• Originates subnets from aggregate for purpose of traffic engineering.
    • Longest prefix.

Configuration:

  • Inject Map
    • More specific subnet to advertise.
    • ‘set ip address prefix-list <list>
  • Exist Map
    • Aggregate to be originated from.
    • ‘match ip address prefix-list <list>
    • ‘match ip route-source prefix-list <list>

In the image below R2, R3 and R5 are all in ASN 100, R8 and R10 in ASN 810.1. R5 R8 and R10 are running EBGP between the two ASNs.

R5 is going to summarize ASN 100 transit links 10.30.2.0 and 10.30.3.0 into ASN 810.1.

Both R8 and R10 are now showing 10.30.0.0/16 for the transit links within ASN 100. To advertise a more specific route that falls within the summary address, a conditional route injection needs to be performed. This is a valid method of traffic engineering. Currently if R10 is trying to reach the subnet 10.30.2.0/30, it will go over the EBGP path directly to R5. This is because AD for EBGP is much lower than AD of iBGP. Injecting a more specific route into BGP originating from R8 would steer traffic from R10 to R8 first because longest prefix is always going to be more preferred.

Configuration:

  • Create Prefix Lists for Aggregate/Summary, Route neighbor/source, and more specific route.

Prefix-list AGGREGATE is the summarized route from R5.

Prefix-list ROUTE-SOURCE is the neighbor advertising the route inbound to ASN 810.1.

Prefix-list Transit_2 is the more specific prefix we want to advertise into BGP/down to R10.

  • Create Route-Maps for ‘INJECT MAP’ and ‘EXIST MAP’.

‘INJECT_MAP’ defines the more specific route, ‘Transit_2’, to be sent into BGP domain.

‘EXIST_MAP’ specifies what’s there, ie. existing Aggregate (10.30.0.0/16) and the neighbor advertising the Aggregate via EBGP.

  • BGP ASN statement

‘bgp inject-map INJECT_MAP exist-map EXIST_MAP’ defines in the global BGP process everything that’s just been discussed. Clearing BGP neighbors might be necessary in lab. The change takes a while to work.

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