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IEWB-DYN Lab 5 – BGP Highlights

Posted by Jo on February 17, 2008

Again, nothing too tricky on the BGP section (this is a lab rated 6!), quite a few peerings to configure. I actually missed on of these (R3 to SW2) out on my first pass of the question, but good job that I checked it and corrected my mistake.

AS-Path Manipulation

This scenario had a number of private BGP AS numbers configured. The task wanted to strip these out when they were being advertised out of AS 300.

router bgp 300
no synchronization
bgp router-id 150.1.3.3
neighbor 162.1.0.4 remote-as 100
neighbor 162.1.0.4 remove-private-as
neighbor 162.1.13.1 remote-as 200
neighbor 162.1.13.1 remove-private-as

This config removes the any private AS numbers when advertising to the specified neighbors, so they appear to be from AS 300.

BGP Filtering

To stop an advertised network form being advertised beyond an AS use the no-advertise keyword. This task had a network advertised on R5 into AS 500 and didn’t want it to be advertised out from R4 beyond AS 100.

First create an ip prefix-list to match the network

ip prefix-list LO15 seq 5 permit 162.1.15.0/24

Then, a route-map to match the prefix-list and set the community value accordingly.

route-map NO-ADVERTISE permit 10
match ip address prefix-list LO15
set community no-advertise

Finally, apply the route map on the peering statement to R4, and set the send-community value

neighbor 150.1.4.4 send-community
neighbor 150.1.4.4 route-map NO-ADVERTISE out

Check this route is received on R4 and not advertised to other AS’s

Rack1R4#sh ip bgp 162.1.15.0
BGP routing table entry for 162.1.15.0/24, version 34
Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to any peer)
Not advertised to any peer
500
150.1.5.5 (metric 65) from 150.1.5.5 (150.1.5.5)
Origin IGP, metric 0, localpref 100, valid, external, best
Community: no-advertise

Rack1R3#sh ip route 162.1.15.0
% Subnet not in table

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2 Responses to “IEWB-DYN Lab 5 – BGP Highlights”

  1. Akash said

    To stop an advertised network form being advertised beyond an AS we use the no-export command. no-advertise keyword will stop advertisement to every bgp peer including ibgp peer.

  2. ccielab said

    Thats a good point but I think in this case R5 is the only router in the AS, so using the no-advertise was fine.

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