User's Manual

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The Overview of BGP-4 operation
Unlike RIP and OSPF protocols, BGP protocol is connection oriented. BGP switches must establish
connection to exchange routing information. The operation of BGP protocol is driven by messages and the
messages can be divided into four kinds:
Open message----It’s the first message which is sent after a TCP connection is established. It is used to
create BGP connecting relation among BGP peers. Some parameters in Open Message are used to negotiate
if a connection could be established among BGP peers.
Keepalive Message ----- it’s the message to check connection availability. It’s usually sent periodically to keep
BGP connection. If this message or Update message is not received within holdtime time, BGP connection is
closed.
Update Message----- it’s the most important message in the BGP system. It’s used to exchange routing
information among peers. The switches exchange not only updated routing information, but also unavailable
or canceled routing information. It consists of three parts: unreachable route, NLRI(Network Layer
Reachability Information) and Path Attributes.
Notification Message------it’s the mistake notification message. When a BGP speaker receives this message,
it shutdowns the BGP connections with its neighbors
BGP-4 is connection oriented. BGP acts as higher protocol and runs on the particular equipments. When
detecting a neighbor, a TCP session is established and maintained. Then the exchanging and synchronization
of the route table will be carried out. By sending the whole BGP route table the routing information is
exchanged only when the system initiates. After that, the routing information is exchanged only when the
updated routing information is available. Only incremental update message is exchanged. BGP-4 maintains
links and sessions periodically through keep alive message. That is sending and receiving keep alive
message periodically to check if the connections are normal.
The switches that participate the BGP session are called BGP speaker. It continuously receives or generates
new routing information and advertises it to other BGP speakers. When a BGP speaker receives a new
routing notification from other AS, if this route is better than the presently known route or there is no
acceptable route, it sends this route to all the other BGP speakers of the AS. A BGP speaker calls other
speakers that exchange route information with it as neighbors or peers. Several relevant neighbors can
constitute a peer group. BGP operates on the switches in the following two manners:
 IBGP：Internal BGP
 EBGP：External BGP
When BGP runs in the same AS, it’s called IBGP. When in the different AS, it’s called EBGP. Generally, the
outer neighbors are connected physically and the inner neighbors can be in any place of the AS. The
difference is finally shown in the dealing manner of BGP to routing information. The equipments may check
the AS numbers of the Open Message from neighbors to decide treating the neighbor switches as the exterior
neighbor or as the interior neighbor.