Wednesday, March 30, 2011

Lab 84 - BGP Next-Hop on Broadcast and NBMA Networks

Prerequisites: CCNP level skills.

Topology
Pic. 1 - Topology Diagram.
Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
Configure BGP on all routers according to the topology diagram. Advertise all loopbacks using 'network' statement. Check the next-hop behavior on LAN link connecting R1, R2 and R4.

Lab Solution

Task 1
Configure BGP on all routers according to the topology diagram. Advertise all loopbacks using 'network' statement. Check the next-hop behavior on LAN link connecting R1, R2 and R4.

R1 Configuration:
!
router bgp 13
 no synchronization
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 network 172.16.101.0 mask 255.255.255.0
 neighbor 10.1.13.3 remote-as 13
 neighbor 10.1.13.3 next-hop-self
 neighbor 10.1.124.4 remote-as 40
 no auto-summary
!

R2 Configuration:
!
router bgp 20
 no synchronization
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 network 172.16.102.0 mask 255.255.255.0
 neighbor 10.1.124.4 remote-as 40
 no auto-summary
!

R3 Configuration:
!
router bgp 13
 no synchronization
 bgp router-id 3.3.3.3
 bgp log-neighbor-changes
 network 172.16.103.0 mask 255.255.255.0
 neighbor 10.1.13.1 remote-as 13
 neighbor 10.1.13.1 next-hop-self
 neighbor 10.1.35.5 remote-as 50
 no auto-summary
!

R4 Configuration:
!
router bgp 40
 no synchronization
 bgp router-id 4.4.4.4
 bgp log-neighbor-changes
 network 172.16.104.0 mask 255.255.255.0
 network 172.16.144.0 mask 255.255.255.0
 neighbor 10.1.124.1 remote-as 13
 neighbor 10.1.124.2 remote-as 20
 no auto-summary
!

R5 Configuration:
!
router bgp 50
 no synchronization
 bgp router-id 5.5.5.5
 bgp log-neighbor-changes
 network 172.16.105.0 mask 255.255.255.0
 neighbor 10.1.35.3 remote-as 13
 no auto-summary
!

Verification:
Pic. 2 - R5' BGP Table.
Pic. 3 - R1's BGP Prefix from AS 20.
Notice!
Even though R1 receives the prefix over EBGP session the next-hop address is R2's address rather than R4's. This is the default behavior on Broadcast and NBMA networks. Pay a special attention to NBMA since the circuits may not be fully meshed and the reachability to the next-hop address may be a problem.

Pic. 4 - Prefix Details.
Notice!
Next-Hop Attribute: 10.1.124.2
Advertising Router: 10.1.124.4
Advertising Router ID: 4.4.4.4