As illustrated in the table, routes being distributed from another routing protocol into RIP or EIGRP must have a configured metric to be eligible for placement in the RIP database or EIGRP topology. Also interesting to note is that BGP will automatically assigned the IGP-learned metric of a redistributed route to the route’s MED in the BGP table.

When RIP or IGRP send an update, they perform certain checks before they advertise the update. This list shows the sequence of events that occurs before Router 1 sends updates to Router 2. The network diagram allows you to examine the sequence of events more closely.

• Is the subnet information part of the same major net as the interface that sources the update?
  • No: Router 1 summarizes at the major net boundary and advertises the network.
  • Yes: Does the network have the same subnet mask as the interface that sources the update?
    • Yes: Router 1 advertises the subnet.
    • No: Does the network have a /32 mask ?
      • Yes: If it is RIP, then the network is advertised. If it is IGRP, then Router 1 drops the network.
      • No: Router 1 drops the network.

Receive Updates

When RIP or IGRP receive an update, they perform certain checks before they accept the update and apply the subnet mask. This is the sequence of events that occurs before Router 2 accepts an update from Router 1:

• Is the subnet received in the update on the same major net as the interface that received the update?
  • Yes: Router 2 applies the mask of the interface that received the update. If the advertised network has a host bit set in the host portion of the update, Router 2 applies the host mask (/32). In the case of RIP, it continues to advertise the /32 route to the subsequent router, but IGRP does not.
  • No: Do any subnets of this major net already exist in the routing table, known from interfaces other than the one that received the update? The network in this update should be a major net unless the link between the two routers is an unnumbered link, in which case it is possible for the update to contain subnet information.
    • Yes: Router 2 ignores the update.
    • No: Router 2 applies a classful mask. If the update came across an unnumbered link and contains subnet information (bits in subnet portion of network are set), then Router 2 applies a host mask. Refer to Understanding and Configuring the ip unnumbered Command for unnumbered case examples.

Specific Case

Send Updates

When Router 1 sends an update to Router 2, it performs these checks:

• Is 131.108.5.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
  • Yes: Does 131.108.5.0/24 have the same subnet mask as 131.108.2.0/24, which sources the update?
    • Yes: Router 1 advertises the network.
• Is 137.99.88.0/24 part of the same major net as 131.108.2.0/24, which sources the update?
  • No: Router 1 summarizes 137.99.88.0/24 at the major net boundary and advertises the route as 137.99.0.0.

This process results in Router 1 including 131.108.5.0 and 137.99.0.0 in its update to Router 2. You can see this in the debug ip rip command output shown on Router 1:

*Mar 25 00:22:46.177: RIP: sending v1 update to 255.255.255.255 via Serial0 (131.108.2.2)  
*Mar 25 00:22:46.178: RIP: build update entries
*Mar 25 00:22:46.182: subnet 131.108.5.0, metric 1
*Mar 25 00:22:46.185: network 137.99.0.0, metric 1

Receive Updates

When you issue the debug ip rip command, you can see the routing update received on Router 2 from Router 1:

*Mar 25 00:22:46.201: RIP: received v1 update from 131.108.2.2 on Serial0
*Mar 25 00:22:46.203:131.108.5.0 in 1 hops
*Mar 25 00:22:46.205:137.99.0.0 in 1 hops

Look at the checks Router 2 performs in order to determine what mask to apply on a received network.

• Is the received major net 137.99.0.0 the same as 131.108.2.0, which is the address assigned to the interface that received the update?
  • No: Do any subnets of this major net already exist in the routing table known from other interfaces?
    • No: Router 2 applies the natural mask (/16) because 137.99.0.0 is a class B address.
• Does subnet 131.108.5.0 belong to the same major net as subnet 131.108.2.0, which is the interface that received the update?
  • Yes: Router 2 applies the mask /24, which is the mask of the interface that received the update.

This process results in these networks and masks in the routing table of Router 2, displayed with the show ip route command:

R    137.99.0.0/16 [120/1] via 131.108.2.2, 00:00:07, Serial0
     131.108.0.0/24 is subnetted, 3 subnets
R    131.108.5.0 [120/1] via 131.108.2.2, 00:00:08, Serial0
C    131.108.2.0 is directly connected, Serial0
C    131.108.3.0 is directly connected, Ethernet0

Solution

In some situations, discontiguous networks are unavoidable. In these situations Cisco recommends that you do not use RIPv1 or IGRP. Routing protocols like EIGRP or OSPF are better suited for this situation.

Establish Connectivity

In the event that you use RIPv1 or IGRP with discontiguous networks, you must use static routes to establish connectivity between the discontiguous subnetworks.

Original Document @ http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a0080093fd8.shtml