Bgp Routing Mechanism - H3C S7500 Series Operation Manual

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches
Data: Used to further determine the cause of errors. Its content is the error data
which depends on the specific error code and error subcode. Its length is
unfixed.
V. Keepalive
In BGP, Keepalive message keeps BGP connection alive and is exchanged
periodically. A BGP Keepalive message only contains the packet header. No
additional field is carried.
VI. Route-refresh
Route-refresh message is used to notify peers that the route refresh function is
available.

6.1.3 BGP Routing Mechanism

When BGP initially starts on a router, it sends the whole BGP routing table to its peers
to exchange routing information. Afterwards, BGP sends only Update messages
instead of the whole table. During this process, BGP also sends/receives Keepalive
messages to determine whether the connections to its peers are normal.
A router running BGP is also called a BGP speaker because it can send BGP
messages. A BGP speaker can receive routing information as well as generate and
advertise routing information to other BGP speakers. When a BGP speaker receives
a route from another AS and finds this is a new route (a route it does not know) or a
route superior to any of its known routes, the BGP speaker advertises the route to all
other BGP speakers in the AS.
Two BGP speakers capable of exchanging BGP messages with each other are peers
of each other. Multiple BGP peers can form one peer group.
I. BGP route advertisement policies
In the implementation on H3C S7500 Series Ethernet Switches (hereinafter referred
to as the S7500 series), BGP adopts the following policies to advertise routes:
When there are multiple optional routes, a BGP speaker chooses only the
optimal one;
A BGP speaker advertises only the local routes to its peers;
A BGP speaker advertises the routes obtained from EBGP to all its BGP peers
(including both EBGP and IBGP peers);
A BGP speaker does not advertise the routes obtained from IBGP to its IBGP
peers;
A BGP speaker advertises the routes obtained from IBGP to its EBGP peers (in
the S7500 series, BGP and IGP does not synchronize with each other);
Once a BGP speaker sets up a connection to a new peer, it advertises all its BGP
routes to the new peer.
6-5
Chapter 6 BGP Configuration