H3C Technologies H3C S12500 Series Switches - Manual (Page 208)

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Configuring BGP
Overview
Border Gateway Protocol (BGP) is an exterior gateway protocol. It is called internal BGP (IBGP) when it
runs within an AS and called external BGP (EBGP) when it runs between ASs.
The current version in use is BGP-4 (RFC 4271).
BGP has the following characteristics:
• Focuses on the control of route propagation and the selection of optimal routes rather than route
discovery and calculation, which makes BGP different from interior gateway protocols (IGPs) such
as Open Shortest Path First (OSPF) and Routing Information Protocol (RIP).
• Uses TCP to enhance reliability.
• Measures the distance of a route by using a list of ASs that the route must travel through to reach the
destination. Therefore, BGP is also called a path-vector protocol.
• Supports classless inter-domain routing (CIDR).
• Reduces bandwidth consumption by advertising only incremental updates, suited to advertise a
great amount of routing information on the Internet.
• Eliminates routing loops by adding AS path information to BGP route updates.
• Uses policies to implement flexible route filtering and selection.
• Has good scalability.
BGP speaker and BGP peer
A router running BGP is a BGP speaker. A BGP speaker establishes peer relationships with other BGP
speakers to exchange routing information over TCP connections.
BGP peers include the following types:
• IBGP peers—Reside in the same AS as the local router.
• EBGP peers—Reside in different ASs from the local router.
BGP message types
BGP has the following types of messages:
• Open—After a TCP connection is established, the first message sent by each side is an Open
message for peer relationship establishment.
• Update—The update messages are used to exchange routing information between peers. Each
update message can advertise a group of feasible routes with identical attributes, and carry
multiple withdrawn routes.
• Keepalive—Keepalive messages are sent between peers to maintain connectivity.
• Route-refresh—A Route-refresh message is sent to a peer to request the specified address family
routing information to be resent.