OSPF / BGP path selection challenge
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:
R2:
router ospf 1
router-id 2.2.2.2
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
R3:
router ospf 1
router-id 3.3.3.3
area 50 nssa
router bgp 65023
bgp log-neighbor-changes
neighbor 10.0.23.2 remote-as 65023
R4:
router ospf 1
router-id 4.4.4.4
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
R5:
router ospf 1
router-id 5.5.5.5
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