Prerequisites: CCNP level skills.
Topology
Pic. 1 illustrates BGP peering between the routers. All loopbacks have been advertised into BGP.
Task 2
Configure OSPF between R1 and R2 using their E0/0 interfaces. Advertise their loopbacks into OSPF. Ensure that they are advertised with /24 network mask. The traffic between them takes the shortest path (through E0/0). The only configuration allowed to accomplish this should be done in the BGP context.
Solution
Task 1
Pic. 1 illustrates BGP peering between the routers. All loopbacks have been advertised into BGP.
Pic. 2 - BGP Table on R1.
Pic. 3 - BGP Table on R2.
Pic. 4 - Traceroute.
Traffic from R1's Loopback towards R2's loopback goes through R3.
Task 2
Configure OSPF between R1 and R2 using their E0/0 interfaces. Advertise their loopbacks into OSPF. Ensure that they are advertised with /24 network mask. The traffic between them takes the shortest path (through E0/0). The only configuration allowed to accomplish this should be done in the BGP context.
R1 Configuration:
!
interface Loopback0
ip address 172.16.101.1 255.255.255.0
ip ospf network point-to-point
!
ip address 172.16.101.1 255.255.255.0
ip ospf network point-to-point
!
router ospf 1
router-id 172.16.101.1
log-adjacency-changes
network 10.1.12.1 0.0.0.0 area 0
network 172.16.101.1 0.0.0.0 area 0
router-id 172.16.101.1
log-adjacency-changes
network 10.1.12.1 0.0.0.0 area 0
network 172.16.101.1 0.0.0.0 area 0
!
R2 Configuration:
!
interface Loopback0
ip address 172.16.102.2 255.255.255.0
ip ospf network point-to-point
ip address 172.16.102.2 255.255.255.0
ip ospf network point-to-point
!
router ospf 1
router-id 172.16.102.2
log-adjacency-changes
network 10.1.12.2 0.0.0.0 area 0
network 172.16.102.2 0.0.0.0 area 0
router-id 172.16.102.2
log-adjacency-changes
network 10.1.12.2 0.0.0.0 area 0
network 172.16.102.2 0.0.0.0 area 0
!
Verification:
Pic. 5 - R1' Routing Table.
eBGP prefix (172.16.102.0/24) has the administrative distance 20 and is better than OSPF (110). Path is suboptimal.
R1 Configuration:
!
router bgp 10
no synchronization
bgp router-id 172.16.101.1
bgp log-neighbor-changes
network 172.16.101.0 mask 255.255.255.0
network 172.16.102.0 mask 255.255.255.0 backdoor
neighbor 10.1.13.3 remote-as 30
no auto-summary
no synchronization
bgp router-id 172.16.101.1
bgp log-neighbor-changes
network 172.16.101.0 mask 255.255.255.0
network 172.16.102.0 mask 255.255.255.0 backdoor
neighbor 10.1.13.3 remote-as 30
no auto-summary
!
R2 Configuration:
!
router bgp 20
no synchronization
bgp router-id 172.16.102.2
bgp log-neighbor-changes
network 172.16.101.0 mask 255.255.255.0 backdoor
network 172.16.102.0 mask 255.255.255.0
neighbor 10.1.23.3 remote-as 30
no auto-summary
no synchronization
bgp router-id 172.16.102.2
bgp log-neighbor-changes
network 172.16.101.0 mask 255.255.255.0 backdoor
network 172.16.102.0 mask 255.255.255.0
neighbor 10.1.23.3 remote-as 30
no auto-summary
!
Verification:
Pic. 6 - R1's Routing Table.
Now, the subnet is known via OSPF with the next hop 10.1.12.2 (out E0/0). BGP backdoor changed its eBGP administrative distance from 20 to 200.
Pic. 7 - Traceroute.
