Today I’ve created a nice path selection challenge for everybody.
Let’s start with the topology:
IP addressing is simple. The subnets used are 10.0.xx.y/24 where xx are the numbers of the two routers on the link (lowest router first). Y is the router number.
The situation is as follows:
- R2 has an eBGP peering with R1. It receives tge default route from R1.
- R2 advertises this default route to R3
- R2 and R3 also form an OSPF network with area 0
- R2 forms an OSPF NSSA area 50 with R4
- R2 redistributes the connected route to R1 into OSPF
- R3 forms an OSPF NSSA area 50 with R5
- R4 redistributes OSPF NSSA 50 into BGP (including NSSA external routes)
- R5 redistributes (i)BGP routes into OSPF
So the questions are:
- Which way will R3 route traffic to R1?
- Why does R3 send the traffic in that direction?
- What three commands will change the behavior on R3?
Below the relevant configuration:
router ospf 1 router-id 220.127.116.11 area 50 nssa redistribute connected subnets route-map CONN_OSPF router bgp 65023 bgp log-neighbor-changes redistribute ospf 1 neighbor 10.0.12.1 remote-as 65001 neighbor 10.0.23.3 remote-as 65023 route-map CONN_OSPF permit 10 match interface GigabitEthernet0/2
router ospf 1 router-id 18.104.22.168 area 50 nssa router bgp 65023 bgp log-neighbor-changes neighbor 10.0.23.2 remote-as 65023
router ospf 1 router-id 22.214.171.124 area 50 nssa router bgp 65045 bgp log-neighbor-changes redistribute ospf 1 match internal external 1 external 2 nssa-external 1 nssa-external 2 neighbor 10.0.45.5 remote-as 65045 neighbor 10.0.45.5 next-hop-self
router ospf 1 router-id 126.96.36.199 area 50 nssa redistribute bgp 65045 subnets router bgp 65045 bgp log-neighbor-changes bgp redistribute-internal network 10.0.45.0 mask 255.255.255.0 neighbor 10.0.45.4 remote-as 65045