static routes in the routing table

[ranairfan]

I have observed two different behavior of static routes in the routing table while working on two different 7513 routers.

On both routers static routes for customers are configured pointing towards their WAN interface ip.

Problem is when Router 1 [ios Version 12.0(18) S] customer’s WAN link goes down static entry remains in the routing table,

but when the Router 2 [ios Version 12.1(8)] customer’s WAN link goes down the static entries immediately removed from the routing table.

Kindly reply what is the actual behavior of static routes in the routing table after the link failure, any ios bug and any specific configuration to resolve this problem.

Thanks

[lgijssel]

How are these static routes specified?

There is a difference in behaviour when you enter an ip-adress or an interface as the next hop. Static routes with an interface as next hop will be removed from the table when the interface goes down.

[ranairfan]

static routes on both routers are defined towards next hop ip address, but these are removed from the routing table of Router 2 when the link goes down.

[a.manosca]

Hope this info and examples help:

Adding a static route to an Ethernet or other broadcast interface (for example, ip route 0.0.0.0 0.0.0.0 Ethernet 1/2) will cause the route to be inserted into the routing table only when the interface is up. This configuration is not generally recommended. When the next hop of a static route points to an interface, the router considers each of the hosts within the range of the route to be directly connected through that interface, and therefore it will send ARP requests to any destination addresses that route through the static route.

The practical implication of configuring "ip route 0.0.0.0 0.0.0.0 Ethernet 1/2" is that the router will consider all of the destinations that the router does not know how to reach through some other route as directly connected to Ethernet 1/2. So the router will send an ARP request for each host for which it receives packets on this network segment. This configuration can cause high processor utilization and a very large ARP cache (along with attendant memory allocation failures). Configuring a default route or other static route that directs the router to forward packets for a large range of destinations to a connected broadcast network segment can cause your router to reload.

Specifying a numerical next hop that is on a directly connected interface will prevent the router from using Proxy ARP. However, if the interface with the next hop goes down and the numerical next hop can be reached through a recursive route, you may specify both the next hop and interface (for example "ip route 0.0.0.0 0.0.0.0 Ethernet1/2 10.1.2.3") with a static route to prevent routes from passing through an unintended interface.

The following example routes packets for network 172.31.0.0 to a router at 172.31.6.6:

ip route 172.31.0.0 255.255.0.0 172.31.6.6

The following example routes packets for network 192.168.1.0 directly to the next hop at 10.1.2.3. If the interface goes down, this route is removed from the routing table and will not be restored unless the interface comes back up.

ip route 192.168.1.0 255.255.0.0 Ethernet0 10.1.2.3

[henrybb]

router A-(10.1.2.3)-------switch---------(e0)-router B

router C-(10.1.2.2)----------+

if router B has following config:

ip route 192.168.1.0 255.255.0.0 Ethernet0 10.1.2.3

ip route 192.168.1.0 255.255.0.0 Ethernet0 10.1.2.2 50

if router A fail,does route change to router C?How does it know this? router A doesn't response to arp which router B send? or anything.

if router A and C both fail,And router B has no route to 192.168.1.0 and no bit bucket. but it has packet which destination is 192.168.1.0. What's the action the router B will do? Drop or other?

thanks! any comment is appreciated.