Lab 100 - BGP Hidden Feature BESTPATH AS-PATH IGNORE

Lab 100 - BGP Hidden Feature BESTPATH AS-PATH IGNORE
Di Posting Oleh : Simple Learning
Kategori : BGP Core Technologies Routing

Prerequisites: CCNP level skills.

Topology

Personal Note!
This week I'm in Mechelen, Belgium. My poor laptop barely handles four dynamips instances :(.
Pic. 1 - Topology Diagram.
 Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
Using AS-Path Prepending try to influence the returning traffic towards 172.16.101.0/24 to flow through AS 20.

Task 2
On R3 check the bgp table and pay attention to the path towards 172.16.101.0/24. Ensure that R3 overrides the path selection choosing the best path to the subnet in question.

Lab Solution


Task 1
Using AS-Path Prepending try to influence the returning traffic towards 172.16.101.0/24 to flow through AS 20.


R1 Configuration:
!
ip prefix-list Lo0 seq 5 permit 172.16.101.0/24
!
route-map AS_PREPEND permit 10
 match ip address prefix-list Lo0
 set as-path prepend 10 10 10
route-map AS_PREPEND permit 999
!
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
 neighbor 10.1.12.2 remote-as 20
 neighbor 10.1.13.3 remote-as 30
 neighbor 10.1.13.3 route-map AS_PREPEND out
 no auto-summary
!


Task 2
On R3 check the bgp table and pay attention to the path towards 172.16.101.0/24. Ensure that R3 overrides the path selection choosing the best path to the subnet in question.

Pic. 2 - BGP Table on R3.

Notice!
R3 prefers AS 20 towards 172.16.101.0/24 due to the shorter AS path.

R3 Configuration:
!
router bgp 30
 no synchronization
 bgp router-id 172.16.103.3
 bgp log-neighbor-changes
 bgp bestpath as-path ignore
 network 172.16.103.0 mask 255.255.255.0
 neighbor 10.1.13.1 remote-as 10
 neighbor 10.1.23.2 remote-as 20
 no auto-summary
!

Notice!
The command is now hidden (as of the time of writing this post) and does NOT show using help. For this command to take effect: 'clear ip bgp *'

Verification:
Pic. 3 - BGP Table on R3.

Notice!
AS-Path length is now ignored. R3 chooses the longer path towards 172.16.101.0/24.

source:
http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_b1gt.html#wp1072077

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