.
To facilitate this decision, the router installed a pre-programmed schedule displayed in all possible resources and default management spacing. Table A provides an example of this table. (Although the manager changes the default management spacing by using the distance command in router configuration mode, this is usually an undesirable approach.)
SOURCE OF THE ROUTE DEFAULT DISTANCEConnected Interface or static route to an Interface 0Static route to an IP Address 1EIGRP Summary 5BGP External 20EIGRP Internal 90IGRP 100OSPF 110IS-IS 115RIP 120EIGRP External 170BGP Internal 200Unknown Source 255
For example, if the router receives a route from OSPF and a route fromRIP, it will select OSPF route. Because the management spacing of OSPF is 110,RIPThe management interval is 120.
Here is another example: For example, your router receives a route from EIGRP Internal, and its management spacing is 90, but you accidentally enter a static route into an IP address, and the spacing management address of this IP address is 1. Then the router will use static routing without EIGRP routing.
Finally, I would like to emphasize: managing spacing is the focus of the CCNA exam. If you are preparing to take this exam, you must know the management spacing of general routing protocols.
Measurement
The routing protocol uses metrics to determine which route to put the routing table into when two valid routes can be sent to the same target file. The router puts the routing table into the least-measured route because it thinks that the route is closest and therefore is the best.
Contrary to administrative spacing, there is only one routing protocol for metrics. They cannot handle routing in multiple repositories.
For example: enter a show ip eigrp topology command:
P 10.55.103.0/24, 1 successors, FD is 6049536
via 10.220.100.1 (6049536/5537536), Serial3/0
via 10.55.100.14 (52825600/281600), Tunnel55
Note that this EIGRP routing protocol has two routes to the network. However, this router only accepts one of the shortest metric routes in the routing table. Here is an example of routing table entries:
Router# show ip route 10.55.103.0
Routing entry for 10.55.103.0/24
Known via "eigrp 100", distance 120, metric 6049536, type internal
Redistributing via eigrp 100
Last update from 10.220.100.1 on Serial3/0, 00:56:12 ago
Routing Descriptor Blocks:
* 10.220.100.1, from 10.220.100.1, 00:56:12 ago, via Serial3/0
Route metric is 6049536, traffic share count is 1
Total delay is 41000 microseconds, minimum bandwidth is 512 Kbit
Reliability 226/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
Different routing protocols have different algorithms for measurement.RIPThe algorithm is based on hop count, OSPF is based on bandwidth, and EIGRP is determined based on bandwidth, delay time, load and reliability.
Article entry: csh Editor in charge: csh