Chapter 50
| IP Routing Commands
Border Gateway Protocol (BGPv4)
– 1075 –
Path Selection
When there are multiple paths to the same prefix (with the same prefix length), the
information included in route advertisement is used to select the best path to a
destination following the rules shown below.
1. Choose the path with the highest WEIGHT. If the value of this attribute is the
same for more than one candidate, go to the next step.
2. Choose the path with the highest LOCAL-PREF. If the value of this attribute is
the same for more than one candidate, go to the next step.
3. Choose the path that was generated by the local router with the network or
aggregate-address command. If the value of this criteria is the same for more
than one candidate, go to the next step.
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
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