Filtering As Paths With A Filter List - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

host1(config-router)#neighbor 192.168.1.158 prefix-tree newyork out
New policy values are applied to all routes that are sent (outbound policy) or
received (inbound policy) after you issue the command.
To apply the new policy to routes that are already present in the BGP routing
table, you must use the clear ip bgp command to perform a soft clear or hard
clear of the current BGP session.
Behavior is different for outbound policies configured for peer groups for which
you have enabled Adj-RIBs-Out. If you change the outbound policy for such a
peer group and want to fill the Adj-RIBs-Out table for that peer group with the
results of the new policy, you must use the clear ip bgp peer-group command
to perform a hard clear or outbound soft clear of the peer group. You cannot
merely perform a hard clear or outbound soft clear for individual peer group
members because that causes BGP to resend only the contents of the Adj-RIBs-Out
table.
IPv6 prefix trees are not supported, Therefore you can specify an IPv6 address
with this command only within the IPv4 address family and when you want to
advertise IPv4 routes to IPv6 peers.
Use the no version to remove the prefix tree.
See neighbor prefix-tree.

Filtering AS Paths with a Filter List

You can use a filter list to filter incoming and outgoing routes based on the value of
the AS-path attribute. Whenever a BGP route passes through an AS, BGP prepends
its AS number to the AS-path attribute. The AS-path attribute is the list of ASs that a
route has passed through to reach a destination.
To filter routes based on the AS-path, define the access list with the ip as-path
access-list command, and apply the list to routes received from or passed to a
neighbor with the neighbor filter-list command. AS-path access lists use regular
expressions to describe the AS path to be matched. A regular expression uses special
characters often referred to as metacharacters to define a pattern that is compared
with an input string. For a full discussion of regular expressions, with examples on
how to use them, see JUNOSe IP Services Configuration Guide.
The router compares each route's AS path against the conditions in the access list
one by one. If the first match is for a permit condition, the route is accepted or
passed. If the first match is for a deny condition, the route is rejected or blocked.
The order of conditions is critical because testing stops with the first match. If no
conditions match, the router rejects or blocks the route; that is, the last action of any
list is an implicit deny condition for all routes.
You cannot selectively place conditions in or remove conditions from an AS-path
access list. You can insert a new condition only at the end of an AS-path access list.
Consider the network structure in Figure 23 on page 88.
Example 1
Chapter 1: Configuring BGP Routing
Configuring BGP Routing Policy
87