BGP uses the UPDATE message to send routing updates to peers. When a BGP session is initialized, UPDATE messages are sent until the complete BGP table has been exchanged.
Every time an UPDATE message is received, the BGP route table is updated and the BGP route table version number is incremented by one.
BGP advertises prefix and length sets and sends out one update message for each AS-path and attribute combination. For each AS-path, a set of NLRI announcements are sent in each update message.
UPDATE MESSAGE FORMAT
- Network Layer Reachability Information
- Path Attributes
- Withdrawn Routes
| SECTIONS | FRAME FORMAT | |
| Unreachable Routes |
Unfeasible Routes Length (2bytes) | |
| Withdrawn Routes (variable) | ||
| Path Attributes |
Total Path Attribute Length (2 bytes) | |
| Path Attributes (variable length) | ||
| NLRI | Length (1 byte) | Prefix (variable length) |
| Length (1 byte) | Prefix (variable length) | |
| additional length/prefix pairs | ||
Unfeasible Routes
The unfeasible routes section starts with a 2 byte field that indicates the length of the list of withdrawn routes. Withdrawn routes are routes that are down or no longer reachable.
Path Attributes
The path attributes section starts with a 2 byte field that indicates the length of the list of path attributes. Path Attributes