Accton Technology ES4626 - Manual (Page 512)

512
2． 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 LayerReachability 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:
z IBGP：Internal BGP
z EBGP：External BGP
When BGP runs in the same AS, it’s called IBGP. When in the different AS, it’s