After I applied the ACL to each vlan that I wanted to block from accessing Vlan 20, communication was blocked. I appreciate everyone's help with this.
Original Message:
Sent: Jul 22, 2025
From: parnassus
Subject: RE: Acl across switches
Hi!
Your question:
"Should I create an ACL on the other vlans with a line to block them from vlan 20?"
Should became:
"Should I create an ACL on the other vlans with a line to block them to reach the vlan 20?"
Answer: Yes, you should.
Example scenario on your Core Switch (doing IP Routing for its VLANs VLAN 10 and VLAN 20):
- VLAN 10 (Net Address: 10.0.10.0 /24 - SVI IP Address: 10.0.10.254)
- VLAN 20 (Net Address: 10.0.20.0 /24 - SVI IP Address: 10.0.20.254)
Create:
- ACL name to protect VLAN 10: ACL-VLAN-10
- ACL name to protect VLAN 20: ACL-VLAN-20
Direction to apply ACLs on each VLAN: Inbound.
== ACL-VLAN-10 (made to block egressing traffic from VLAN 10 to VLAN 20, permitting all the rest to any other possible destination routed by Core) ==
config
ip access-list extended ACL-VLAN-10
10 remark "Deny VLAN 10 net to VLAN 20 net - IP traffic"
10 deny ip 10.0.10.0 0.0.0.255 10.0.20.0 0.0.0.255 <-- This ACE will DENY any IP traffic egressing the VLAN 10 with 10.0.20.0 as destination network, mind you: ICMP is still permitted.
999 remark "Final explicit Permit of unmatched traffic before final implicit Deny ACE"
999 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
write memory
exit
== ACL-VLAN-20 (made to block egressing traffic from VLAN 20 to VLAN 10, permitting all the rest to any other possible destination routed by Core) ==
config
ip access-list extended ACL-VLAN-20
10 remark "Deny VLAN 20 net to VLAN 10 net - IP traffic"
10 deny ip 10.0.20.0 0.0.0.255 10.0.10.0 0.0.0.255 <-- This ACE will DENY any IP traffic egressing the VLAN 20 with 10.0.10.0 as destination network, mind you: ICMP is still permitted.
999 remark "Final explicit Permit of unmatched traffic before final implicit Deny ACE"
999 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
write memory
exit
Now...if your Core is the router of many other VLANs (say you have VLAN 10, 20, 30, 40 and so on...)...you should replicate the ACL-VLAN-10 structure to cope with that, at the point that you will have various other ACLs with a similar setup (avoid their egressing traffic to the VLAN 20). The ACL protecting the VLAN 20 needs other ACEs in order to deny its egressing traffic to those additional VLANs, example:
== ACL-VLAN-20 (made to block egressing traffic from VLAN 20 to VLAN 10 and VLAN 30, permitting all the rest to any other possible destination routed by Core) ==
config
ip access-list extended ACL-VLAN-20
10 remark "Deny VLAN 20 net to VLAN 10 net - IP traffic"
10 deny ip 10.0.20.0 0.0.0.255 10.0.10.0 0.0.0.255 <-- This ACE will DENY any IP traffic egressing the VLAN 20 with 10.0.10.0 as destination network, mind you: ICMP is still permitted.
20 remark "Deny VLAN 20 net to VLAN 30 net - IP traffic"
20 deny ip 10.0.20.0 0.0.0.255 10.0.30.0 0.0.0.255 <-- This ACE will DENY any IP traffic egressing the VLAN 20 with 10.0.30.0 as destination network, mind you: ICMP is still permitted.
999 remark "Final explicit Permit of unmatched traffic before final implicit Deny ACE"
999 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
write memory
exit
== ACL-VLAN-30 (made to block egressing traffic from VLAN 30 to VLAN 20, permitting all the rest to any other possible destination routed by Core) ==
config
ip access-list extended ACL-VLAN-30
10 remark "Deny VLAN 30 net to VLAN 20 net - IP traffic"
10 deny ip 10.0.30.0 0.0.0.255 10.0.20.0 0.0.0.255 <-- This ACE will DENY any IP traffic egressing the VLAN 30 with 10.0.20.0 as destination network, mind you: ICMP is still permitted.
999 remark "Final explicit Permit of unmatched traffic before final implicit Deny ACE"
999 permit ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255
write memory
exit
Original Message:
Sent: Jul 21, 2025 01:11 PM
From: mg14
Subject: Acl across switches
I updated the acl and added the ip access-group line to the vlan on the core switch. If I try to connect to a web server on a device on that vlan I can still connect. Do you have to get granular with the protocols, etc.. in the ACL? I appreciate your help.
aclvlan20
10 deny ip any 10.0.20.0 0.0.0.255
20 permit ip any any
vlan 20
name "test vlan"
tagged Trk1
untagged 1/5,1/6
ip helper-address 192.168.1.254
ip address 10.0.20.254 255.255.255.0
ip access-group "aclvlan20" in
ip igmp
exit
Original Message:
Sent: Jul 21, 2025 12:07 AM
From: mg14
Subject: Acl across switches
I am trying to create an ACL that blocks any traffic to any devices in vlan 20, but lets vlan 20 out to any devices. The devices are on a switch switch in another closet connected to the core over a fiber trunk connection. Do I need to create the ACL on the core switches and also on the other switches? I have tried going into the vlan 20 and applying it in, but nothing was blocked. Appreciate any help.
Below is my ACL info
ip access-list extended block20
10 deny ip any 10.0.20.0 0.0.0.255
20 permit ip 10.0.20.0 0.0.0.255 any
exit
Below is vlan 20 config on my 2930m Core switch stack. This is the vlan I am working on.
vlan 20
name "test vlan"
tagged Trk1
untagged 1/5,1/6
ip helper-address 192.168.1.254
ip address 10.0.20.254 255.255.255.0
ip igmp
exit
Remote 2930m switch stack config.for vlan 20 connected over fiber trunk to core switch stack.
vlan 20
name "test vlan"
tagged Trk1
untagged 1/48,3/37
no ip address
ip igmp
no ip igmp querier
exit