High availability gateways between multiple routers.  A few options to choose from are typically HSRP (Hot Standby Routing Protocol) more common to our Cisco friends, or the more widely used standards based VRRP (Virtual Route Redundancy Protocol), of course there’s GLBP too but i’ll leave that one for a rainy day, all of which are FHRP (First Hop Routing Protocols).

Ultimately, what you want is for a router to fail and a standby router to become the active gateway, and forward packets to the next hop right?

HSRP Configuration Example

R1(config)# interface GigE 0/1
R1(config-if)# ip address 10.1.1.2 255.255.255.0
R1(config-if)# standby 1 ip 10.1.1.1
R1(config-if)# standby 1 priority 200
R1(config-if)# standby 1 preempt

R2(config-if)# ip address 10.1.1.3 255.255.255.0
R2(config-if)# standby 1 ip 10.1.1.1
R2(config-if)# standby 1 preempt

VRRP Configuration Example

R1(config)# interface GigE 0/1
R1(config-if)# ip address 10.1.1.2 255.255.255.0
R1(config-if)# vrrp 1 ip 10.1.1.1
R1(config-if)# vrrp 1 priority 110

R2(config)# interface GigE 0/1
R2(config-if)# ip address 10.1.1.3 255.255.255.0
R2(config-if)# vrrp 1 ip 10.1.1.1