From: naveen M S (navin.ms@gmail.com)
Date: Sat Mar 14 2009 - 19:18:52 ARST
Hi Brian,
If you still have the setup, can you please paste the "show ip route" here ?
Wanted to know if router selected R1 or R4 to reach 192.168.100.0. From what
I remember about OSPF Path Selection is -
1) OSPF prefers Intra-area path than Inter-area path. "O" route path
preferred more than "O-IA" route path.
2) If 2 Type-5 LSAs are received with different ASBRs, then OSPF goes
through following path selection rule (per your Cisco Link)
a) If metric is different on both LSAs, choose the one with Lowest
metric.
b) If metric is same on both LSAs, check the Forwarding address in the
LSAs.
(i) If Forwarding address on both LSAs are set to 0.0.0.0, then
choose the nearest ASBR.
(ii) If Forwarding address on one is 0.0.0.0 and the other is set
to non-zero,
-> compare metric to ASBR (which set 0.0.0.0) to the metric to
reach non-zero Fwd'ing address.
-> choose the path towards ASBR which has lowest metric.
(iii) If Forwarding address is set on both to non-zero values, then
choose the path towards
the forwarding address with the lowest metric.
3) If same external network is learnt as both Type-7 and Type-5 (which is
Gaurav's scenario), then ???
No cisco documentation is available on this. But very likely it checks
for lowest metric first and if they are
the same on both LSAs I guess it goes through same selection process
described in item (2) above.
Rgds,
-Naveen.
In Gaurav's case, since the external network are learnt as both LSA-7 and
LSA-5, I think OSPF would choose lowest metric of both for forwarding
traffic.
The Cisco link shows the ca
On Sat, Mar 14, 2009 at 8:30 AM, Bryan Bartik <bbartik@ipexpert.com> wrote:
> Hi Guarav,
>
> Not sure I understand, but if you had an NSSA, E routes wouldn't be
> allowed,
> you would only have N routes redistributed by the ASBR. E routes wouldn't
> make it in, but the N would be converted to E at the ABR...
>
> Now I guess you could have a scenario where the same route was
> redistributed
> as a N in one nssa area and E in another regular area and they both showed
> up at the NSSA ABR like this:
>
> R1 and R4 connected to same LAN 192.168.100.0 (no OSPF on it, just
> redistributing)
> ---R1---R2----R3---R4---
>
> R2(config-router)#do sho ip osp dat | be pe-7
> Type-7 AS External Link States (Area 1)
>
> Link ID ADV Router Age Seq# Checksum Tag
> 192.168.100.0 1.1.1.1 13 0x80000001 0x000E40 0 <----
> From R1 in an NSSA
>
> Type-5 AS External Link States
>
> Link ID ADV Router Age Seq# Checksum Tag
> 192.168.100.0 2.2.2.2 2 0x80000001 0x0084CF 0
> <---- Converted to Type 5 by R2
> 192.168.100.0 4.4.4.4 14 0x80000001 0x006D55 0 <----
> From R4 in another area
> R2(config-router)#
>
> In this case, the it looks like the same rules would apply as here:
>
>
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml
>
> -hth
>
> On Sat, Mar 14, 2009 at 8:29 AM, GAURAV MADAN <gauravmadan1177@gmail.com
> >wrote:
>
> > Hi
> >
> > Can someone cearify the rpeference order that ospf follows while
> selecting
> > between NSSA ; E1 routes (especially when P bit is set ) .
> >
> > Thnx in advance
> > Gaurav Madan
> >
> >
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>
> --
> Bryan Bartik
> CCIE #23707, CCNP
> Sr. Support Engineer - IPexpert, Inc.
> URL: http://www.IPexpert.com
>
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