Gyan –
As RFC 5302 and RFC 7775 clearly state, IS-IS also has “a pecking order for
route preference before the equal cost time breaker comes into play”.
The differences between IS-IS and OSPF largely derive from the fact that a
single OSPF instance may include multiple areas – whereas a single IS-IS
instance only supports a single area – with the option of participating in an
L2 backbone.
So, the set of route types available in the two protocols differ – but the
notion of route type preference first and cost second is consistent.
The reason it makes sense for (as an example) an L1 intra-area route to be
treated the same as an L1 external route is because the path calculations for
the two route types are performed over the same topology.
A historical note: The sharing of preference for internal/external routes at
the same level can be traced back to RFC 1195 (circa 1990). See the NOTE in
Section 3.10.2:
“NOTE: Internal routes (routes to destinations announced
in the "IP Internal Reachability Information" field),
and external routes using internal metrics (routes to
destinations announced in the "IP External Reachability
Information" field, with a metric of type "internal")
are treated identically for the purpose of the order of
preference of routes, and the Dijkstra calculation.”
RFC 5302 (circa 2000 in its original form RFC 2966) carefully maintained this
route type preference when defining preference for route types advertised using
TLVs 128 and 130.
RFC 7775 owes a great deal to RFC 5302. We simply updated the rules based on
the different set of route types available when using TLVs 135/236 et al.
HTH
Les
From: Gyan Mishra <[email protected]>
Sent: Friday, February 18, 2022 6:52 PM
To: Les Ginsberg (ginsberg) <[email protected]>
Cc: Muthu Arul Mozhi Perumal <[email protected]>; lsr <[email protected]>
Subject: Re: [Lsr] Preference among IS-IS IPv6 route types
Les
Just as a comparison and I believe that maybe what Muthu is alluding to is that
OSPF on the other hand does have a pecking order for route preference before
the equal cost time breaker comes into play.
intra-area, inter-area followed by external
So the result is not an ECMP with two different route types as the route has to
be the same type.
With ISIS that is one of the many differences between OSPF and ISIS.
I agree with your point that no loops would exist as the L1 route would be
advertised by the same or set of router(s) to get to the destination thus no
loops.
So the use of either path is valid and does not result in any loops as you
stated.
Kind Regards
Gyan
On Fri, Feb 18, 2022 at 9:09 AM Les Ginsberg (ginsberg)
<[email protected]<mailto:[email protected]>> wrote:
Muthu –
Let me try to be more complete in my response.
What RFC 7775 is addressing is defining the route preference between different
route types. It is necessary for interoperability that all implementations use
the same preference rules in this regard.
(One of the motivations for the RFC was a real world interoperability issue
that occurred because of inconsistency in this regard – see Appendix A.)
Within the preference groups defined in Sections 3.3 and 3.4, an implementation
has to apply the criteria used in the SPF algorithm. If this is the well known
Dijkstra lowest cost algorithm described in Annex C.2 of ISO 10589, then within
the set of routes with highest preference we choose the path(s) with lowest
cost.
If there are multiple paths with the same cost, an implementation may install
any or all of them in forwarding even if all routes are NOT of the same type.
In your example, so long as the cost to reach the destination using the L1
intra-area route is the same as the cost to reach the destination via the L1
External route, use of either path will NOT result in looping.
And one node could install both paths – another could install only one path –
and still no looping would occur.
Lack of use of ECMP might result in link utilization that is not to the liking
of a customer, but no interoperability issue will occur.
So the prioritization you mention below is NOT required to avoid looping.
Les
From: Muthu Arul Mozhi Perumal
<[email protected]<mailto:[email protected]>>
Sent: Thursday, February 17, 2022 11:59 PM
To: Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>>
Cc: lsr <[email protected]<mailto:[email protected]>>
Subject: Re: [Lsr] Preference among IS-IS IPv6 route types
Hi Les,
If we do ECMP, we'll have a traffic loop in the topology described in Appendix
A of RFC7775 b/w R1 and R2, assuming all routes are L1, right?
Seems prioritizing one of the routes (intra-area vs external) or honouring the
metric is required for avoiding this loop..
Regards,
Muthu
On Thu, Feb 17, 2022 at 9:46 PM Les Ginsberg (ginsberg)
<[email protected]<mailto:[email protected]>> wrote:
Muthu –
Use of Equal Cost Multipath (ECMP) is commonplace.
Les
From: Muthu Arul Mozhi Perumal
<[email protected]<mailto:[email protected]>>
Sent: Thursday, February 17, 2022 7:51 AM
To: Les Ginsberg (ginsberg) <[email protected]<mailto:[email protected]>>
Cc: lsr <[email protected]<mailto:[email protected]>>
Subject: Re: [Lsr] Preference among IS-IS IPv6 route types
Hi Les,
Thanks for your response. Please see inline..
On Thu, Feb 17, 2022 at 8:56 PM Les Ginsberg (ginsberg)
<[email protected]<mailto:[email protected]>> wrote:
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.
Ok, but RFC7775 says "Note that all types of routes listed for a given
preference are treated equally". How is that to be interpreted when there is an
L1 intra-area route of metric a and an L1 external route of metric b for the
same IPv6 prefix during comparison?
Regards,
Muthu
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]<mailto:[email protected]>> On Behalf Of
Muthu Arul Mozhi Perumal
Sent: Thursday, February 17, 2022 6:49 AM
To: lsr <[email protected]<mailto:[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
_______________________________________________
Lsr mailing list
[email protected]<mailto:[email protected]>
https://www.ietf.org/mailman/listinfo/lsr
--
[Image removed by sender.]<http://www.verizon.com/>
Gyan Mishra
Network Solutions Architect
Email [email protected]<mailto:[email protected]>
M 301 502-1347
_______________________________________________
Lsr mailing list
[email protected]
https://www.ietf.org/mailman/listinfo/lsr
