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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 35 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. When chosen,
the selected path is entered into 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 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, instead of a single best path, 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.
Tab le 35-10 Advantages and Disadvantages of Hard and Soft Resets
Type of Reset Advantages Disadvantages
Hard reset No memory overhead The prefixes in the BGP, IP, and FIB tables
provided by the neighbor are lost. Not
recommended.
Outbound soft reset No configuration, no storing of routing table
updates Does not reset inbound routing table updates.
Dynamic inbound soft reset Does not clear the BGP session and cache
Does not require storing of routing table updates
and has no memory overhead
Both BGP routers must support the route
refresh capability (in Cisco IOS Release 12.1
and later).
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 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.
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 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.
Step 4 show ip bgp
show ip bgp neighbors Verify the reset by checking information about the routing table and about BGP
neighbors.