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=62253&t=62247 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
