Type 5 LSA

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?.

  1. 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.

Reply to
srinivas.kandalam
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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

Reply to
ts

I earlier wrote: ...

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

Reply to
ts

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.

Allows ou to influence routing better.

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.

Reply to
Paul Matthews

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? :) )

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.

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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.

Reply to
stephen

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