Content

| 9.2 |  | Basic BGP Operation |
| 9.2.5 |  | Network-layer reachability information (NLRI) |

Rather than advertise reachable destinations as a network and a subnet mask, BGP advertises them using network-layer reachability information (NLRI), which consists of prefixes and prefix lengths. The prefix represents the reachable destination, and the prefix length represents the number of bits set in the subnet mask. For example, 10.1.1.0 255.255.255.0 has a prefix of 10.1.1.0 and a prefix length of 24.

> Note: There are 24 bits set in the subnet mask. Therefore it would be advertised by BGP as 10.1.1.0/24.

The NLRI consists of multiple instances of the 2-tuple <length, prefix>. A tuple is a mathematical term for a set of elements. In this case, the two (2) refers to the fact that there are only two elements in the set. Therefore, the NLRI <19, 192.24.160.0> represents the prefix of 192.24.160.0, and the length is a 19-bit mask. In decimal terms, this NLRI refers to the supernet 192.24.160.0 255.255.224.0.

Withdrawn Routes
Withdrawn routes provide a list of routing updates that are no longer reachable and that need to be withdrawn or removed from the BGP routing table. Withdrawn routes have the same format as NLRI.

An update message that has no NLRI or path attribute information is used to advertise only routes to be withdrawn from service.