Default Route Generation in RIP

Default Route Generation in RIP: A default route is the route that takes effect when no other route is available for an IP destination address.If a packet is received on a routing device, the device first checks to see if the IP destination address is on one of the device’s local subnets. If the destination address is not local, the device checks its routing table. If the remote destination subnet is not listed in the routing table, the packet is forwarded to the next hop toward the destination using the default route. The default route generally has a next-hop address of another routing device, which performs the same process. The process repeats until a packet is delivered to the destination.

The route evaluation process in each router uses the longest prefix match method to obtain the most specific route. The network with the longest subnet mask that matches the destination IP address is the next-hop network gateway.

The default route in IPv4 is designated as 0.0.0.0/0 or simply 0/0. Similarly, in IPv6, the default route is specified as ::/0. The subnet mask /0 specifies all networks, and is the shortest match possible. A route lookup that does not match any other route uses this route if it is configured and active in the routing table. To be active, the configured next-hop address must be reachable.

Administrators generally point the default route toward the routing device that has a connection to a network service provider. Therefore, packets with destinations outside the organization's local area network, typically destinations on the Internet or a wide area network, are forwarded to the routing device with the connection to that provider. The device to which the default route points is often called the default gateway.

Default Route Generation in RIP

Let us understand default route generation in RIP with help of below topology. The aim is to generate default route on R2 towards R1 and R3 (RIP Domain). R2 is connected to a Service Provider(MPLS VPN or and ISP).  R1 and R3 should be able to ping/reach the Internet IP connected behind R4 (66.66.66.66/32) using the RIP default route.

On R2, In order to generate default route towards LAN network routers R1 and R3, we have to  configure below command in RIP

 

R2(config)#
R2(config-router)#default-information originate

We can see default route (denoted by R*) on R1 and R3 pointing to R2.

 

R1#show ip route | i 0.0.0.0/0
R*    0.0.0.0/0 [120/1] via 10.10.12.2, 00:00:19, FastEthernet1/0

R3#show ip route | i 0.0.0.0/0
R*    0.0.0.0/0 [120/1] via 10.10.23.2, 00:00:12, FastEthernet1/0

Let us try to ping an internet IP connected to R4 (66.66.66.66/32) from R1 and R2. Though R1 and R3 do not have route to this Internet IP, but they will be able to reach this using RIP default route.

 

R1#sh ip route 66.66.66.66
% Network not in table

R3#sh ip  route 66.66.66.66
% Network not in table


R1#ping 66.66.66.66
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 66.66.66.66, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 84/168/220 ms

R3#ping 66.66.66.66
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 66.66.66.66, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 136/159/236 ms

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I hope you have found this article informative and useful and now have a fair understanding of Default Route Generation in RIP and how to configure it. For any of the related queries or feedback, kindly write to us at networkurge@gmail.com