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Configuring BGP
Configuring Basic BGP Features
IPC-308
Cisco IOS IP Configuration Guide
Clearing the Hit Count Table of Prefix List Entries
To clear the hit count table of prefix list entries, use the following command in EXEC mode:
Configuring BGP Path Filtering by Neighbor
In addition to filtering routing updates based on network numbers, you can specify an access list filter
on both incoming and outbound updates based on the BGP autonomous system paths. Each filter is an
access list based on regular expressions. To specify the access list filter, define an autonomous system
path access list and apply it to updates to and from particular neighbors. See the “Regular Expressions”
appendix in the Cisco IOS Terminal Services Configuration Guide for more information on forming
regular expressions.
To configure BGP path filtering, use the following commands beginning in global configuration mode:
See the “BGP Path Filtering by Neighbor Examples” section at the end of this chapter for an example of
BGP path filtering by neighbor.
Disabling Next Hop Processing on BGP Updates
You can configure the Cisco IOS software to disable next hop processing for BGP updates to a neighbor.
Disabling next hop processing might be useful in nonmeshed networks such as Frame Relay or X.25,
where BGP neighbors might not have direct access to all other neighbors on the same IP subnet. There
are two ways to disable next hop processing:
• Provide a specific address to be used instead of the next hop address (manually configuring each
address).
• Use a route map to specify that the address of the remote peer for matching inbound routes, or the
local router for matching outbound routes (automatic method).
Router# show ip prefix-list prefix-list-name
[network/length] longer
Displays all entries of a prefix list that are more specific than
the given network and length.
Router# show ip prefix-list prefix-list-name
[network/length] first-match
Displays the entry of a prefix list that matches the given
prefix (network and length of prefix).
Command Purpose
Router# clear ip prefix-list prefix-list-name
[network/length]
Clears the hit count table of the prefix list entries.
Command Purpose
Step 1
Router# ip as-path access-list access-list-number
{permit | deny} as-regexp
Defines a BGP-related access list.
Step 2
Router# router bgp as-number
Enters router configuration mode.
Step 3
Router(config-router)# neighbor {ip-address |
peer-group-name} filter-list access-list-number {in
| out}
Establishes a BGP filter.