[Isis-wg] Clarification needed on domain-wide prefix distribution

[Henk Smit]

> Folks,
> I am surprised to know that there can be L1 external routes also.
> I thought that only L2 routers can advertise the external routes.

  That is what rfc1195 says.
 Real world is different.

  One of the things I like about ISIS is that fact that it is
 simple and elegant. One of the reasons for that is that almost
 all protocol rules are within a single area (L1 or L2 area).
 Communication between L1 and L2 is maybe not always well defined,
 but it is easy to understand. This will make multiple levels (L3/L8)
 easy to do when the time has come.

  It is simple to take external routes from L2 and put them into L1.
 Just put TLV130 into your L1 LSPs.
 In the past some customers had network which ran in one flat area,
 which was a L1 area. A problem was that they could not do redistibution
 into L1. Yup, they should have migrated to a L2 area. But it was
 easier to just put TLV130 into L1 LSPs. Heck, we could have even
 put the redistributed routes into TLV128 if we wanted to.

> If there are L1 external routes, then how does a router in L2
> knows about the advertising router present in L1 domain for 
> calculating the metric. I do not see equivalent of OSPF type 4
> summary LSA in IISIS.

  Knowing where the ASBR is is not necessary for external routes.
 It is only necessary when the external routes are advertised
 with external metrics.
 As you probably have noticed yourself:
        external route-type != external metric-type.

  Yet another reason not to do external metrics. They add more
 troubles then that they solve real issues.

> Also the domain-wide prefix distribution draft talks about 
> the following types of routes.
> - L1 intra-area
> - L2->L1 inter-area
> - L2->L1 external
> - L2 intra-area
> - L2 external
> - L1->L2 inter-area
> and each can be distinguished according to the table given
> in the draft. 
>  
> Is there any proposal to have L1 external routes and redistribution
> of such routes into L2? 

  I have promised to adjust draft-ietf-isis-domain-wide-01.txt to
 be more specific about route preferences. I am sorry that I have
 not found the time yet to do so.

        Henk.