Discussion

aviva@RouterF# set term accept-null-path then accept
aviva@RouterF# set term else-reject then reject
This policy accepts all routes learned from BGP and that have no AS path, and
rejects all other routes. A policy like this is useful when the only routes you want to
advertise to a particular EBGP peer are those that originated in your AS. For
instance, if another AS is advertising routes to you and you don’t want to readvertise
them, you can apply this null AS path policy with a set export null-path command.
13.9 Restricting the Number of Routes Advertised
to a BGP Peer
Problem
You want to control the number of routes that your peers send you.
Solution
Set the maximum number of routes that you will accept from each of your peers:
[edit protocols bgp group session-to-AS65505 neighbor 10.0.31.1]
aviva@RouterF# set family inet unicast prefix-limit maximum 7500
aviva@RouterF# set family inet unicast prefix-limit teardown
Discussion
As an ISP, you keep track of how many routes each of your peers and customers normally
send you. This number generally increases slowly over time. To place a limit
on the number of routes a peer or customer can send you, set a maximum number of
routes to accept. This type of administrative policy guards against an inadvertent
policy misconfiguration, which, in the worst case, could result in a peer or customer
redistributing the full Internet routing table to you. You decide on the maximum
number of prefixes you accept based on the normal number of routes exchanged
with the peer, and, when the limit is reached, BGP tears down the session with the
peer. Typically, you take the current number of routes exchanged and add about 50
percent.
In this recipe, we know that neighbor 10.0.31.1 typically sends 5,000 prefixes, so we
set the limit to 7,500 prefixes. For example, if the peer tries to send the entire Internet
routing table (on the order of 170,000 prefixes), BGP on the local router will shut
down the peering session with the neighbor. This shutdown tells both you and the
peer that something has gone wrong at his end.
To verify the configuration, look at the BGP neighbor’s information:
aviva@RouterF> show bgp neighbor 10.0.31.1
Peer: 10.0.31.1+4051 AS 65505 Local: 10.0.31.2+179 AS 65500
Description: EBGP to Customer A
448 | Chapter 13: BGP
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.