IP routing table - candidate default route

The default route that appears in the IP routing table of a Cisco IOS router is called the gateway of last resort. It always appears at the beginning of the output of the routing table with the phrase "Gateway of last resort is..."

The following is an example of a routing table that indicates that the gateway of last resort is 192.168.12.254

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.12.254 to network 0.0.0.0

C    192.168.12.0/24 is directly connected, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 192.168.12.254, FastEthernet0/1
D    192.168.23.0/24 [90/307200] via 192.168.12.2, 00:00:07, FastEthernet0/0

This default route is statically configured. Note the static route to 0.0.0.0/0 that is defined along with the "*" that indicates that the particular entry is a candidate default route. If it is the only candidate default route, then it becomes the default route.

Similarly, the following routing table shows how a route learned via EIGRP has become the candidate default route:

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is 192.168.12.2 to network 0.0.0.0

C    192.168.12.0/24 is directly connected, FastEthernet0/0
D*EX 0.0.0.0/0 [170/307200] via 192.168.12.2, 00:00:07, FastEthernet0/0

There can be more than one candidate default route, but only one default route.

Links:

https://forum.networklessons.com/t/ip-routing-explained/1472/45?u=lagapides

https://www.ciscopress.com/articles/article.asp?p=2180210&seqNum=12

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