I love interesting problems that we face when studying for our CCIEs. Especially fun are those that come from an unusual behavior of protocols.
Couple of days ago, a student sent me this little problem. It looks simple, but let’s see how you do solving it.
The setup is really straight-forward. Routers R2 and R5 are running OSPF in area 0. Router R2 has two static routes pointing to Null0 interface (0.0.0.0/0 and 10.0.0.0/8). This is the task:
- Redistribute only 0.0.0.0/0 into OSPF on R2
- Make sure the default route is received in OSPF on R5 and advertised in BGP to R4
Here is the relevant configuration on all three routers.
hostname R2 ! interface Loopback0 ip address 192.168.0.2 255.255.255.255 ! interface Serial0/2/0 ip address 192.168.25.2 255.255.255.0 ! ip route 0.0.0.0 0.0.0.0 Null0 ip route 10.0.0.0 255.0.0.0 Null0 ! ip prefix-list Redistribute seq 10 permit 0.0.0.0/0 ! route-map Connected permit 10 match ip address prefix-list Redistribute ! router ospf 1 router-id 192.168.0.2 network 192.168.0.0 0.0.255.255 area 0 redistribute static subnets route-map Connected !
hostname R5 ! interface Loopback0 ip address 192.168.0.5 255.255.255.255 ! interface Serial0/0/0 ip address 172.16.45.5 255.255.255.0 ! interface Serial0/2/0 ip address 192.168.25.5 255.255.255.0 ! router ospf 1 router-id 192.168.0.5 network 192.168.0.0 0.0.255.255 area 0 ! router bgp 5 bgp router-id 192.168.0.5 neighbor 172.16.45.4 remote-as 4 redistribute ospf 1 !
hostname R4 ! interface Loopback0 ip address 172.16.0.4 255.255.255.255 ! interface Serial0/1/0 ip address 172.16.45.4 255.255.255.0 ! router bgp 4 bgp router-id 172.16.0.4 neighbor 172.16.45.5 remote-as 5 !
We can see that both OSPF and BGP are operational:
R5#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 192.168.0.2 0 FULL/ - 00:00:34 192.168.25.2 Serial0/2/0 R5#show ip bgp summary | begin ^Neighbor Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 172.16.45.4 4 4 31 33 4 0 0 00:29:56 0
However, not everything works as expected. Take a look at the OSPF routes on R5:
R5#show ip route ospf 192.168.0.0/32 is subnetted, 2 subnets O 192.168.0.2 [110/65] via 192.168.25.2, 00:31:28, Serial0/2/0
Also, let’s take a look at the BGP routes on R4:
R4#show ip route bgp B 192.168.25.0/24 [20/0] via 172.16.45.5, 00:30:00 192.168.0.0/32 is subnetted, 2 subnets B 192.168.0.2 [20/65] via 172.16.45.5, 00:30:00 B 192.168.0.5 [20/0] via 172.16.45.5, 00:30:00
We can clearly see that the default route is missing on both R5 and R4. What’s wrong and how do we fix this problem? Post your thoughts and solutions in the comments below and in a few days, I’ll follow-up with the solution.