Cisco IE 4000, Software Configuration Manual

Page: 858 / 1066

854
Configuring IP Unicast Routing
Configuring BGP
DETAILED STEPS
EXAMPLE
In the following example, an outbound soft reset is initiated for sessions with all routers in the autonomous system
numbered 35700:
Switch# clear ip bgp 35700 soft out
Configuring BGP Decision Attributes
When a BGP speaker receives updates from multiple autonomous systems that describe different paths to the same
destination, it must choose the single best path for reaching that destination. The decision is based on the value of
attributes that the update contains and other BGP-configurable factors. The selected path is entered into the BGP routing
table and propagated to its neighbors.
When a BGP peer learns two EBGP paths for a prefix from a neighboring AS, it chooses the best path and inserts that
path in the IP routing table. If BGP multipath support is enabled and the EBGP paths are learned from the same
neighboring autonomous systems, multiple paths are installed in the IP routing table. Then, during packet switching,
per-packet or per-destination load balancing is performed among the multiple paths. The
maximum-paths router
configuration command controls the number of paths allowed.
These factors summarize the order in which BGP evaluates the attributes for choosing the best path:
1. If the path specifies a next hop that is inaccessible, drop the update. The BGP next-hop attribute, automatically
determined by the software, is the IP address of the next hop that is going to be used to reach a destination. For
EBGP, this is usually the IP address of the neighbor specified by the
neighbor remote-as router configuration
command. You can disable next-hop processing by using route maps or the neighbor next-hop-self router
configuration command.
2. Prefer the path with the largest weight (a Cisco proprietary parameter). The weight attribute is local to the router and
not propagated in routing updates. By default, the weight attribute is 32768 for paths that the router originates and
zero for other paths. You can use access lists, route maps, or the
neighbor weight router configuration command
to set weights.
Command Purpose
1. show ip bgp neighbors Display whether a neighbor supports the route refresh capability. When
supported, this message appears for the router:
Received route refresh capability from peer.
2. clear ip bgp { * | address |
peer-group-name }
Reset the routing table on the specified connection.

Enter an asterisk (*) to specify that all connections be reset.

Enter an IP address to specify the connection to be reset.

Enter a peer group name to reset the peer group.
3. clear ip bgp { * | address |
peer-group-name } soft
out
(Optional) Perform an outbound soft reset to reset the inbound routing
table on the specified connection. Use this command if route refresh is
supported.

Enter an asterisk (*) to specify that all connections be reset.

Enter an IP address to specify the connection to be reset.

Enter a peer group name to reset the peer group.
4. show ip bgp
show ip bgp neighbors Verify the reset by checking information about the routing table and
about BGP neighbors.