Lab 134 - BGP Backdoor

Prerequisites: CCNP level skills.

Topology

Pic. 1 - Topology Diagram.

 Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
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.

Notice!
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
!

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

!

R2 Configuration:

!

interface Loopback0
 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

!

 
Verification:

Pic. 5 - R1' Routing Table.

Notice!
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

!

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

!

Verification:

Pic. 6 - R1's Routing Table.

Notice!
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.