1. Static routing failure
1. Static routing and category search
When the routing table process checks for a parsable static route that uses an intermediate address (the IP address referenced by the next hop in the routing table), this check is always done in a category mode. Regardless of whether or not using the ip classless command, if the intermediate address in a category mode cannot be parsed in the routing table, the static route is deleted.
Use show ip route to view the routing table.
Use debug to show that a certain network is down.
If you use the classless method and have a default route exists, a backup stance route with a high management distance will never be loaded into the routing table when the main static route fails. This is because any static route, even a static route pointing to a non-existent intermediate address, will be parsed using the default route.
The CISCO routing table process calls a static routing function that checks the routing table every 60S to install or delete static routes according to dynamically changing routing tables.
2. Static routing and intermediate addresses
Static routing can be created using intermediate network addresses or outgoing interfaces. In most cases, it is more efficient to use outgoing interfaces to resolve static routes in the routing table process.
As long as the intermediate IP address can be parsed in the routing table, it does not have to be the interface of the real next-hop router. Static network routing (such as the intermediate address) must eventually be parsed into a route with an outgoing interface in the routing table.
Whenever the routing table process needs to use a static routing table entry for a .0/24 network, it also needs to parse the intermediate address, called a recursive lookup. One additional routing lookup may not have much impact on the performance of the routing process. However, taking multiple recursive lookups to obtain the parsed static route may affect performance.
3. Static routing optimization
To avoid recursive search: Serial network: Use outgoing interface
Ethernet network: Use both intermediate address and outgoing interface
4. Repeated static route installation and deletion
Use outbound interfaces instead of intermediate addresses as much as possible to configure static routing.
5. Use discarded routes
Sometimes loops are generated in the network. By periodically viewing the counters on the router interface, you can see the results of the routing loop.
clear counters serial0/0
show interface serial0/0
The problem with routing loops creates a black hole in the network. Once the lifetime (TTL) in the IP header is reduced to 0, the packet is discarded.
Solution 1: No ip classless with class mode----Use no ip classless on user network routers. The router does not use any hypernet or default routes when at least one known subnet exists. But it is not preferred. Because it changes the routing table lookup behavior of all packets.
Solution 2: Use a discarded route---When there is no specific match in the routing table, and it is not appropriate to use a hypernet or default route to forward those packets, a discarded route sends the packet to null0, that is, the bitbucket.
ip route .0.0 255.255.0.0 null0
ip route .0.0 255.255.0.0 null0 200
The latter command line configuration only another discarded route used when the main route fails. It is achieved by changing the default management distance of the static route to a value higher than the management distance of the dynamic routing protocol used.
1. Static routing and category search
When the routing table process checks for a parsable static route that uses an intermediate address (the IP address referenced by the next hop in the routing table), this check is always done in a category mode. Regardless of whether or not using the ip classless command, if the intermediate address in a category mode cannot be parsed in the routing table, the static route is deleted.
Use show ip route to view the routing table.
Use debug to show that a certain network is down.
If you use the classless method and have a default route exists, a backup stance route with a high management distance will never be loaded into the routing table when the main static route fails. This is because any static route, even a static route pointing to a non-existent intermediate address, will be parsed using the default route.
The CISCO routing table process calls a static routing function that checks the routing table every 60S to install or delete static routes according to dynamically changing routing tables.
2. Static routing and intermediate addresses
Static routing can be created using intermediate network addresses or outgoing interfaces. In most cases, it is more efficient to use outgoing interfaces to resolve static routes in the routing table process.
As long as the intermediate IP address can be parsed in the routing table, it does not have to be the interface of the real next-hop router. Static network routing (such as the intermediate address) must eventually be parsed into a route with an outgoing interface in the routing table.
Whenever the routing table process needs to use a static routing table entry for a .0/24 network, it also needs to parse the intermediate address, called a recursive lookup. One additional routing lookup may not have much impact on the performance of the routing process. However, taking multiple recursive lookups to obtain the parsed static route may affect performance.
3. Static routing optimization
To avoid recursive search: Serial network: Use outgoing interface
Ethernet network: Use both intermediate address and outgoing interface
4. Repeated static route installation and deletion
Use outbound interfaces instead of intermediate addresses as much as possible to configure static routing.
5. Use discarded routes
Sometimes loops are generated in the network. By periodically viewing the counters on the router interface, you can see the results of the routing loop.
clear counters serial0/0
show interface serial0/0
The problem with routing loops creates a black hole in the network. Once the lifetime (TTL) in the IP header is reduced to 0, the packet is discarded.
Solution 1: No ip classless with class mode----Use no ip classless on user network routers. The router does not use any hypernet or default routes when at least one known subnet exists. But it is not preferred. Because it changes the routing table lookup behavior of all packets.
Solution 2: Use a discarded route---When there is no specific match in the routing table, and it is not appropriate to use a hypernet or default route to forward those packets, a discarded route sends the packet to null0, that is, the bitbucket.
ip route .0.0 255.255.0.0 null0
ip route .0.0 255.255.0.0 null0 200
The latter command line configuration only another discarded route used when the main route fails. It is achieved by changing the default management distance of the static route to a value higher than the management distance of the dynamic routing protocol used.
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