Cisco Cisco IOS Software Release 12.0(22)S

BGP routing information includes the following items:

A network number (prefix), which is the IP address of the destination.

Autonomous system (AS) path, which is a list of the other ASs through which a route passes on its 
way to the local router. The first AS in the list is closest to the local router; the last AS in the list is 
farthest from the local router and usually the AS where the route began. 

Path attributes, which provide other information about the AS path, for example, the next hop. 

MPLS labels are included in the “update” messages that a router sends. Routers exchange the following 
types of BGP messages:

Open Messages—After a router establishes a TCP connection with a neighboring router, the routers 
exchange “open” messages. This message contains the AS number to which the router belongs and 
the IP address of the router who sent the message. 

Update Messages—When a router has a new, changed, or broken route, it sends an “update” message 
to the neighboring router. This message contains the Network Layer Reachability Information 
(NLRI), which lists the IP addresses of the usable routes. The update message also includes any 
routes that are no longer usable. The update message also includes path attributes and the lengths of 
both the usable and unusable paths. Labels for VPNv4 routes are encoded in the update message as 
specified in RFC 2858. The labels for the IPv4 routes are encoded in the update message as specified 
in RFC 3107. 

Keepalive Messages—Routers exchange “keepalive” messages to determine if a neighboring router 
is still available to exchange routing information. The router sends these messages at regular 
intervals. (Sixty seconds is the default for Cisco routers.) The keepalive message does not contain 
routing data; it only contains a message header. 

Notification Messages—When a router detects an error, it sends a “notification” message. 

When BGP (External BGP (EBGP) and Internal BGP (IBGP)) distributes a route, it can also distribute 
an MPLS label that is mapped to that route. The MPLS label mapping information for the route is carried 
in the BGP update message that contains the information about the route. If the next hop is not changed, 
the label is preserved. 

When you issue the neighbor send-label command on both BPG routers, the routers advertise to each 
other that they can then send MPLS labels with the routes. If the routers successfully negotiate their 
ability to send MPLS labels, the routers add MPLS labels to all outgoing BGP updates.