Muthu –
RFC 7775 is defining preference rules between routes of different types – it is
NOT discussing preference rules within a (set of) route types that have the
same preference.
Such a discussion is out of scope.
Use of “lowest cost” is part of the well known Dijkstra Shortest Path First
(SPF) algorithm – though there are many example of constrained SPF calculations
that incorporate attributes other than cost in the choice of “best path”.
All of this is out of scope for RFC 7775.
Les
From: Lsr <[email protected]> On Behalf Of Muthu Arul Mozhi Perumal
Sent: Thursday, February 17, 2022 6:49 AM
To: lsr <[email protected]>
Subject: [Lsr] Preference among IS-IS IPv6 route types
Hi,
Need some clarification on the preference among IS-IS IPv6 route types
described in RFC7775 section 3.4 and RFC5308 section 5.
RFC7775 places L1 intra-area routes and L1 external routes at the same
preference level and says that all types of routes listed for a given
preference are treated equally. There is no mention of metric.
<snip>
This document defines the following route preferences for IPv6 routes
advertised in TLVs 236 or 237. Note that all types of routes listed
for a given preference are treated equally.
1. L1 intra-area routes; L1 external routes
2. L2 intra-area routes; L2 external routes; L1->L2 inter-area
routes; L1-L2 external routes; L2-L2 inter-area routes
3. L2->L1 inter-area routes; L2->L1 external routes; L1->L1 inter-
area routes
</snip>
RFC5308 however says:
<snip>
If multiple paths have the same best preference, then selection
occurs based on metric.
</snip>
It is not clear whether metric is to be used for selection among L1 intra-area
and external routes or is to be used for selection only with a given route
type. Can someone please clarify?
Regards,
Muthu
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