The introduction of a routing protocol across a GRE tunnel can cause some interesting
problems. Take, for example, our network, now altered as shown in Figure 12-3. This time we have the links between the routers
updating routes using RIPv2. The other interfaces on Router A and Router D are included in
RIP using the
redistribute connected command. EIGRP is
running on all interfaces on Routers A and D with the exception of the serial links, which
are running RIPv2.
Figure 12-3. Recursive routing example
While this may look a bit odd, consider the possibility that the Routers B and C are owned and operated by a service provider. We cannot control them, and they only run RIPv2. We run EIGRP on our routers (A and D) and want to route between them using EIGRP.
Here are the pertinent configurations for Router A and Router D (remember, in this scenario, Routers B and C are beyond our control):
interface Loopback0 ip address 10.100.100.100 255.255.255.255 ! interface Tunnel0 ip address 172.16.0.1 255.255.255.0 tunnel source Loopback0 tunnel destination 10.200.200.200 ! interface Ethernet0/1 ip address 10.10.10.1 255.255.255.0 ! interface Serial0/1 ip address 192.168.1.1 255.255.255.0 router eigrp 100 network 10.10.10.0 0.0.0.255 network 10.100.100.0 0.0.0.255 network 172.16.0.0 0.0.0.255 no auto-summary ! router rip version 2 redistribute connected passive-interface ...