BGP Attribute AS Path Ignore

Posted by September 22, 2011 in BGP

Scenario:

You are responsible for a International network. All routers are connected through BGP. One of the network administrators in NewDelhi configure AS Path prepending for their AS so Paris will send all traffic through Sydney. You heard about a hidden option to ignore AS Path information, maybe this is just what you need…

Goal:

  • All IP addresses have been preconfigured for you as specified in the topology picture.
  • Configure EBGP between all the ASes.
  • Advertise the loopback0 interface on router Barcelona in BGP.
  • Router Paris should have two paths to reach network 1.1.1.0 /24.
  • Configure AS Path prepending on router NewDelhi so router Paris sends all traffic to network 1.1.1.0 /24 through router Sydney.
  • Configure router Paris to ignore the added ASes so it will send traffic for network 1.1.1.0 /24 to router NewDelhi. Don’t use any of the other BGP attributes.

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IOS:

c3640-jk9o3s-mz.124-16.bin

Topology:

BGP AS Path Attribute Ignore

Configuration Files

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Once you are logged in you will find the configuration files right here.

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11 Comments

  1. hi

    when i opened the topology file it just came with the peer group lab.

    Can you check the file?

  2. I cannot get the follow as-path ignore command to work

    R2(config-router)#bgp bestpath ?
    compare-routerid Compare router-id for identical EBGP paths
    cost-community cost community
    med MED attribute

    Cisco IOS Software, 3600 Software (C3640-JK9O3S-M), Version 12.4(16a), RELEASE SOFTWARE (fc2)

    Any idea why this might be?

  3. Just try the command, it will work even if it is not shown.
    “bgp bestpath as-path ignore”
    you will then see it in the running config.

  4. Hi,
    To complete this lab, use the same topology as “BGP Attribute Router ID”
    1) NewDelhi —Router-ID = 2.2.2.2
    neighbor 192.168.23.3 route-map as-prepend out —- towards Paris
    route-map as-prepend permit 10
    set as-path prepend 2 2 2
    2)
    bgp bestpath as-path ignore ———— just past the command even it is showing up
    bgp bestpath compare-routerid
    based on AS-path lenght the Sydney should be the prefered path, but with ” ingore command” Paris will prefer Newdelhi as it has a lower-ID ( 2.2.2.2)
    !
    Paris#sh ip bgp 1.1.1.0
    BGP routing table entry for 1.1.1.0/24, version 2
    Paths: (2 available, best #2, table Default-IP-Routing-Table)
    Advertised to update-groups:
    1
    4 1
    192.168.34.4 from 192.168.34.4 (4.4.4.4)
    Origin IGP, localpref 100, valid, external
    2 2 2 2 1
    192.168.23.2 from 192.168.23.2 (2.2.2.2)
    Origin IGP, localpref 100, valid, external, best
    Paris#

  5. !!! Rectifictaion!!!
    2) Paris config
    bgp bestpath as-path ignore ———— just past the command even it is NOT showing up with ? mark

  8. Thanks once again for your struggle,

    sorry to say, the topology is change.

    show me show to upload the topology to your site.
    i create to many topology , i wish to share with all of you.
    i can help to upload the topology when missing is there like this one.

    i will appreciate you email to me saidullahdata@gmail.com

    thanks to your struggle and help to others

  9. i don’t read any of the lab comments before i have completed the lab myself.
    Cisco hidden command… doh!
    what did people do before prolific Internet search was even a thing ?! 🙂

    good lab.

Comments are closed.