Once a BGP router has routes in it’s BGP table, in order to share those routes with BGP neighbors, it sends them the routing information via BGP update messages.
Each message has three parts:
- Withdrawn Routes field – allows neighbors to inform each other of failed routes
- Path Attributes field – lists the PAs for each route
- Prefix, and Prefix Length fields – defines each individual NLRI
Update messages will include NLRIs that all have the same PA values. If PA values differ between NLRIs then an individual update message will be required for each NLRI.