Re: OSPF distance-vector behavior

From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Wed Jul 16 2008 - 07:10:01 ART

Adam,
OSPF distance-vector behavior ony manifests itself when a router receives
type 3,5,7 LSAs and calculates the resulting metric. It does not affect LSA
generation. Behaving like DV protocol means that the router witll simply add
the metric to reach the advertising router (ABR or ASBR) to the metric
received in LSA (for type 5 and type 7 it's only valid for E1 metric type)
and will not perform any SPF computations (since this not not a type 1/2
LSA)

In your scenario, R1 is an ABR. When R1 receives type 1 LSA describing R3
Loopback interface, it generates a summary LSA (type 3) with a proper metric
value and floods it through Area 0 so R4 can learn it (R4 will use the above
DV behavior to compute the new route metric - i.e. it will add metrci to
reach R1 to the metric in LSA 3 update). However, when R1 will try to inject
the route to 10.0.0.0 into routing table, it will fail, since EIGRP has the
same route but with a better distance.

Therefore, R4 will learn the route to R3 Loopback subnet as OSPF, but the
packet will follow R4->R1->R2->R3 path due to the fact EIGRP has better AD
on R1. 

-- 
Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice)
petr@internetworkexpert.com

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2008/7/16 Adam Elghafri <ccie.adam@gmail.com>:

> hi group
>
> i have a question regarding redistribution
>
> [[A]] Jeff Doyle wrote in Routing TCP/IP vol. I that ospf behave like
> a distance vector protocol between areas
>
>
> [[B]] and what i cam to know that distance vector routing protocols
> like rip and eigrp will not advertise a route that is not in the
> routing table ...
>
> now take a look at this picture
>
> http://tinypic.com/view.php?pic=smwxep&s=4
>
> 1) notice R3 is redistributing ospf into eigrp
> 2) we changed the AD of the route to 10.0.0.0 into 89 on R1 eigrp process
>
> now R1 is learning the loopback 10.0.0.0/24 from R3 and from R2... now
> it will prefer the path through R2... and it will be like this
>
> R1#sh ip route 10.0.0.0
> D 10.0.0.0/24 is subneted, 1 subnet
>   10.0.0.0, via s1/0
>
> now the question is, when R1 tries to send an LSA type 3 to R4... will
> it include the 10.0.0.0/24 route???? considering the abovementioned
> points A and B
>
> i havnt test this yet.. as it just triggered in my head.. i will do n
> provide the answer soon...
>
> Adam Elghafri
>
>
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