Re: OSPF filtering

David,

Thank you for your comprehensive explanation.

This was also my thought, that I filter the only one best route. I see that
this is actually what is happening
but i don't really understand why, because I expected the second best
route(the one through Area0) to be
placed in the routing table if I filter the best route.

Thanks,
Rares

On Fri, Dec 10, 2010 at 7:48 AM, David Mahler <dave.mahler_at_gmail.com> wrote:

> Hello Rares,
>
> I investigated this myself after seeing your post since I remember whizzing
> past this INE lab without thinking too much about it. So to summarize what I
> see is happening....
>
> I looked at what's happening to the global routing table via:
>
> access-list 3 permit 150.1.3.3
> debug ip ospf local 3 (installation of routes into OSPF RIB)
> debug ip ospf global 3 (installation of routes into the Global RIB as seen
> in 'show ip route')
>
> "Issue":
> R6 has 2 possible routes to 150.1.3.3. One via R4 or not directly through
> area 0, this is only allowed when "capability transit" is enabled. The
> other route is via R1 a backbone/area 0 router which is allowed weather or
> not "capability transit" is enabled/disabled.
>
> So with "capability transit" enabled:
>
> OSPF RIB sees both routes as below from debugging:
>
> Via R1 (area 0)
> *Dec 9 22:53:04.060: OSPF-RIB-LOCAL: Add 150.1.3.3/255.255.255.255, area
> 0, type Intra, dist 3783, forward 0, tag 0x0, via 155.1.146.1
> GigabitEthernet0/0.146, route flags (None), path flags (none), source
> 150.1.3.3, spf 34, list-type change_list
>
> Via R4 (non area 0, only allowed because of "capability transit")
> *Dec 9 22:53:04.064: OSPF-RIB-LOCAL: Add 150.1.3.3/255.255.255.255, area
> 0, type Intra, dist 1564, forward 0, tag 0x0, via 155.1.146.4
> GigabitEthernet0/0.146, route flags (None), path flags (none), source
> 150.1.3.3, spf 34, list-type transit_list
>
> The path Via R4 is preferred because its metric, noted in the debugging as
> "dist" is lower - 1564 and so it is chosen as the sole best path in the OSPF
> RIB
>
> Rack1R6#show ip ospf rib 150.1.3.3
>
> OSPF local RIB for Process 1
> Codes: * - Best, > - Installed in global RIB
>
> * 150.1.3.3/32, Intra, cost 1564, area 0
> SPF Instance 38, age 00:00:03
> Flags: HiPrio, Transit, OldTrans
> via 155.1.146.4, GigabitEthernet0/0.146, flags: none
> LSA: 1/150.1.3.3/150.1.3.3
>
>
> However - even though this is the best path, it can't be used because of
> your distribute list which filters this best route from getting into the
> Global RIB
>
> Seen in debugging "update failed":
> *Dec 9 22:53:09.064: OSPF-RIB-GLOBAL: Network update failed
> 150.1.3.3/255.255.255.255, via 155.1.146.4 on GigabitEthernet0/0.146,
> distance 1564, flags (None), source 150.1.3.3, tag 0x0, type Intra, return:
> 3.
>
>
> Now with "no capability transit":
>
> Now the OSPF RIB only gets one path Via R1, the path via R4 is not allowed
> because it is a non-backbone path and you just said thats not allowed with
> "no capability transit":
>
> via R1 - backbone ONLY
> *Dec 9 22:52:37.428: OSPF-RIB-LOCAL: Add 150.1.3.3/255.255.255.255, area
> 0, type Intra, dist 3783, forward 0, tag 0x0, via 155.1.146.1
> GigabitEthernet0/0.146, route flags (None), path flags (none), source
> 150.1.3.3, spf 33, list-type change_list
>
> R4 not allowed
>
> now the OSPF RIB has the path via R1 (despite its worse metric) and not R4
> (b/c it's not allowed now)
>
> Rack1R6#show ip ospf rib 150.1.3.3
>
> OSPF local RIB for Process 1
> Codes: * - Best, > - Installed in global RIB
>
> *> 150.1.3.3/32, Intra, cost 3783, area 0
> SPF Instance 36, age 00:00:34
> Flags: RIB, HiPrio, OldTrans
> via 155.1.146.1, GigabitEthernet0/0.146, flags: RIB
> LSA: 1/150.1.3.3/150.1.3.3
>
>
> Now we get the ">" as in installed in the global table:
>
> *Dec 9 22:52:37.428: OSPF-RIB-GLOBAL: Network update succeeded
> 150.1.3.3/255.255.255.255, via 155.1.146.1 on GigabitEthernet0/0.146,
> distance 3783, flags (None), source 150.1.3.3, tag 0x0, type Intra, return:
> 0
>
>
> And that's because this time your distribute list didn't block the route
> this time since it wasn't from R4.
>
>
> Phew - so after all that my conclusion is this (feel free to correct anyone
> with more skills than me :-) ). When you allow the non backbone path with
> "capability transit" (default) it is the best path in the OSPF RIB, but then
> you blocked that path from being installed in the Global RIB with your
> distribute list. When you don't allow the non backbone path with "no
> capability transit" the best path is now not impacted/blocked by your
> distribute list and is properly installed.
>
>
>
> Regards,
> David Mahler

Blogs and organic groups at http://www.ccie.net
Received on Fri Dec 10 2010 - 11:17:53 ART