From: kenza@kenza.nl
Date: Mon Apr 02 2007 - 05:16:49 ART
The way I understand them is as follows:
Distribute-lists allow you to filter certain routes from making into the RIB
of your router, or from propagating to other routers. The concept is pretty
straight forward, but basically it boils down to if you don't want Router B
to know how to reach Router C through Router A, you would set up a
Distribute-list to filter outgoing routing updates to Router B that contain
router C's subnet - forcing Router B to choose an alternate path to the
destination.
Prefix-lists allow you to specify what specific subnets you want your router
to learn about for placement into its RIB. For instance, if you only want
/25 subnets to make it into your RIB, you could setup a prefix-list that
will filter out all other routes except those that have a /25 mask on them.
This, in itself, wasn't that useful to me, but is more powerful when you
team it with the "gateway" option. In that case, you can tell your router
to only accept routing updates from specific gateways - forcing it to only
route traffic through the gateway you specify.
Offset-lists are just a way of manipulating route selection within the
router. In the case of RIP, you could setup an offset-list to automatically
add 5 to whatever metric is being used by the routing protocol (hop-count in
RIP's case). Therefore, any route that your router receives from another
RIP router with a hop count of 5, will be "offset" to a hop-count of 10. If
the router had another route to the destination with a hop-count of 7, it
would choose that route first, while recording the other route as an
alternate. You can also use offset-lists as a way of route poisoning. For
instance, if you don't want your router to use a route that a downstream
router has propagated to it through RIP, you could set an offset-list of 15
to any incoming RIP update from that router. This would make any incoming
RIP update from that router unusable as its hopcount would be 15 + whatever
the real hopcount is (15 hops is considered a dead route).
rgds,
Kenza
http://cisco.pagina.nl
----- Original Message -----
From: "Daniel Fredrick" <dfredrick@gmail.com>
To: "dennis lin" <74dennis@gmail.com>
Cc: <ccielab@groupstudy.com>
Sent: Monday, April 02, 2007 4:51 AM
Subject: Re: Confused about the BGP command: neighbor
prefix-list/distribute-list/filter-list
>I have a simple answer for you... I don't know how detailed of an answer
>you
> want... but pretty much they all do the same thing. One of them you use an
> access-list to filter, one a regexp list, and the other a prefix list.
>
> In the real world you would just use the one that fits with what you are
> already using or feel more comfortable using.
>
> In the "Real World" of the CCIE lab exam... you would just use the one
> that
> doesn't break some restrictions given by the exam.
>
> HTH,
>
> Dan
>
>
> On 4/1/07, dennis lin <74dennis@gmail.com> wrote:
>>
>> Guys
>>
>> I am quite confused about the following BGP commands, In what kind
>> of sceneriao I can use it? Can someone help me out? Thanks.
>>
>> neighbor distribute-list<
>> http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_bgh2.htm#wp1071658
>> >
>>
>> neighbor filter-list<
>> http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_bgh2.htm#wp1071967
>> >
>>
>> neighbor prefix-list<
>> http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_bgh2.htm#wp1072874
>> >
>>
>> Dennis
>>
>> _______________________________________________________________________
>> Subscription information may be found at:
>> http://www.groupstudy.com/list/CCIELab.html
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
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