ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
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4. Choose the path with the shortest AS_PATH. If the value of this
attribute is the same for more than one candidate, go to the next step.
Note that this attribute may be disabled in the selection process using
the bgp bestpath as-path ignore command.
5. Choose the path with the lowest ORIGIN (IGP < EGP < Incomplete). If
the value of this criteria is the same for more than one candidate, go to
the next step.
6. Choose the path with the lowest MED. By default, the MED attribute is
considered only when a prefix is received from neighbors in the same
AS. If the value of this criteria is the same for more than one candidate,
go to the next step.
7. Choose an eBGP path over an outer confederation, and an outer
confederation over an iBGP path. If the value of this criteria is the same
for more than one candidate, go to the next step.
8. Choose the path with the lowest IGP metric to the next hop. If the
value of this criteria is the same for more than one candidate, go to the
next step.
9. Choose the path originated by the BGP router with the lowest router ID.
MESSAGE TYPES
Four message types are used by BGP. The OPEN message is used by BGP
peers to identify their capabilities, the UPDATE message is used to
advertise/withdraw prefixes, the NOTIFICATION message is used to send
errors or close the session, and the KEEPALIVE messages is used to keep
the BGP session up. These message types are described below.
◆ OPEN – BGP routers normally wait for BGP connections on TCP port
179. A router that wants to establish an association will first open a TCP
connection leading to that port on the peer router. Once the connection
has been set, each side sends an OPEN message to negotiate the
association’s parameters based on the capabilities advertised in these
messages. Open messages include information about the BGP version
number in use, the peer’s AS number, the hold time, the BGP identifier
(i.e., loopback address or the highest value of all the BGP speaker’s
interfaces), and optional parameter length.
◆ UPDATE – These messages are used to announce or withdraw IP
prefixes, and include the following components: withdrawn route
length, withdrawn routes, total path attributes length, path attributes,
and network layer reachability information.
◆ NOTIFICATION – These messages are used to indicate error conditions.
The underlying TCP session is closed after a notification message is
sent.
◆ KEEPALIVE – These messages are sent at a set interval and are used to
verify that the BGP session is active. The hold timer is reset upon
receipt of a KEEPALIVE or UPDATE message. If the hold time is set to