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Community Member

ACL processing and Cisco CEF

Hello,

I have a need to block access to a few specific devices on a vlan within my Data Center ServerFarm. I am going to create an access-list and apply it to the gateway interface for that vlan or vlan interface, which resides on the ServerFarm Distribution switches. Note: the ServerFarm Distribution switches consist of (2) 6509 switches.

The question that I have is this. Do I need to turn off CEF and fast switches for this vlan interface so the all packets are forced to the Processor? If not, traffic will be express forwarded by a table instead of proccessed.

Does this sound right?

Thanks in advance for your help,

Tony

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: ACL processing and Cisco CEF

Tony

"So if I understand this correctly, the ip unreachables is strictly to permit ICMP unreachable traffic to be returned. Is that right?"

Yes and if you disable "ip unreachables" on the vlan interface then all your denies will be preformed in hardware.

Jon

6 REPLIES
Hall of Fame Super Blue

Re: ACL processing and Cisco CEF

Tony

No you do not need to disable CEF. ACL processing on the 6500 is done in hardware and not by the route processor although there are some exceptions to this -

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/acl.html#wp1033602

Jon

Community Member

Re: ACL processing and Cisco CEF

Jon,

Thank you for your answer. Is this the same for CAT OS in Hybrid mode?

I am running L2 in CAT OS and LS MSFC with IOS? (SUP720-3BXL with PFC3BXL).

Thanks again,

Tony

Hall of Fame Super Blue

Re: ACL processing and Cisco CEF

Tony

Yes it should be the same for Hybrid mode. It is the PFC that does the acl processing in hardware and you have that regardless of native/hybrid. Managed to find a link white paper on ACL processing in 6500 -

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a00800c9470.shtml#wp42096

Jon

Community Member

Re: ACL processing and Cisco CEF

Jon,

Wow...This is great info. Thanks for digging it out for me!

Also, this is much more complicated than I originally thought.

To break it down in simplistic terms though, here is the way I think it will be working for me: (Please correct if needed) (since I am not NATing or Policy Based Routing or using any QOS on this vlan interface)

Per the White Paper:

All traffic will be hardware switched (or fast switched) except ACL denied traffic.

-Supervisor 720 with PFC3-ACL denied packets are leaked to the MSFC3 if unreachables are enabled. Packets requiring ICMP unreachables are leaked at a user-configurable rate (500 pps by default)

So if I understand this correctly, the ip unreachables is strictly to permit ICMP unreachable traffic to be returned. Is that right?

With an ACL applied to the interface such as this:

access-list 100 permit ip host 10.10.10.1 host 10.1.1.1

access-list 100 permit ip host 10.10.10.1 host 10.1.1.2

access-list 100 deny ip any host 10.1.1.1

access-list 100 deny ip any host 10.1.1.2

access-list 100 permit ip any any

In this case all traffic is hardware switched. Any traffic that is denied will get an ICMP unreachable returned, which is processed by software on the MSFC. Also, this could be turned off by configuring no ip unreachables.

Thanks again for your assistance.

Tony

Hall of Fame Super Blue

Re: ACL processing and Cisco CEF

Tony

"So if I understand this correctly, the ip unreachables is strictly to permit ICMP unreachable traffic to be returned. Is that right?"

Yes and if you disable "ip unreachables" on the vlan interface then all your denies will be preformed in hardware.

Jon

Community Member

Re: ACL processing and Cisco CEF

Jon,

Thank you again for your help! I greatly apprecaite it.

Tony

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