[Isis-wg] draft-ietf-isis-traffic-01.txt

[Christian E. Hopps]

Henk Smit <hsmit@cisco.com> writes:
>   If you want to mark an IP prefix as external, we can do it via sub-TLVs.
>  My collegues Naiming Shen has been working on tagging/coloring IP
>  prefixes. Similar to OSPF tags or BGP communities. Maybe we can
>  introduce some "well known" tags, to indicate that some prefixes
>  were redistributed from outside into ISIS.

What about support of incomparable metrics (i.e, ospf's type 2 metric).
The idea being you import the metric form the other protocol into the
`external' metric and it takes precedence over the IGP metric (which is
used to break ties).

You can add this as a new sub-TLV in another draft; however, then you
run into transition issues.  Routing loops are easy to come by if you
give the external metric precendence in the SPF and then some router
ignores that sub-tlv.  By defining this in the main draft you at least
eliminate trying to transition twice.

Since I don't have a lot of operational experience I'd like to ask: is
an incomparable external metric perceived as a gratuitous addition?  I
was under the impression that it was used just about anytime you wanted
to import an EGP (or some aggregates of the EGP) into the IGP.

Thanks,
Chris.