Communications
BGP uses TCP on port 179 for communication between neighbours. RIP and OSPF either run directly on top of IP or use UDP.
A BGP header consists of;
A 16-bit marker set all ‘1’s
A 2-byte length field (19-4096) indicating the total length of the message
A type field; there are several different types, originally four.
1
Open
2
Update
3
Notification
4
Keep-alive
Both sides of a BGP session begin with an open message, including the BGP version, routers AS number and IP address.
Updates are used to list new or withdrawn routes. They use a format called Network Layer Reachability Information or NLRI.
Notifications are generally sent to indicate errors.
When you register your AS, you are required to declare information about your AS in a Routing Registry (RR). These records are in a format specified by Routing Policy Specification Language (RPSL) defined in RFC 2622.
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