Type 5 LSA

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Hi,

I have two basic question.


1. Say we have three routers R1, R2 and R3 connected thru multiaccess
segement. The DR and BDR selection is done (assume DR=R1, BDR=R2).
   We have one more setup on which routers R4,R5 and R6 connected thru
multiaccess segment.In this case also DR and BDR selection is done. Now
what
happens if we join these two segements, does routers automatically
initiate
DR/BDR selecton after coming to know the existence of another DR/BDR in
the
segment (all routers are part of same area, say area 0), OR do we need
to
manually clear the ospf process so that the DR/BDR selection happens
again?.

2. What exactly is the use of having two types of path types(E1 and E2)
for
Type-5 LSA.

   Suppose we have routers connected in this fashion with the below
costs.


        R10---5------R20----10------R50-----network 10.2.1.0
        R10---15-----R40----5-------R50-----network 10.2.1.0
(All routers are part of Area 0.)

In case of Type 1 the cost to reach network 10.2.1.0 from R10 is 15
(least
cost). The path choosen is R10-R20-R50.
Suppose we distribute network 10.2.1.0 as type-2, then also the path
choose
will be R10-R20-R50, because R10 chooses least cost path to reach ASBR.

So in both cases the path choosen will be same. So why do we need this
differentiation of Type1 and Type2. What exactly is the purpose for
this.

Any help on this would be appriciated.

Thanks,
-Srini.


Re: Type 5 LSA

srinivas.kandalam@gmail.com wrote:
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...
E2 is the default route type for routes learned via redistribution. The
key thing is that the cost of E2 routes reflects only the cost of the
path from the ASBR to the final destination.

If you want the cost of the routes to reflect the entire path, ie not
just the path between the ASBR and the destination network, the routes
must be redistributed into OSPF as E1 routes on the ASBR.That's the
important difference.

This is essential knowledge (especially for CCNP success, i think).

//Regards Tony


Re: Type 5 LSA
I earlier wrote:
...
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To clarify things in plain english:

The cost of a type 2 route is always the external cost. The interior
cost to
reach that route is irrelevant... err, not reflected, in this context.

A type 1 cost is the external cost and additionally the internal cost
used
to reach that route.

Furthermore, a type 1 route is always preferred over a type 2 route for
the same destination.

//Regards Tony


Re: Type 5 LSA
srinivas.kandalam@gmail.com wrote:


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DR will not normally relect. If you want to influence them you need to set
appropriate priorities, then clear OSPF - just the DR and BDR should do it.

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Allows ou to influence routing better.

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Delberately oversimple network. Imagine a large corporate network, with routes
to the internet at opposite "ends". The T1 lets you encourage devices to use
their nearest exit. T2 lets you have a primary and backup, so that even people
near the backup will use the primary.

P.
--
Paul Matthews                          
paul@cattytown.me.uk
http://www.hepcats.co.uk

Re: Type 5 LSA
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if they are all set up to be in the same subnet etc, then it should sort
itself out..

or it did last time this happened in anger to me - but with Nortel / IBM
gear (but standards mean everything works the same - right? :) )
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you got a good explanation for this from another poster.

if you want some examples on why you might want that feature, read the OSPF
RFC - it goes thru a lot of this stuff.
http://www.ietf.org/rfc/rfc2328.txt

if you prefer to read about it, the best book i know of on OSPF is "Anatomy
of a routing protocol" by John Moy, although some of the sections are out of
date now.
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--
Regards

stephen_hope@xyzworld.com - replace xyz with ntl



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