From: Scott Morris (swm@emanon.com)
Date: Sat Mar 14 2009 - 20:41:32 ARST
When you do a "show ip ospf database nssa (ip-route#)" do you see anything
about the P-bit or not?
If the p-bit is present I would expect the N1 to be chosen. If the p-bit is
not present, I would expect the E1 to be chosen.
RFC 3101 details the process that is supposed to happen.
I'll try to get some time to lab that up though as it would be intriguing if
not following expected behavior! :)
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Bryan Bartik
Sent: Saturday, March 14, 2009 6:21 PM
To: smorris@internetworkexpert.com
Cc: naveen M S; GAURAV MADAN; GS CCIE-Lab
Subject: Re: E1 Vs N OSPF routes
Hmmm....Here is the route entry, metric is 84. Later I will add 64 to make
it even to the one coming from R4:
R2#sho ip rou 192.168.100.0
Routing entry for 192.168.100.0/24
Known via "ospf 1", distance 110, metric 84, type NSSA extern 1
Last update from 192.168.12.1 on Serial1/1, 00:01:13 ago
Routing Descriptor Blocks:
* 192.168.12.1, from 1.1.1.1, 00:01:13 ago, via Serial1/1
Route metric is 84, traffic share count is 1
This makes the N1 equal to the E1, and E1 is chosen (from ASBR R4)
R2(config)#int s1/1
R2(config-if)#ip osp cost 128
R2#sho ip route 192.168.100.0
Routing entry for 192.168.100.0/24
Known via "ospf 1", distance 110, metric 148, type extern 1
Last update from 192.168.23.3 on Serial1/0, 00:05:33 ago
Routing Descriptor Blocks:
* 192.168.23.3, from 4.4.4.4, 00:05:33 ago, via Serial1/0
Route metric is 148, traffic share count is 1
We reach your step 3 (or perhaps 2-b-ii)...which shows R2 is picking the LSA
as advertised from R4 (4.4.4.4) instead of R1 (1.1.1.1). R2's costs to both
routers is 128...(R2-->R1=128, R2-->R4=64+64=128). E1 is chosen over N1
instead of multiple paths.
The behavior changes if the area between R1 and R2 is a normal area and not
an NSSA, two routes are installed:
R1(config)#router ospf 1
R1(config-router)#no are 1 nssa
R2(config)#router osp 1
R2(config-router)#no are 1 nssa
R2#sho ip rou 192.168.100.0
Routing entry for 192.168.100.0/24
Known via "ospf 1", distance 110, metric 148, type extern 1
Last update from 192.168.23.3 on Serial1/0, 00:01:51 ago
Routing Descriptor Blocks:
192.168.23.3, from 4.4.4.4, 00:01:51 ago, via Serial1/0
Route metric is 148, traffic share count is 1
* 192.168.12.1, from 1.1.1.1, 00:01:51 ago, via Serial1/1
Route metric is 148, traffic share count is 1
So it appears that in a scenario where two equal cost external routes would
cause multiple paths to be installed, if one of those happened to be a
type-7 instead, the type-5 would win. I don't find any documentation and I
hesitate to say it is 100% without testing more. Maybe you could lab it as
well and share your thoughts.
On Sat, Mar 14, 2009 at 4:17 PM, Scott Morris <
smorris@internetworkexpert.com> wrote:
> Nope.
>
> Re #2, E1s will be preferred over E2's. Then the steps you outline.
>
> Re #3, there are still other criteria in here. Type 1's over Type 2's...
> But if an NSSA route with the P-bit is received, then it will even take
> precedence over Type 5's. The RFC points this out. I'm not aware of any
> particular Cisco doc that goes into it.
>
> After that, the steps are similar to what you list for #2.
>
> Cheers,
>
> Scott
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> naveen M S
> Sent: Saturday, March 14, 2009 5:19 PM
> To: Bryan Bartik
> Cc: GAURAV MADAN; GS CCIE-Lab
> Subject: Re: E1 Vs N OSPF routes
>
> 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<http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008
0124c7d%0A.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
> > >
> > >
> > > Blogs and organic groups at http://www.ccie.net
> > >
> > >
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