Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Prefix list + Route map + BGP

Hi All,

I need understanding the lines below. For some reason the "ge" option in the prefix list is throwing me off. So far, I understand it as BGP will NOT advertise X.X.160.0 out. Is that correct?

PREFIX LIST

ip prefix-list OUT seq 5 permit X.X.160.0/20 ge 21

ROUTE MAP

route-map OUT deny 20

match ip address prefix-list OUT

!

route-map OUT permit 100

match as-path 2

BGP

Router bpp xxxx

neighbor x.x.x.x route-map OUT out

4 REPLIES
New Member

Re: Prefix list + Route map + BGP

Yes, that is correct. Basically GE means greater than, so GE 21 means anything that is greater than /21 will be denied such as /22, /23, etc. So this prefix-list blocks more specific routes to x.x.160.0/20

New Member

Re: Prefix list + Route map + BGP

Thanks for the reply. Ok I think I understand.. but a few more questions to help me understand better. Does this mean 1.1.168.0 - 1.1.167.254 will be advertised? See output below. (BGP line added)

BGP

router bgp xxxx

network 1.1.160.0 mask 255.255.240.0

PREFIX INFO

whatmask 1.1.160.0/20

------------------------------------------------

TCP/IP NETWORK INFORMATION

------------------------------------------------

IP Entered = ..................: 1.1.160.0

CIDR = ........................: /20

Netmask = .....................: 255.255.240.0

Netmask (hex) = ...............: 0xfffff000

Wildcard Bits = ...............: 0.0.15.255

------------------------------------------------

Network Address = .............: 1.1.160.0

Broadcast Address = ...........: 1.1.175.255

Usable IP Addresses = .........: 4094

First Usable IP Address = .....: 1.1.160.1

Last Usable IP Address = ......: 1.1.175.254

whatmask 1.1.160.0/21

------------------------------------------------

TCP/IP NETWORK INFORMATION

------------------------------------------------

IP Entered = ..................: 1.1.160.0

CIDR = ........................: /21

Netmask = .....................: 255.255.248.0

Netmask (hex) = ...............: 0xfffff800

Wildcard Bits = ...............: 0.0.7.255

------------------------------------------------

Network Address = .............: 1.1.160.0

Broadcast Address = ...........: 1.1.167.255

Usable IP Addresses = .........: 2046

First Usable IP Address = .....: 1.1.160.1

Last Usable IP Address = ......: 1.1.167.254

Hall of Fame Super Silver

Re: Prefix list + Route map + BGP

Hello Merrylle,

prefix 1.1.160.0/21 will match the prefix-list.

May you post also the as-path access-list 2, what is permitted by this as-path filter ?

The only warning is that if prefix 1.1.160.0/21 matches as-path access-list 2 it could be advertised by the second route-map block.

All prefixes that don't match the second block will be denied (implicit deny at the end of the route-map, in some applications a last block with only permit is needed to avoid this)

Hope to help

Giuseppe

Hall of Fame Super Silver

Re: Prefix list + Route map + BGP

Hello Merrylle,

the prefix-list has the following meaning:

all IP prefixes that are contained in x.x.160.0/20 and have a prefix len Greater Equal of 21 will be permitted

Then the prefix-list is used under first route-map block with deny so those IP prefixes permitted by the prefix-list will be actually filtered

All prefixes matching the as path access-list 2 and not matching the prefix-list OUT will be advertised to the neighbor x.x.x.x

Hope to help

Giuseppe

189
Views
7
Helpful
4
Replies
CreatePlease login to create content