From: David Wolsefer (dwolsefer@xxxxxxxx)
Date: Mon May 07 2001 - 20:34:29 GMT-3
Prefix lists are much simpler to understand and more straightforward than
access-lists for BGP. They are also much easier to change since there is a
sequence number, so you don't have to remove and re-enter the entire list
the way you do with access-lists. Here is an example. Suppose we wanted to
implement a prefix list to deny obviously bogus networks while only allowing
our own netblocks to be announced. We could do this as follows assuming our
AS is 111:
router bgp 111
! Our eBGP neighbor is 10.10.10.1 with an ASN of 222.
neighbor 10.10.10.1 remote-as 222
neighbor 10.10.10.1 description eBGP with ISP222
neighbor 10.10.10.1 prefix-list bogons in
neighbor 10.10.10.1 prefix-list announce out
!
! The bogons prefix list prevents the acceptance of obviously bogus
! routing updates. This can be modified to fit local requirements.
ip prefix-list bogons description Bogon networks we won't accept.
ip prefix-list bogons seq 5 deny 0.0.0.0/8 le 32
ip prefix-list bogons seq 10 deny 10.0.0.0/8 le 32
ip prefix-list bogons seq 15 deny 127.0.0.0/8 le 32
ip prefix-list bogons seq 20 deny 172.16.0.0/12 le 32
ip prefix-list bogons seq 25 deny 169.254.0.0/16 le 32
ip prefix-list bogons seq 30 deny 192.168.0.0/16 le 32
ip prefix-list bogons seq 35 deny 192.0.2.0/24 le 32
ip prefix-list bogons seq 40 deny 224.0.0.0/3 le 32
ip prefix-list bogons seq 45 permit 0.0.0.0/0 le 32
!
! The announce prefix list prevents us from announcing anything beyond
! our aggregated netblock(s). Note: Our netblock is 1.88.0.0 netmask
! 255.255.224.0
ip prefix-list announce description Our allowed routing announcements
ip prefix-list announce seq 5 permit 1.88.0.0/19
ip prefix-list announce seq 10 deny 0.0.0.0/0 le 32
I hope this helps clarify prefix-lists.
Regards,
David Wolsefer, CCIE #5858
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
garry baker
Sent: Monday, May 07, 2001 3:26 PM
To: ccielab@groupstudy.com
Subject: prefix lists
Guys,
I am having problems finding some info on and
understanding prefix lists. Can anyone give a plain
english explanation of prefix lists.
Garry
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 10:30:36 GMT-3