BGP AS-Override Lab

Contents

Images used in lab: VIOS-ADVENTERPRISEK9-M), Version 15.9(3)M2

Topology Interfaces Only

Topology With IP Addressing

The last octet is the router number unless specified otherwise. Example: PE11's G0/0 is 10.10.111.11/24. Access to the initial configs, EVE-NG lab file and solution is below.

Intro

If a BGP router recieves an update and sees its own AS number in the AS Path, it will not accept the prefix. This is how loop prevention works in eBGP. It works well but imagine a scenario where multiple customer sites connect to the service provider using the same AS number. When a prefix is advertised from CE1 in AS100, it goes into the provider network in a different AS and it’s then received by CE2 that also used AS100, the prefix will never be accepted into the BGP table. In this lab you’ll see this problem and fix it by using the BGP AS-Override feature.

Tasks

  • Configure iBGP peerings between PE11 and PE12
  • Configure eBGP peerings between PE11 and CE1.
  • Configure eBGP peerings between PE12 and CE2.
  • Use their directly connected links for each peering.
  • On each CE router, advertise their loopback prefix into BGP.
  • Use next-hop-self to fix any next-hop processing issues.
  • Use the AS-Override feature so CE1 and CE2 accept eachothers Loopback prefix in their BGP tables.
  • There must be full reachability between the CE's Loopbacks.

Download Lab

To download the EVE-NG topology file you'll need to be a member. You can register here, it's free! It will be right here once you log in.

Initial Configs

Solution

AS Override in is very similar to Allow-AS in. Key points to remember: "Allow-AS in" is done on the customer side. "AS Override" is done on the provider side.

Hope you enjoyed this lab. If you have questions or need any help at all please leave a comment below and I’ll get back to you. 

Discussion

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