IGP next hop unreachable

[Islam Abulaynain]

Say that  router has the below routes:

ip route 10.230.0.0 255.255.0.0 10.12.12.2

ip route 10.230.10.0 255.255.255.0 10.13.13.3

a packet with destination "10.230.10.20" should be matched with the 2nd route, as the longest match

but what if the interface connected to "next hop of the 2nd route" is up while there is no arp entry for the next-hop itself !?

what will the router do to forward this packet, will router drop it if next hop is unreachable !!?

[andreslgx_101]

If the next hop IP is unreachable, but the subnet to which that next hop ip belongs to is reachable, then static route will be placed in the routing table and, the router will try to send the packet through this route. If the subnet is not unreachable, the entry for this static route is removed from the routing table.

[Georg Pauwen]

Hello,

below is the ARP table with the IP address 10.13.13.3 removed from the neighboring router, but the interface itself still being up. The static route through 10.13.13.3 will still be used, as it remains in the routing table.

Router#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 10.12.12.1 - 5296.9ec2.c900 ARPA GigabitEthernet0/0
Internet 10.12.12.2 0 5296.9e47.6400 ARPA GigabitEthernet0/0
Internet 10.13.13.2 - 5296.9ec2.c901 ARPA GigabitEthernet0/1
Internet 10.13.13.3 0 Incomplete ARPA

Router#ping 10.230.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.230.10.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

[Richard Burts · Accepted solution]

If you had enabled debug ip packet and did the ping you would see error messages about encapsulation failure. The route is still considered viable and will remain in the routing table but the next hop is not reachable and the ping packets fail.

HTH

Rick

[Richard Burts]

I am glad that our responses have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. These forums are excellent places to ask questions and to learn about networking. I hope to see you continue to be active in the community.

HTH

Rick