Does Access-List block Routing updates

Answered Question
Nov 12th, 2008

hi,


If I were to put an access-list deny ip any any, would it block any routing updates as well via the dynamic routing protocols.


Correct Answer by Istvan_Rabai about 8 years 3 months ago

For OSPF:

access-list 100 permit ospf any any


For EIGRP:

access-list 100 permit eigrp any any


For BGP:

access-list 100 permit tcp any any eq 179

or

access-list 100 permit tcp any eq 179 any

depending on the direction you apply the access-list.


Cheers:

Istvan



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Istvan_Rabai Wed, 11/12/2008 - 22:37

Yes, definitely.


You should permit the proper routing protocol packets in the access-list if you want them to pass the access-list.

The acl statement permitting the routing protocol should be before the "deny ip any any" statement, of course.


Cheers:

Istvan

new_networker Wed, 11/12/2008 - 23:19


So if I were to permit routing updates via BGP, OSPF and RIP what would the access-list be like.


Thanks.

Correct Answer
Istvan_Rabai Thu, 11/13/2008 - 08:07

For OSPF:

access-list 100 permit ospf any any


For EIGRP:

access-list 100 permit eigrp any any


For BGP:

access-list 100 permit tcp any any eq 179

or

access-list 100 permit tcp any eq 179 any

depending on the direction you apply the access-list.


Cheers:

Istvan



Richard Burts Thu, 11/13/2008 - 10:25

If the access list is applied inbound then Istvan is correct that an access list would deny the routing protocol packets unless there is a permit statement for them. However if the access list is applied outbound then the access list will not block the routing protocol packets (no matter whether there is a permit statement for them or not).


This is one of the odd things about access lists. An outbound access list does not examine (and will not block) any traffic that is generated by the router itself.


HTH


Rick

Istvan_Rabai Thu, 11/13/2008 - 11:08

Hi Rick,


Thank you for your addition.

I really forgot to mention this.


Cheers:

Istvan

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