The Open message plays a critical role in establishing a BGP neighbor (peer) relationship. It is the first BGP message exchanged after a TCP connection is successfully established between two BGP routers.
Purpose of the Open Message
The Open message is used to:
- Initiate a BGP session
- Exchange basic parameters between BGP peers
- Verify that both routers are compatible before forming a full BGP relationship
Without a successful Open message exchange, a BGP session cannot progress to the Established state.
When the Open Message Is Sent
- After TCP session establishment on port 179
- During the OpenSent and OpenConfirm states of the BGP finite state machine
- Each router sends its own Open message to the peer
Information Carried in the Open Message
The Open message includes several important fields:
- BGP Version
Ensures both routers use a compatible BGP version (typically BGP-4). - Autonomous System (AS) Number
Identifies the router’s AS and confirms whether the session is iBGP or eBGP. - Hold Time
Proposes how long a router will wait without receiving a keepalive or update before declaring the session dead. - BGP Identifier (Router ID)
A unique 32-bit value that identifies the BGP router. - Optional Parameters
Used to negotiate additional capabilities, such as:- Route refresh
- Multiprotocol extensions (MP-BGP)
- Graceful restart
Timer Negotiation
- The Hold Time is negotiated using the Open message.
- The smaller hold time proposed by the two peers is selected.
- The Keepalive timer is automatically set to one-third of the agreed hold time.
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