eBGP policy and ACL in interface

Posted on 2016-11-18
Last Modified: 2016-11-21
Hi Anyone can answer the question? Two routers (RouterA and B) have eBGP connection. RouterA sends the routes and to routerB. The routerB’ interface allow in and routerB’ eBGP allow in. My question is which route can come into the routerB? Thank you
Question by:eemoon
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LVL 29

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 500 total points
ID: 41894114
Since ACLs in your case are assigned to interface and traffic from other router is permitted - All routes that are advertised from other eBGP peer will be accepted. Filtering routes is performed by distribution lists in BGP configuration. Distribution lists filter routes according to ACL list and direction in which filtering is applied.

Author Comment

ID: 41894959
Hi Predrag Jovic, Thank you so much for your fast reply.
If the ACL is used in interface or in route-map(and then used in bgp), i think the former is only filter traffic/data and do not filter routes. The later can directly filter routes. Do you think i am right?
LVL 29

Accepted Solution

Predrag Jovic earned 500 total points
ID: 41895019
Not sure that I can understand properly last post, so here comes example. :)
This one will filter traffic, but not routes:
access-list 100 permit ip x.x.x.x y.y.y.y
interface gi0/1
 ip access-group 100 in

Open in new window

This one will filter routes (there are also other ways to achieve the same (link below)):
router bgp 1567
 neighbor x.x.x.x remote-as 2065
 neighbor x.x.x.x distribute-list 101 in
access-list 101 permit ip x.x.x.0

Open in new window

This access-list permits routes that are subnets of x.x.x.0/24. (permits x.x.x.0/24, x.x.x.0/25, x.x.x.128/25, x.x.x.0/26, x.x.x.64/26  ..... etc...)

For more details please read Cisco article - Block One or More Networks From a BGP Peer
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Author Comment

ID: 41895153
We agree on the issue. so I can summarize it like this: All ACL only impact on traffic directly and do not impact routes if the ACL is associated with interface. And all the ACL only impact on routes and do not impact traffic directly if the ACL is associated with routing protocol and not associated with interface, right?
LVL 29

Assisted Solution

by:Predrag Jovic
Predrag Jovic earned 500 total points
ID: 41895355
Sounds good. The way it is written I would remove word "all" from both sentences, but than again, I am not native speaker.

Author Closing Comment

ID: 41895936
Thank you!
LVL 29

Expert Comment

by:Predrag Jovic
ID: 41895953
You're welcome.

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