Routing

Hi,

Is it possible to have generation of summary LSAs by non-backbone ABR like in the following case,

Area 1 ------- ABR ----- Area 2

Would appreciate if anybody share some reasons on it as why it happens even in the absence of Virtual Link.

Regards,

Khurram

Hello there,

AFAIK, summary LSA generation by non-backbone ABR is/was the default JUNOS behaviour, please see "OSPF and ISIS: Choosing an IGP for Large-Scale networks" book by Jeff Doyle ISBN 0-321-16879-8 chapter 7.3 pages 224-228.

I haven't verified yet if this is still the case with newest JUNOS, so YMMV.

HTH

Regards

Alex

Alex,

This is exactly the case till JUNOS8.5 as i tested it on mentioned version.

Regards,

Khurram

Hi Khurrum,

Yes Summery LSA can be generate by non backbone ABR and in your example area 1 summery can reach area 2 as i have tested in the lab and found that summery LSA from R2 to R5 with out backbone area. following senerio i tested...

R2----area 2-----R1-----area 1-----R3------area 3-----R5

Backbone area benefits:::

OSPF was originally designed to Separate linkstate databases are maintained for each area, with area 0 acting as a sort of interconnect, but it's possible to have area-based routing in general without such a 'backbone area'. IS-IS, for example, has no such restriction.

Also, while the required area 0 might be a hurtle, you can create virtual links with transit areas to get around that restriction in a lot of cases.The requirement to have area 0, and the requirement to plan everything else around it, is one of my bigger peeves with OSPF in general. Physically there are better ways to organize routing for a network backbone.

Virtual link only requried if backbone area is configured, because in the presence of the backbone area summery LSAs only exchange through backbone. e.g.

R2----area 2-----R1-----area 1-----R3------area 0-----R5

Area 2 summery reaches R3 but R3 not install it in the routing table as it expect it from backbone area i.e. area 0 and that's why not advertise to area 0 therefore we configure virtual link.

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If this worked for you please flag my post as an "Accepted Solution" so others can benefit.
A kudos would be cool if you think I earned it

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thanks

Farooq

Thanks Farooq for elaboration but below mentioned statement doesn't match my findings

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R2----area 2-----R1-----area 1-----R3------area 0-----R5

Area 2 summery reaches R3 but R3 not install it in the routing table as it expect it from backbone area i.e. area 0 and that's why not advertise to area 0 therefore we configure virtual link.

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In my case, R3 is not only accepting Area 2 summary LSAs but also put them in the routing table. However, i failed to reach area 2 routes which is right as per theory.

Regards,

Khurram

Hi Khurrum,

Yes you are right but this is the case if we advertise R3 loopback in area 0 then R3 install it in the routing table.

but

in my case i have configured R3 Lo0 address in area 1 that's why R3 not install in the routing table.

thanks

Farooq