Lab 102 - BGP Path Selection using DMZLINK-BW

Prerequisites: CCNP level skills.

Topology

Personal Note!

Back home! Resuming my bgp command study and posts. My company seem to have planned a lot of travels for me (well, beggars can't be choosers). These kind of study distractions I equate to a tcp slow start: they're inevitable impediments and must be factored in during study.
Solution: increase the dose of tenacity ;)

Pic. 1 - Topology Diagram.

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

Task 1
Assuming the Serial connections to R3 is 512 kbps, configure BGP on routers so that R4 does an equal load balance towards 172.16.105.0/24.

Solution

Task 1

Assuming the Serial connections to R3 is 512 kbps, configure BGP on routers so that R4 does an equal load balance towards 172.16.105.0/24.

Notice!
BGP does not allow to perform a traffic sharing by default. Using dmzlink-bw bgp option we can allow both equal and unequal cost load balancing. For this feature to work the following are necessary:

• prefixes must have the same attributes: Weight, Local Preference, Origin, MED.
• prefixes must be learned either through IBGP or EBGP sessions.
• Have the same IGP metric towards the next-hop routers

If you want to achieve unequal cost load balancing, the bandwidth parameter must be properly configured (ratio).

source: http://www.cisco.com/en/US/docs/ios/12_2t/12_2t11/feature/guide/ft11b_lb.html

Pic. 2 - BGP Table on R4 Before Applying Configuration.

Pic. 3 - Routing Table on R4 Before Applying Configuration.

Notice!
Only one path is installed in the routing table.


R1 Configuration:

!

interface Serial1/1
 bandwidth 512
 ip address 10.1.13.1 255.255.255.0
 serial restart-delay 0

!

router bgp 124
 no synchronization
 bgp router-id 172.16.101.1
 bgp log-neighbor-changes
 bgp dmzlink-bw
 network 172.16.101.0 mask 255.255.255.0
 neighbor 10.1.13.3 remote-as 30
 neighbor 10.1.13.3 dmzlink-bw
 neighbor 10.1.14.4 remote-as 124
 neighbor 10.1.14.4 next-hop-self
 neighbor 10.1.14.4 send-community extended
 no auto-summary

!

R2 Configuration:

!

interface Serial1/2
 bandwidth 512
 ip address 10.1.23.2 255.255.255.0
 serial restart-delay 0
!

router bgp 124
 no synchronization
 bgp router-id 172.16.102.2
 bgp log-neighbor-changes
 bgp dmzlink-bw
 network 172.16.102.0 mask 255.255.255.0
 neighbor 10.1.23.3 remote-as 30
 neighbor 10.1.23.3 dmzlink-bw
 neighbor 10.1.24.4 remote-as 124
 neighbor 10.1.24.4 next-hop-self
 neighbor 10.1.24.4 send-community extended
 no auto-summary

!

Notice!
On R1 and R2 (Edge routers) the following have been configured:

• router configuration:  bgp dmzlink-bw
• EBGP neighbor dmzlink-bw
• Extended communities sent towards R4 which carry additional parameter allowing to install two paths in the routing table

R4 Configuration:

!

router bgp 124
 no synchronization
 bgp router-id 172.16.104.4
 bgp log-neighbor-changes
 bgp dmzlink-bw
 network 172.16.104.0 mask 255.255.255.0
 neighbor 10.1.14.1 remote-as 124
 neighbor 10.1.14.1 route-reflector-client
 neighbor 10.1.24.2 remote-as 124
 neighbor 10.1.24.2 route-reflector-client
 maximum-paths ibgp 2
 no auto-summary

!

Notice!
Maximu-paths increased to 2.

Pic. 4 - BGP Prefix Details on R4.

Pic. 5 - Routing Table on R4.

Notice!
Only one path shows as best in the BGP table. However, both are installed in the routing table.

Pic. 6 - Prefix Details in the Routing Table.