From: Bryan Bartik (bbartik@ipexpert.com)
Date: Sat Mar 14 2009 - 20:21:10 ARST
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_note09186a0080124c7d%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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> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> > --
> > Bryan Bartik
> > CCIE #23707, CCNP
> > Sr. Support Engineer - IPexpert, Inc.
> > URL: http://www.IPexpert.com
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
>
>
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>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
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>
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>
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>
>
-- Bryan Bartik CCIE #23707, CCNP Sr. Support Engineer - IPexpert, Inc. URL: http://www.IPexpert.comBlogs and organic groups at http://www.ccie.net
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