Re: [Lsr] draft-ppsenak-lsr-igp-ureach-prefix-announce

[Peter Psenak <[e-mail]>]

Hi Bruno,

thanks for your feedback, please see inline (##PP):

On 15/06/2022 16:09, [e-mail] wrote:
> Hi Peter, authors, all
> 
> Thanks for the draft. I find it a useful contribution to the problem space.
> 
> IMHO the use of MAX_PATH_METRIC is a good idea in particular since it 
> can be made backward compatible and provides incremental benefits with 
> incremental deployment.
> 
> I also have two disagreements on the current draft. Both are minor IMO, 
> but authors may have a different opinion.
> 
>  1. This draft defines a new signaling from an egress ABR to all ingress
>     ABR/PE. As such, this require all these nodes to agree on this
>     signaling. So I’d call for a STD track document.

##PP
there is no new signalling defined in the draft. We are using what has 
been defined in the RFC5305/RFC5308


>  2. IMO, the behavior defined in this draft is not backward compatible
>     with the specification of MAX_PATH_METRIC in an IP prefix.

##PP
I see no backward compatibility issue

> 
> RFC5305 says:
> 
> If a prefix is advertised with a metric larger then MAX_PATH_METRIC
> 
> (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered
> 
> during the normal SPF computation.This allows advertisement of a
> 
> prefix for purposes other than building the normal IP routing table.
> 
> As per the above, one (ABR) may (is allowed and free to do so) already 
> advertise both an aggregate prefix IP1/N with a regular metric and a 
> more specific prefix IP2/32 with MAX_PATH_METRIC.
> 
> With the above advertisement, all nodes compliant with RFC 5305 will 
> install IP1/N in their FIB and not consider IP2/32 during their SPF and 
> as a consequence not install it in their FIB.
> 
> In term of reachability, all nodes have IP reachability to all IP @ in 
> IP1 including IP2.
> 
> If one node now implements the draft, it will remove reachability for 
> IP2. And hence all my BGP routes using IPv2 for next-hop will be removed.

##PP
there is no such thing specified in the draft. What the drafts says is 
that if the receiver is configured to do so, it can pass the UPA to the 
applications that may be interested in it. How they act on it is outside 
of the draft and ISIS as such.

I'm not sure where did you get the "remove reachability for IP2".


> 
> This looks clearly like a change in behavior to me, plus one which 
> introduce loss of reachability in an existing network.
> 
> 3) The solution that I would suggest is:
> 
> - advertise the “unreachable prefix” with metric MAX_PATH_METRIC
> 
> - define a new “Extended Reachability Attribute Flags” ([RFC 7794]) 
> explicitly signaling that the reachability to this prefix has been lost:
> 
> Unreachable Flag (U_flag). Set if this prefix is to be considered 
> unreachable.

##PP
I see no reason for the new flag. RFC5305/RFC5308 already provide a way 
to signal unreachable prefix. That's all we need.

thanks,
Peter
> 
> This would allow for explicit signaling of the reachability (vs implicit 
> currently) and would be backward compatible with existing specification 
> and deployments.
> 
> Regards,
> 
> --Bruno
> 
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