BGP-NLRI

• Network Layer Reachability Information.
  • In CCIE context, the routes passed between hosts via BGP.
• Uses UPDATE and WITHDRAW messages to exchange NLRI.

• BGP NLRI Origination.
  • ‘network’ statement.
    • Requires exact match in routing table.
  • ‘redistribute statement.
    • Won’t include OSPF External by default.
  • ‘aggregate-address’ statement.
    • Requires one subnet in BGP table.
  • ‘bgp inject-map’
    • Opposite of aggregation command.

Network Statement:

• Originates prefixes with ORIGIN of IGP (i)
• Requires exact match in RIB.
  • Does not need to be connected, can be learned via IGP.
• Assumes classful if mask keyword not used.
• Sets weight to 32768.

BGP Redistribute Statement:

• Originates prefixes with ‘ORIGIN INCOMPLETE (?).
• Originates classful summary if auto-summary enabled.
• Auto copies IGP metric to BGP MED.
• Won’t include OSPF External by default.
  • ‘redistribute ospf <process> match internal external’
    • Required to redistribute OSPF external.
• Sets weight to 32768.
• ‘bgp redistribute internal’
  • By default only external BGP routes are redistributed into IGP with redistribution.
  • Command allows for internal BgP routes to be redistributed into IGP.
  • Can result in routing loop.

BGP Conditional Advertisement:

• ‘neighbor advertise-map map 1 <non-exist or exist>’
• Advertise prefix matched in advertise-map
  • If prefix matched in non-exist map does not exist.

• Inject map
  • Subnet to be advertised.
  • 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>

The image below shows six routers all running BGP. R1, R2, R3 and R5 are all in ASN 100, while R8 and R10 are in ASN 810.1. R5 and R8 are EBGP peers.

Transit /30 subnets are setup for each link in ASN 100, 10.30.1.0, 10.30.2.0, and 10.30.3.0. These transits are being advertised in the local IGP and in BGP to ASN 810.1. This can be seen in R8’s routing table.

If we wanted to summarize these three subnets into one advertisement, the ‘aggregate-address’ command can be used under the BGP process on R5.

This results in one route showing under R8’s route table.

Similar to EIGRP, summarization in BGP can be performed nearly anywhere. The portion of the command ‘summary-only’ says that the summary will be the only route advertised, no more specific prefixes.