ip prefix-lists

[Sharin Taib]

Hi, I'm trying to manipulate traffic for my WAN. Basically, I'm trying to stop traffic for a particular /24 block from coming in via Link 1 and instead come in via Link 2 and 3.

after adding "ip prefix-list asprepend seq 25 permit 1.1.169.0/24" into our router facing Link 1's router, the traceroute is still showing that traffic is still present. Is there anything that I have missed out on this?

[johnlloyd_13]

Hi,

Perform a 'clear bgp soft reset' to take effect and then try again.

Sent from Cisco Technical Support iPhone App

[Sharin Taib]

Thanks. did that as well after I did the prefix list. No dice.

[milan.kulik]

Hi,

a) Are you sure your router is using the prefix 1.1.169.0/24 to forward the traffic?

There might be a prefix with a different subnet mask used, e.g.

You can try to issue

show ip route 1.1.169.1

e.g., and the output should show you which prefix is being used.

b) Are you sure you are applying the prefix-list (or route-map?) on the right router and right direction?

As you say you are "...trying to stop traffic for a particular /24 block from coming in via Link 1 and instead come in via Link 2 and 3."

Don't forget blocking the prefix 1.1.169.0/24 from receiving on the interface would make the router not to forward traffic targeted  to that prefix through out that interface, and not to receive the traffic sent from that source on the interface!

HTH,

Milan

[Sharin Taib]

Hi,

Router#sh ip route1.1.169.0

Routing entry fo 1.1.169.0/24

  Known via "bgp 1", distance 200, metric 0, type internal

  Last update from 10.10.10.1 1w6d ago

  Routing Descriptor Blocks:

  * 10.10.10.1, from 192.168.168.1, 1w6d ago

      Route metric is 0, traffic share count is 1

No mention of the prefix-list.

for your 2nd part, I believe that's why I was trying to achieve

[Reza Sharifi]

Hi,

Can you post the BGP as path preped config, route map and access list?

[Sharin Taib]

Hi,

route-map addasnumber permit 10

match ip address prefix-list asprepend

set as-path prepend 1234 1234 1234 1234

access-list 10 permit 100.100.100.48

access-list 10 permit 100.100.100.48    

ip prefix-list asprepend seq 15 permit 9.9.9.0/23

ip prefix-list asprepend seq 20 permit 50.50.50.0/24

ip prefix-list asprepend seq 25 permit 1.1.169.0/24

[cadet alain]

Hi,

AS-PATH prepending is used to influence how traffic will get routed to us by another AS  and so the route-map must be applied outbound as it influences inbound traffic.But the AS-PATH is not the first attribute that is looked upon by the BGP algorithm when deciding which is the best path so if the other AS is preferring this path because of local preference your prepending won't change anything.

Is it an experiment or a real life situation? if this is real implementation I would ask your ISP if we can set a community when sending the prefixes so on their side they set the local preference for this community and so influence the inbound path on your side.

Regards.

Alain

[Sharin Taib]

Hmm ok. we are testing an unused IP block to see if we are able to manipulate the traffic.

We have a pretty big sized block that we have to get them to assist with, as we are testing our other ISP's.

is there any way we can manipulate it on our side?

[cadet alain]

Hi,

did you ask them how they were coping with AS-PATH prepend and if they had special communities to influence local preference ?

Are they seeing this prefix with the prepending ?

Regards.

Alain.