offset list

[The_guroo_2]

guys we always use access-list for an offset list....now i have seen in eigrp that offset list 0 has been used but there is no access-list??? can someone please xplain why is that....

[guruprasadr]

HI, [Pls RATE if HELPS]

If the offset is 0, no action is taken.

Hope I am Informative. PLS RATE if HELPS

Best Regards,

Guru Prasad R

[Richard Burts]

Guru

I think that you misunderstand the question. Your answer would be appropriate if the offset list specified an offset of 0. But I believe that the original post is not asking about that. I believe that the question is about the offset list specifying the access list as 0 not the offset as 0.

In configuring an offset list, which is used to change the metric of some or all of the prefixes being advertised, there is an option to specify an access list. The purpose of using the access list is to allow the offset list to affect some prefixes (the ones that are permitted) but not others (the ones that are denied). If the offset list specifies 0 as the access list then there is no access list and the offset value is applied to all prefixes being advertised.

HTH

Rick

[Giuseppe Larosa]

Hello Rick,

nice info I didn't know this

Standard access list number or name to be applied. Access list number 0 indicates all access lists. If the offset value is 0, no action is taken.

see

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_eig2.html#wp1012264

Best Regards

Giuseppe

[The_guroo_2]

sorry guys didnt get no action is taken if no action is taken whats the point of putting offset-list 0??? i m confused

[Richard Burts]

Giuseppe

That is usually a good link but in this particular instance it is not so good. I would say that in this case the documentation is unclear and a bit misleading. It says:"Access list number 0 indicates all access lists. If the offset value is 0, no action is taken"

I would argue that it is more appropriate to say that 0 indicates no access list rather than all access lists. And the next sentence is copied from the definition of the offset value. It is inappropriate to have that sentence in this paragraph talking about the value of the access list. The first 5 times I read it I kept thinking that it was saying that if the access list number was 0 no action was taken - which is absolutely not true.

The second sentence is literally true (when talking about the value of the offset). But its placement into the paragraph about access list is misleading.

To clarify let me insert the output of online help:

R1(config-router)#offset-list ?

<0-99> Access list of networks to apply offset (0 selects all networks)

<1300-1999> Access list of networks to apply offset (expanded range)

WORD Access-list name

This makes it clear that using a value of 0 for access list will apply the offset to every network being advertised.

HTH

Rick

[Giuseppe Larosa]

Hello Rick,

I agree this is a case where documentation is unclear, however applying all ACLs defined in a router is something that doesn't make sense.

Your show is clear a 0 in access-list position means a permit any= all IP networks.

Hope to help

Giuseppe

[ullasupendran]

See the following example, the router applies an offset of 5 to the delay component of the router only to access list 15

offset-list 15 out 5

What Rick meant is that inyour case if the offset value is 0 then there wont be any addition of the metric value,no change on the existing metric value.

Ullas

[Richard Burts]

Ullas

There seems to be a lot of confusion about what happens when the offset value is 0 and what happens when the access list value is 0. You and Guru and the documentation found by Giuseppe talk about the value of offset being 0. And I agree for that situation there is no action (and I seriously question why anyone would code an offset list using a value of 0 for the offset). The original question as I understand it was about what happens when the access list is 0. And in that case the offset is applied to every network being advertised.

HTH

Rick

[David Salazar]

Hi, guroo.


I used this option on the configuration of a client and previously I test this feature on a lab.
To the access-list number you can use a access-list number = 0 this mean all-prefixes; other thing is the offset value that can be set to 0 and not action or change of metric is doing.


Is important recall the following:


On IOS 12.4 o previous:


router(config-router)# offset-list ? -> show the option <0-99> standard access-list


On IOS 15.0 and later:

router(config-router)# offset-list ? -> show the option <1-99> standard access-list the option of use 0 is not more possible.

http://www.cisco.com/en/US/docs/ios/iproute_eigrp/command/reference/ire_i1.html#wp1039675

offset-list (EIGRP)

To add an offset to incoming and outgoing metrics to routes learned via Enhanced Interior Gateway Routing Protocol (EIGRP), use the offset-list command in router configuration mode or address family topology configuration mode. To remove an offset list, use the no form of this command.

offset-list {access-list-number | access-list-name} {in | out} offset [interface-type  interface-number]

no offset-list {access-list-number | access-list-name} {in | out} offset [interface-type  interface-number]

Syntax Description

http://www.cisco.com/en/US/docs/ios/iproute_eigrp/command/reference/ire_i1.html#wp1039675

access-list-number | access-list-name	Standard access list number or name to be applied. Access list number 0  indicates all networks (networks, prefixes, or routes). If the offset value is 0, no action is taken.
in	Applies the access list to incoming metrics.
out	Applies the access list to outgoing metrics.
offset	Positive offset to be applied to metrics for networks matching the access list. If the offset is 0, no action is taken.
interface-type	(Optional) Interface type to which the offset list is applied.
interface-number	(Optional) Interface number to which the offset list is applied.