Cisco ACLs

The source I used was the Sybex CCNA book, which did a very good job of explaining Access Lists. The O'Reilly Cisco IOS Access Lists book is an extensive reference (especially if you want to do more advanced stuff). This is some really great stuff once you get the hang of it...

Online, take a look here for some decent info and guides:

http://www.cisco.com/warp/public/105/acl_wp.html

http://www.networkcomputing.com/907/907ws1.html

http://www.oreilly.com/catalog/cisrtlist/chapter/ch05.html

Hope these help....good luck!

The other poster gave good sites. Here's just a few practical tips ...

1. Always create the access list first, then apply the list to the interface.

2. Access lists work from top down. If you have two different rules for the same type of packet, the first one in the list will be used, and the second one will never see the packet.

3. There is an implicit deny at the end of all access lists. This means that if you have not specified a particular type of packet, it will be dropped.This causes a lot of headaches for beginners. :)

4. Cisco has access lists and extended access lists. Access lists compare source addresses. Extended lists compare source and destination addresses.

Overall, ACLs are a very handy tool for filtering traffic. The best part is it cuts down the load on the router because the packet is compared first, before any route processing. You can drop a good bit of unnecessary traffic. Security is another big issue lately with ACLs. That is foranother post. :)

Steve Olson

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