Edge-Core ES4626 User Manual page 558

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.
IBGP are used in the AS. It sends message to all the BGP neighbors in the AS. IBGP
exchanges AS routing information in a big organization. Attention, the switches in the AS
needn't be connected physically. Only if the switches are in the same AS, they can be
neighbors each other. Because BGP can't detect route, the route tables of other inner
route protocols (such as static route, direct route, OSPF and RIP) need contain neighbor
IP addresses and these routes are used to exchange information among BGPs. In order
to avoid routing loops, when a BGP speaker receives a route notification from inner
neighbor, it would not notify this route to other inner neighbors.
EBGP is used among the AS, and it transmits routing information to the BGP
neighbors of outer ASes. EBGP need physical connection and share the same medium.
Because EBGP need physical connection, the boundary equipments between two AS are
usually running EBGP. When a BGP speaker receives routing information from outer
neighbors, it notifies these routes to other inner neighbors.
3．Route attribute
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