Cisco Offset-List Command

(Rene Molenaar) #1

This topic is to discuss the following lesson:

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(Don S) #2

Hey rene.
Could you explain, why would you use offset-list in a working network?

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(Rene Molenaar) #3

Hi Don,

The main reason to use it is for path control of distance vector routing protocols (RIP or EIGRP). If you have two links between routers then you could use an offset-list to increase the metric of 1 prefix, making it prefer the other link.

For outbound traffic I wouldn’t use the offset-list, you can use policy based routing for this (it has more options). An offset-list might be useful for inbound traffic perhaps…for example, let’s say that you have a router with two links connected to another router that is out of your control. You could use an offset-list to increase the metric for 1 prefix so that the router on the other side prefers the other link for this particular prefix. This allows you to have “path control” even without access to the other router.

Rene

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(Jason W) #4

You applied off-set list to 1.1.1.1 and 11.11.11.11. The result – R2 show ip route rip – 1.1.1.1 has a new hop count of 6 and 11.11.11.11 has new hop count of 11. What if we had a R3 connected to R2… would it show 1.1.1.1 [120/7] and 11.11.11.11 [120/12]. Does R3 take into account the off-set list between R1 and R-2? Does that off-set promulgate throughout the network?

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(Rene Molenaar) #5

Hi Jason,

That’s correct. R1 is advertising these networks with a certain hop count to R2 so that’s what R2 is installing in its routing table.

When it’s advertising these prefixes to R3, it will advertise the metric that it has stored for these networks.

Rene

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(Abdool Y) #6

How would the offset work for eigrp since it doesnt use hopcount.

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(Andrew P) #7

Abdool,
Good question! If you lab up offset lists with EIGRP you will discover that whatever value you enter as the offset will be added to the composite metric of EIGRP. As you may know, the composite metric of EIGRP is the result of a complex formula, but by default, it is a result of Bandwidth and Delay values. Fortunately, Cisco is smart enough to know that it is best not to mess with bandwidth values for the purpose of EIGRP traffic engineering (because this can mess up things like QoS).

So what you will see is that the Delay used to calculate the Composite Metric will be manipulated such that the final Composite Metric will increase to include your offset list value.

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(Sergei K) #8

Hi, lads!
The offset list works only with standard ACL, not extended? With reference to EIGRP?
Thanx!

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(Lazaros Agapides) #9

Hello Sergei

The offset does not work with extended access lists. This is because the offset list modifies or increases the metric of a network prefix. When used, the access list determines the network prefixes that should be modified. Only the network prefix is necessary and thus only a standard access list would be used. A source and destination IP address would be meaningless in such a situation, so extended access lists are not used.

I hope this has been helpful!

Laz

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