Link to home
Start Free TrialLog in
Avatar of davorin
davorinFlag for Slovenia

asked on

Lost with Cisco access lists...


I'm checking the configuration of one Cisco 890 series router and I'm a little bit confused about how access lists are set to interfaces.

The router has configured 7 extended access lists, but in the configuration I see that only one access list is set on vlan1 interface. On no other interfaces is set any other access list.
Also if I execute "show ip interface | include line protocol|access list" the situation is no different.
On the VLAN1 I can see set same one inbound access list. No others have set access list.
FE0 and FE8 have line protocol up, all others are down.

The strange thing is that if I execute "sh access-list" command, I can see that on TWO (one set on VLAN1 and another one) access lists are counted matches and the numbers are changing - so they are active and in use...

I know, that my cisco knowledge is a little rusty, but that much...

I would be very happy if someone could help me with this enigma.

Thank you!
Avatar of Predrag Jovic
Predrag Jovic
Flag of Poland image

You can assign only two access lists per interface (one ACL in each direction).

ACL in Cisco world are used for many things...
One ACL is probably for NAT, other who knows... (route-maps , class-maps, SNMP access......)
Can you attach the config for us to have a look at, suitably sanitised to hide any personal information of course.
Avatar of davorin


Thanks for your answers.
Yes, I know, that I can assign max two access lists to each interface - inbound and outbound.
What is confusing me that I have configured multiple access lists, but only one is assigned to an interface. All others have no assignation, but at least another one is active - which I presume from changes in number of matched packets.

If access list is used for let say NAT, it is probably not assigned to an interface?
Or they always needs to be assigned to an interface?
If not, how do I know it is used for NAT or something else?

Right now I don't have access to the configuration, but in VLAN1 configuration section is only line something like: "access-group stop_in in"
And there are then just configurations of access-lists themself
access-list extended stop_in...
access-list extended inbound
access-list extended vpn_client_name

(Sorry for bad example)
If access list is used for let say NAT, it is probably not assigned to an interface?
No, it is not. ACl applies to NAT in statement like
ip nat inside source list 10 ....
Many access lists are never applied to interfaces...
route map PBR permit 10
match  ip  address 10
set next-hop

in both cases access-list 10 is used and it is not applied to any interface
If you post the config, that would help.

But I see this all the time.  People create a new ACL, apply it to an interface that had an old ACL.  But then they leave the old ACL's.

I've run across routers that have literally dozens of unused ACL's.  Some dating back 3 or 4 years.
Avatar of davorin


I will need to check the configuration again, but knowing that some access lists have very specific names like NOSPAM, VPN-CLIENT I'm pretty sure that are not mentioned anywhere except at acl configuration.
But there are some not so specific like NAT, INBOUND that I will need to double check. I think that at least acl named NAT is the one that is confirmed to be active. But at least they they are not named with numbers :)
Do you have any link where are listed most of the examples of how can be access lists used?

@Don: That was also my thought, but then I saw that another presumably unused acl is in fact active. That confused me. Is it possible that some acls do not count matched packets?

I will post my findings tomorrow. Thank you for your help!
Avatar of Predrag Jovic
Predrag Jovic
Flag of Poland image

Link to home
This solution is only available to members.
To access this solution, you must be a member of Experts Exchange.
Start Free Trial
Avatar of davorin


Here are results of the access list lookup:


Can you please confirm my thinking:
- acl MAIL is active and applied to VLAN1 int.
- acl EXTERNAL is inactive
- acl NAT is active and applies to NAT config on int Fe8
- acl NOMAIL is inactive
- acl OUTSIDE is inactive
- both acl VPN-CLIENT1 & 2 are active, applied to VPN dial-in, but apparently not in use lately - no matches displayed in "sh access-lists"

I hope I'm correct.
Yes, you are correct.

In backup we trust!
So, always create one before you make any changes.
Avatar of davorin


Thank you very much for your help!
You are welcome.
Avatar of davorin


Yes, it is always good to have backup...
Can I pigtail this thread with another related yes/no question:

If there is configured another access list with name "inside" it probably has nothing to do with "ip nat inside" commands?

interface Vlan1
 description inside
 ip address
 ip inspect fw out
 ip nat inside


ip nat inside source list nonat interface Dialer1 overload
ip nat inside source static tcp 443 interface Dialer1 443
ip nat inside source static tcp 1723 interface Dialer1 1723
Access-list named inside would not have anything with # ip nat inside command.