Overview of BGP Messages

BGP systems send four types of messages:

Open
Update
Keepalive
Notification

All BGP messages have the same fixed-size header, which contains a marker field indicating the total length of the message and a type field indicating the message type.

This section discusses the following topics:

Open Messages

After a TCP connection is established between two BGP systems, they exchange BGP open messages to create a BGP connection between them. Once the connection is established, the two systems can exchange BGP messages and data traffic.

Open messages consist of the BGP header plus the following fields:

Version—The current BGP version number is 4.
Local AS number—You configure this by including the autonomous-system statement at the [edit routing-options] or [edit logical-systems logical-system-name routing-options] hierarchy level, as described in Specifying the Local Routing Device’s AS Number.
Hold time—Proposed hold-time value. You configure the local hold time with the BGP hold-time statement, as described in Configuring the Delay Before BGP Peers Mark the Routing Device as Down.
BGP identifier—IP address of the BGP system. This address is determined when the system starts up and is the same for every local interface and every BGP peer. You can configure the BGP identifier by including the router-id statement at the [edit routing-options] or [edit logical-systems logical-system-name routing-options] hierarchy level, as described in Assigning a BGP Identifier. By default, BGP uses the IP address of the first interface it finds in the router.
Parameter field length and the parameter itself—These are optional fields.

Update Messages

BGP systems send update messages to exchange network reachability information. BGP systems use this information to construct a graph that describes the relationships among all known ASs.

Update messages consist of the BGP header plus the following optional fields:

Unfeasible routes length—Length of the field that lists the routes being withdrawn from service because they are no longer deemed reachable
Withdrawn routes—IP address prefixes for the routes being withdrawn from service
Total path attribute length—Length of the field that lists the path attributes for a feasible route to a destination
Path attributes—Properties of the routes, including the path origin, the multiple exit discriminator (MED), the originating system’s preference for the route, and information about aggregation, communities, confederations, and route reflection
Network layer reachability information (NLRI)—IP address prefixes of feasible routes being advertised in the update message

Keepalive Messages

BGP systems exchange keepalive messages to determine whether a link or host has failed or is no longer available. Keepalive messages are exchanged often enough so that the hold timer does not expire. These messages consist only of the BGP header.

Notification Messages

BGP systems send notification messages when an error condition is detected. After the message is sent, the BGP session and the TCP connection between the BGP systems are closed. Notification messages consist of the BGP header plus the error code and subcode, and data that describes the error.