Margaret Wasserman wrote:
>
>>
>> Good catch Margaret. I should have noticed that the example given
>> actually violates the scoped addressing architecture doc. The
>> forwarding logic is still correct, but you can only have, at most,
>> one zone id per scope per interface. Otherwise you would have
>> overlapping scope zones.
>
>
> Are you sure?
>
> I originally began an answer to Robert's third example with something
> like "this is an invalid configuration". However, I realized that I
> was mistaken.
If the rule stands that an interface can have no more than one
zone id per scope, then it is invalid if we expect a subnet-local
zone id to automatically appear when a prefix is configured on
an interface.
So, going back to Robert's example:
A --------- B --------- C
- A-B is prefix1::/64 and prefix3::/64
- B-C is prefix2::/64 and prefix3::/64
- (prefix1 != prefix2, prefix1 != prefix3, prefix2 != prefix3)
As long as B has one subnet-local scope zone id per interface,
the forwarding logic I described will work.
You could make it work with multiple zone ids on an interface,
but the logic would require a complicated RPF check on the
source address in order to determine which zone id to use in
the forwarding decision. I don't think I want to go there.
>
> Assuming that router B has support for multi-link subnets, Robert's
> example is a valid configuration showing a subnet-local zone
> that encompasses two links.
It is valid from the base architecture. It is valid from the
scoping architecture if there is no more than one subnet-local
scope zone id.
>
> There is no requirement that all hosts within a given zone need
> to configure addresses from (all of) the unicast prefix(es)
> within that zone, or that all routers need to advertise the same
> unicast prefixes on all interfaces within a given zone. Is there?
Not that I know of.
>
> The real question is this:
>
> By default, if a router is configured to advertise the same prefix
> on two interfaces, should the router assume that the two interfaces
> are in the same subnet-local zone? Should the router assume that
> the two interfaces are on the same link (i.e. in the same link-local
> zone)?
The more I think about it, the more I realize that "automagically"
creating the subnet-local scope zone id isn't going to work.
Especially with multiple prefixes per interface.
>
> These are questions that need to be answered in the scoped address
> architecture.
I think the addressing architecture needs to address the comment
on automatically creating the subnet-local zone id.
>
> I think it would be a reasonable default for a router that is
> configured with the same prefix on two interfaces to assume that
> those interfaces are on the same link (same link-local zone), and
> in the same subnet-local zone.
But if they aren't on the same link, forwarding could break.
>
> In other words, I think that routers should default to the
> single-link subnet case, unless mutli-link subnetting has been
> explicitly configured.
This is slightly different than assuming that the interfaces are
on the same link. If the router treats them as unique prefixes,
then forwarding and routing should work.
Brian
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