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ACLs come in two varieties: numbered and named and standard and extended Numbered and named ACLs define how the router will reference the ACL You can think of this as something similar to an index value A numbered ACL is assigned a unique number among all ACLs, whereas a named ACL is assigned a unique name among all named ACLs These are then used by the router to filter traffic Each of these references to ACLs supports two types of filtering: standard and extended Standard IP ACLs can filter only on the source IP address inside a packet, whereas an extended IP ACLs can filter on the source and destination IP addresses in the packet, the IP protocol (TCP, UDP, ICMP, and so on), and protocol information (such as the TCP or UDP source and destination port numbers or ICMP message types)
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TABLE 22-1
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Source address Destination address IP protocol (ie, TCP or UDP) Protocol information (ie, port number)
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Standard and Extended ACL Comparison
Remember the ltering abilities of standard versus extended ACLs in Table 22-1
With an extended ACL, you can be very precise in your filtering For example, you can filter a specific telnet session from one of your user s PCs to a remote telnet server Standard ACLs do not support this form of granularity With a standard ACL, you can either permit or deny all traffic from a specific source device Table 22-1 compares the two types of filtering for IP traffic
Processing
ACLs are basically statements that are grouped together by either a name or number Within this group of statements, when a packet is processed by an ACL, the IOS will go through certain steps in finding a match against the ACL statements ACLs are processed top-down by the IOS Using a top-down approach, a packet is compared to the first statement in the ACL, and if the IOS finds a match between the packet and the statement, the IOS will execute one of two actions included with the statement: permit or deny If the IOS doesn t find a match of packet contents to the first ACL statement, the IOS will proceed to the next statement in the list, again going through the same matching process If the second statement matches the packet contents, the IOS executes one of the two actions If there is no match on this statement, the IOS will keep on going through the list until it finds a match If the IOS goes through the entire list and doesn t find a match in the ACL statements to the ACL contents, the router will drop the packet The top-down processing of ACLs brings out the following very important points:
Once a match is found, no further statements are processed in the list The order of statements is important, since after the first match, the rest of
the statements are not processed
If no match is found in the list, the packet is dropped
ACL Overview
Statement Ordering
If a match is found on a statement, no further statements are processed Therefore, the order of the statements is very important in an ACL If you have two statements, one denying a host and one permitting the same host, whichever one appears first in the list will be executed and the second one will be ignored Because order of statements is important, you should always place the most specific ACL statements at the top of the list and the least specific at the bottom of the list Let s take a look at an example to illustrate this process In this example, you have an ACL on your router with two statements in this order: 1 Permit traffic from subnet 1721600/16 2 Deny traffic from host 1721611/32 Remember that the router processes these statements top-down Let s assume that a packet is received on the router with a source IP address of 1721611 Given the preceding ACL, the router compares the packet contents with the first statement Does the packet have a source address from network 1721600/16 Yes Therefore, the result indicates that the router should permit the packet Notice that the second statement is never processed once the router finds a match on a statement In this example, any traffic from the 1721600/16 subnet is permitted, even traffic from 1721611 Let s reverse the order of the two statements in the ACL and see how this reordered ACL will affect traffic flow: 1 Deny traffic from host 1721611 2 Permit traffic from subnet 1721600/16 If 1721611 sends traffic through the router, the IOS first compares these packets with the first ACL statement Since the source address matches 1721611, the router drops the packet and stops processing statements in the ACL In this example, it doesn t matter what traffic 1721611 is sending, because it s dropped If another device, say 1721612, sends traffic through the router, the router compares the packet contents to the first ACL statement Since the source address in the packet doesn t match the source address in the ACL statement, the router proceeds to the next statement in the list Comparing the packet contents to the statement, there is a match Therefore, the router will execute the results, permitting the traffic from 1721612 As you can see from both of these ACL examples, the order of statements in the ACL is very important and definitely impacts what traffic is permitted or denied
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