Describe the types, features, and applications of ACLs

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

In this section we will discuss the Cisco implementation of IP access control lists (ACLs). The ACLS is used to filter the network traffic. The ACLs can also be used for define traffic to the NAT (network address translate). It can also be used to encrypt and filter non IP- protocols. The non IP- protocols include Apple talk.

The IP protocols can be mainly of the following types:

Internet Control Message Protocol (ICMP)
Open Shortest Path First (OSPF)
Internet Protocol (IP)
User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)

Some common concepts that are used in ACLs are:

Masks – These are used in IP ACLs to clearly mention what should be allowed and what should not be. The masks usually start with 255 and on the left they have a huge number.
ACL summarisation – this is a concept that is used to sum up a large range of network into one particular network.

One can always define the ACLs and keep. The ACLs do not how any affect unless they are applied on an interface or a router. It is suggested that the ACL should be applied at the interface that is situated nearest to the source of the traffic. This will help you to block and allow the traffic as and when you please. For the UDP traffic to pass the ACL must be given exclusive permission.

You can also edit the ACLs as you want and even delete the same if the requirement arises. As soon as you put a no in front of the access group command the ACL will be removed from the interface.

We will now discuss each type of the ACL in details so that you understand the concept better:

Standard (editing and sequence numbers)

The first type of the ACL is the standard ACL. It is also the oldest form of ACL. The standard ACL can control the traffic by comparing the source address of the IP packet with the address that is already configured in the ACL. The command for standard ACL is

access-list access-list-number {permit|deny}

{host|source source-wildcard|any}

The access list number in this case can vary from 1 to 99. Now even list name can be used in standard ACL. After the ACL is defined the direction must be specified too.

Extended

The next type of ACL that we will discuss is the extended ACL. In this case the traffic is controlled by destination and source address of the IP packets with the address that is already configured in the ACL. The access list number can be in this case anything from 100 to 199. Now even list names can be used in the extended ACLs. The defined ACL must be applied clearly to the interface. The command used for this purpose is

interface <interface>

ip access-group {number|name} {in|out}

The extended ACL can ensure that unwanted pings from outside is prevented from entering the interface.

Named

The named ACL is also known the IP named ACLs. They are called named as they do not use numbers but names. They are alpha numeric in nature too. You can configure up to 100 named ACLs on a server. The command used for this named ACL is

ip access-list {extended|standard} name

The named ACLs can be used to block traffic other than the telnet connection that comes from host to host.

Some examples of named ACLs are:

permit host 5.6.7.8

permit any

deny host 1.2.3.4 and the list just goes on

Numbered

We will now try to discuss in details what a numbered ACL is. Any ACL that is represented by a number is called a numbered ACL. Both standard and extent ACLs are numbered ACL as they are numeric in nature. In standard ACL the numbers range from 1- 99 and in extended the number range from 100 to 199. When an ACL is numbered it can be edited easily. We have already mentioned before that an ACL can be edited as per the requirement. We will now explain how exactly the ACL can be edited.

The extended numbered ACL will deny or allow packets based on the following information:

Source of IP address
IP protocol
Destination of IP address
The source of the UDP or the TCP
Destination of the UDP or the TCP

One can also add ACL lines to the numbered or standard ACLs by using sequence numbers. The new entries made can be checked with the show list command. The new ACLs are supported with security appliances. Unless you remove the crypto map now you will not be able to make the changes in ACL. Not removing the crypto map can also leads to strange behaviour.

Log option

The log option is used in ACL quite often. The log option allows the SNMP trap and the Syslog messages for packets that are usually denied by the ACL. One can use the log option even when the ACL and filters are already in use. All that one needs to do is enter again the ACL or the filter command. After that you need to add the log parameter to the very end of the ACL or the filter. The software is designed in such a way that it will replace the ACL or the filter command with a new command. The new ACL with the log option will start to function with immediate effect.

Apart from these there are many more types of ACLs and they are lock and key or dynamic ACL, reflexive ACL, time based ACL, context based access control and commented IP ACL entries, the list just goes on. However, the ones that we have explained above are essential from the exam point of view. We hope that this chapter will help you to clearly understand and describe the types, features, and applications of ACLs. Just keep these terms and points in mind and you will be able to answer most of the questions in this particular section.

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