|
| |
BGP operation
BGP neighbors, or peers, are established by manual configuration between routers
to create a TCP session on port 179. A BGP speaker will periodically send
19-byte keep-alive messages to maintain the connection (every 60 seconds by
default). Among routing protocols, BGP is unique in using TCP as its transport
protocol.
When BGP is running inside an autonomous system (AS), it is referred to as
Internal BGP (IBGP Interior Border Gateway Protocol). When BGP runs between ASs,
it is called External BGP (EBGP Exterior Border Gateway Protocol). Routers that
sit on the boundary of an AS and that use EBGP to exchange information with the
ISP are border or edge routers. In the Cisco operating system, iBGP routes have
an administrative distance of 200. which is less preferred than either external
BGP or any interior routing protocol. Other router implementations also prefer
eBGP to IGPs, and IGPs to iBGP.
Optional Extensions negotiated at Connection Setup
During the OPEN handshake, BGP speakers can negotiate optional capabilities of
the session, including multiprotocol extensions and various recovery modes. If
the multiprotocol extensions to BGP are negotiated at the time of creation, the
BGP speaker can prefix the Network Layer Reachability Information (NLRI) it
advertises with an address family prefix. These families include the default
IPv4, but also IPv6, IPv4 and IPv6 Virtual Private Networks, and multicast BGP.
Increasingly, BGP is used as a generalized signaling protocol to carry
information about routes that may not be part of the global Internet, such as
VPNs .
| |
|
