- Enterasys Security Router User's Guide

Overview
6-12 Configuring the Border Gateway Protocol
Access Control Lists
Access Control Lists (ACLs) are filters which permit or deny access to one or more IP addresses.
ACLs generally apply to both route updates and packet filtering but with BGP, route update
filtering is emphasized. Prefix-based ACLs control access by specifying which IP addresses are
permitted or denied via the network prefix number.
The XSR filters BGP advertisements as follows:
• with AS-path filters using the
ip as-path access-list and neighbor filter-list
commands.
• with ACLs using the
neighbor distribute-list {access-list} {in | out} command.
Routing data the XSR learns or advertises can be filtered by controlling BGP routing updates
through ACLs applied to the updates.
Filter Lists
As-path filter lists control access by specifying which AS paths to permit or deny. They are
configured with the
ip as-path access-list <ACL#> {permit | deny} as-regular-
expression
command. To further filter BGP paths by neighbor, use the neighbor filter-list
access-list-number {in | out}
command.
Community Lists
Community lists control access by specifying which communities are permitted or denied.
Community-based ACLs are configured with the
ip community-list command.
Route Maps
Route maps act with BGP to control and modify routing data and define the conditions by which
routes are redistributed between routing domains. Route maps are similar to ACLs in that they
both have rules for matching packets and when matches are found, act to permit or deny the
packet. Route maps are flexible and powerful in that they not only match, permit and deny, they
also change route attributes.
The XSR performs a match on AS-path, community, and network numbers for both incoming and
outgoing updates with the
match as-path, match community-list, and match ip address
commands, respectively. You add a route map to in/outbound routes with the
neighbor {ip-
address | peer-group-name} route-map <route-map#> {in | out}
command.
Refer to “BGP Community with Route Maps Examples” on page 6-26 for route-map examples.
Each route map includes sets of instructions that include:
• A permit or deny statement
• A sequence number
• An optional match clause
• An optional set clause
Route maps used with BGP can perform the following:
• Apply a weight to a specific route with
set weight
Note: Distribute-list filters are applied to network numbers, not AS paths.