Subject: [OSPF] Multi-topology OSPF, RFC : 4915

From: Abhijit Chaudhary <[email-masked]>
Date: Wed, 18 Feb 2009 21:20:34 +0530

I have a question Regarding Section 5 of OSPF-MT

< RFC SNIPPET>
5.  Interoperability between MT-Capable and Non-MT-Capable Routers

  The default metric field is mandatory in all LSAs (even when the
   metric value is 0).  Even when a link or prefix does not exist in the
   default topology, a non-MT router will consider the zero value in the
   metric field as a valid metric and consider the link or prefix as
   part of the default topology.

   In order to prevent the above problem, an MT-capable router will
   include all links as part of the default topology.  If links need to
   be removed from the default topology, an MT-capable router must be
   configured in DefaultExclusionCapability mode.  In this mode, routers
   will ensure that all other routers in the area are in the
   DefaultExclusionCapability mode before considering the MT-ID#0 metric
   in the SPF calculation.  Only then can the TOS 0 metric field in
   Router-LSAs be safely ignored during the default topology SPF
   computation.

</RFC SNIPPET>

Now, how does an MT-capable router automatically ensure that all other routers in the same area are also in DefaultExclusionCapability mode?
If I understand correctly, one solution can be for the calculating router to ensure that the router-LSAs from all other routers in the area have the MT Option set. But then, it is not mandatory that router-LSAs MUST have MT-bit set.

Is there any other solution?

Thanks,
- Abhijit

---

From: Peter Psenak <[email-masked]>
Date: Sat, 28 Feb 2009 21:19:02 +0100

Abhijit,

In section 4.3:

    A link may cease participating in the default topology if
    DefaultExclusionCapability is set to enabled.  In this state, a
    router will only form adjacency with routers that set the MT-bit in
    their Hello packets.  This will ensure that all routers have
    DefaultExclusionCapability enabled before the default topology can be
    disabled on a link.

thanks,
Peter

Abhijit Chaudhary wrote:
> Now, how does an MT-capable router automatically ensure that all other 
> routers in the same area are also in DefaultExclusionCapability mode?
> If I understand correctly, one solution can be for the calculating 
> router to ensure that the router-LSAs from all other routers in the area 
> have the MT Option set. But then, it is not mandatory that router-LSAs 
> MUST have MT-bit set.
>  
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