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ACL question

Posted on 2003-10-28
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Last Modified: 2011-08-18
Hi guys,

Whats the differnce between ip access-group 101 in and ip access-group 101 out?

Cheers,
Blue Print
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Question by:blueprint123
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td_miles earned 50 total points
ID: 9638766
"in" applies the access list to traffic "entering" the interface from an external source
"out" applies the access list to traffic leaving the interface from your router

as an example, if you have the access list:

access-list 101 permit ip w.x.y.z any
access-list 101 deny ip any any

==========
you could then apply this:
interface ethernet0
  ip access-group 101 in

which would permit any traffic from the device w.x.y.z that is connected to the ethernet0 interface (eg. via a switch) to enter the ethernet0 interface of the router. All other traffic would be denied.
==========
if you applied it:
interface ethernet0
  ip access-group 101 out

it would allow any traffic that was from the device w.x.y.z to LEAVE the ethernet0 interface. This traffic would have to be either generated locally by the router, or have entered the router from another interface and have been routed to ethernet0.
==========

In summary, think of the "in" and "out" in relation to the viewpoint of that router interface and an external device. If the external device is sending traffic to the router, then the traffic is "inbound". If the router is sending traffic to the other device, then the traffic is "outbound".

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Expert Comment

by:NicBrey
ID: 9639932
Just to add to td_miles comment:
Outgoing  ACL's  do not apply to traffic generated by the router itself. So, if you deny outgoing ICMP for example, you would still be able tto ping from the router console.
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