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BGP Peering & Path Selection

Two autonomous systems, two paths to the same prefix, and R1 keeps choosing the one with the longer AS path. Walk the best-path algorithm to the attribute that overrides it.

Download .unlNo lab server? Practise it in your browser
The lab as it opens in PNETLab: two autonomous systems, two eBGP paths, and the facts you need to compare them.

What you'll practise

  • Read eBGP and iBGP sessions between two autonomous systems
  • Walk the BGP best-path algorithm step by step on real routes
  • Influence path selection with local-preference and AS-path prepending
  • Read show ip bgp to see which attribute chose the best path
  • Find a local-preference override that beats a shorter AS path

Set it up

  1. 1

    Have the node image ready

    Images are not included. The file expects your own licensed copy of:

    • L3-ADVENTERPRISEK9-M-15.4-2T.bin

    If your copy has a different name, open each node's Edit dialog after importing and pick your image from the list.

  2. 2

    Copy it to the server and fix permissions

    Replace <your-server> with the IP address or hostname of your EVE-NG or PNETLab server.

    $ scp bgp-peering-path-selection-lab.unl root@<your-server>:/opt/unetlab/labs/
# then, logged in to the server:
$ /opt/unetlab/wrappers/unl_wrapper -a fixpermissions
  3. 3

    Open it and start the nodes

    The canvas notes list the objectives, and the workbook walks through the fix.

Free to use and share under CC BY 4.0. Please credit LabFabric with a link to this page.