BGP Update Message
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 table is updated and the 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- Withdrawn Routes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| Withdrawn Routes (variable) | ||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) | ||
| NLRI | Length (1 byte) | Prefix (variable) |
| Length (1 byte) | Prefix (variable) | |
| additional length/prefix pairs | ||
| << NLRI | NLRI >> |