Prefix-list

Q1: What is prefix-list?

A1: The prefix-list is a group of EXTERNAL (ebgp) ROUTES.

Q2: Why the prefix-list filtering is important?

A2: Without prefix-list filtering, the router will learn ALL routes from the bgp peers.

The router can become a transit router, which cause unnecessary traffic flow and poor performance.
The worst is the router may stop working because of overload.

Q3: How to use prefix-list?

A3: below is the example;

# ip prefix-list MYLIST seq 7 permit 172.16.0.0/16 ge 16 ( ge = greater than or equal to)

# ip prefix-list MYLIST seq 8 permit 172.16.0.0/16 le 24  ( le  = less than or equal to)

#

# route-map EBGP-FILTER permit 10

# match ip address prefix-list MYLIST

# 

# router bgp 100

# neighbor 1.1.1.1 remote-as 200

# neighbor 1.1.1.1 route-map EBGP-FILTER in

# neighbor 1.1.1.1 soft-reconfiguration inbound (request neighbor to resend all routes)