Thanks, I understand it now. Eric
----- Original Message ----- From: "bergenpeak" To: Sent: Friday, January 31, 2003 4:24 PM Subject: Re: Why is distribute-list in not supported in OSPF? [7:62247] > I think the general ideas are as follows: > > - OSPF provides a mechanism to filter LSA (routing information). This > is done within the OSPF spec via area boundaries (ABRs and ASBRs). > > - the OSPF area construct works best when all devices in the area > have the same view of toplogy and cost within the area. When this is > not the case, issues may arise (see below) > > - With the right topological and OSPF area design, one can support > most requirements for the controlling of routing information. > > Filtering of LSAs within an area has the following issues: > > - first, since LSAs are flooded, filtering LSAs at one location > might have no impact as these filtered LSAs might reach all corners > of the area any way (based on other paths which aren't filtering). > > - if filtering is "successful" and devices within an area have different > views of the toplogy and costs, sub-optimal routing and/or routing loops > will likely be present. > > So, in general, controlling routing information really should be done > at the designated locations, namely where area's interconnect. That's > not to say that there might not be times when one does want to filter > LSAs within an area. Looks like the command "neighbor database-filter" > exists to do this. > > > > > > > > ericbrouwers wrote: > > > > Hello, > > > > The "distribute-list access-list in [interface]" command is not supported > in > > IS-IS and OSPF. Why??? > > > > I tried to find an explanation in Cisco's OSPF design guide: > > > > ....Filtering information with link-state protocols such as OSPF is a > tricky > > business. Distribute-list out works on the ASBR to filter redistributed > > routes > > into other protocols. Distribute-list in works on any router to prevent > > routes > > from being put in the routing table, but it does not prevent link-state > > packets from being propagated, downstream routers would still have the > > routes. > > It is better to avoid OSPF filtering as much as possible if filters can be > > applied on the other protocols to prevent loops.... > > > > Why can router not prevent link-state packets from being propagated??? > > > > Hope someone can explain this to me. Thanks, > > > > Eric Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=62714&t=62247 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
