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Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
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Chapter 38 Configuring IP Unicast Routing
Configuring BGP
Beginning in privileged EXEC mode, follow these steps to learn if a BGP peer supports the route-refresh
capability and to reset the BGP session:
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 selected
path is then entered in the BGP routing table and propagated to its neighbors. The decision is based on
the value of attributes that the update contains and other BGP-configurable factors.
When a BGP peer learns two EBGP paths for a prefix from a neighboring autonomous system, 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, instead of a single best path,
multiple paths are entered 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 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. Routes with the largest weight are preferred.
You can use access lists, route maps, or the
neighbor weight
router configuration command to set
weights.
Command
Purpose
Step 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.
Step 2
clear ip bgp
{
*
|
address
|
peer-group-name
}
Reset the routing table on the specified connection.
•
Enter an asterisk (*) to reset all connections.
•
Enter an IP
address
to reset a specific connection.
•
Enter a peer group name to reset the peer group.
Step 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 reset all connections.
•
Enter an IP
address
to reset a specific connection.
•
Enter a peer group name to reset the peer group.
Step 4
show ip bgp
show ip bgp neighbors
Verify the reset by reviewing the routing table information and BGP neighbor
information.
