Ccna Ccnp Training Offset Lists

As a CCNA / CCNP candidate you are expected to understand Offset lists.

During your career as a Cisco network engineer you will have to deal understanding offset lists on which you use to manipulate the EIGRP metrics. Offset lists are a great tool for the network engineer to use to simply add a offset metric to for any given prefix inbound or out-bound. To make use of an offset list all that a network engineer has to do it create and enable an EIGRP offset list which will specify the value to add to the EIGRP metric, there are a couple of rules that you need to observe when matching prefixes that need to be matched.

To start your offset list configuration you need to determine the following:

1. Which prefixes are you going to match, offset lists refer to access control lists. The access control lists are used to match the prefixes.

2. On each router that you want to change the EIGRP metric on youll have to determine if the metric is to be changed inbound or outbound, this you can determine based on the direction that the route is being learnt. If the prefix is being advertised out then the offset list will need to be set in an out direction, if the prefix is being advertised into the router the offset list will need to be set to the in direction.

3. The offset list needs to be configured to specify the interface on which the prefix is being either sent or received.

4. Set the integer metric that you want added to the EIGRP FD and the AD for the prefix

If you wanted to add the integer value of 1000 to the prefix of 172.16.1.0/24 going out of a router on say serial 1/1 do the following:

1. Create the access control list

Access-list 1 permit 172.16.1.0 0.0.0.0

2. Go to the EIGRP routing process on the router and enter the following command, the first number you see 1 refers to the access-control list shown in step 1, the out specifies the direction. The 1000 determines the value to increase the EIGRP metric by and finally the serial 1/1 configures the offset list to apply the metric increase when this prefix is being advertised in the outbound direction.

Router eigrp 1

Offset-list 1 in 1000 serial1/1

You could use an extended access-control list, if you do use an extended ACL you can use the destination address field to match the prefix length, for example if we wanted to match 20.20.20.0/24 we could write the ACL as:

Access-list 100 permit ip host 20.20.20.0 host 255.255.255.0

The above extended ACL would match the prefix of 20.20.20.0/24 precisely.

by: Joe Spoto

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