Subject: Re: [rbridge] Questions and comments to draft-ietf-trill-rbridge-af-02

From: Radia Perlman <[email-masked]>
Date: Tue, 10 May 2011 16:28:33 -0700

Hi Linda,

On Tue, May 10, 2011 at 4:00 PM, Linda Dunbar <[email-masked]> wrote:
> Hi Radia,
>
> I have a few questions and comments to the draft-ietf-trill-rbridge-af-02.
> Hope you can help me out.
>
>
>
> For a RBridge connected with point to point link to an end station (or
> multiple end stations when a 802.1 bridge/switch is used), does this RBridge
> automatically become the Appointed Forwarder for all the VLANs enabled on
> the ports?

Yes.  Since there is no other RBridge on the link, there's nobody to
delegate to.

> Section 2, 3rd paragraph: “while the cases of multiple ports on a link for
> one RBridge and multiple port with the same MAC address are fully
> accommodated, …”
>
> Do you mean “while the cases of one Bridge with multiple ports with same MAC
> address on one link are fully accommodated, …”?
> If yes, can you change the text a bit to make it more clear?
>

No.  It does not mean that. It is meant to cover both the cases of:
a) a single RBridge with multiple ports on the same link with the same
MAC address, and
b) multiple RBridges using the same MAC address on the link...i.e.,
both R1 and R2 are using MAC address M on the same link.

I agree that paragraph will not win any journalism awards and will
think about how to reword it.

> Section 2.2.1, 2nd paragraph: “if the TRILL Hello is from the winning DRB
> port …, then it loses all Appointed Forwarder status”
>
> Do you mean the DRB loses its Appointed Forwarder status or the receiving
> Rbridge loses its AF status for the VLAN not included in the Hello message?
> Since the Receiving RBridge wasn’t AF, why you call “lose its status”?
>

Again, you've found another case of less than clear wording.  There's
probably a simpler way of saying what is intended, which is that the
RBridge will only be AF for the VLANs specified in that Hello.

> Section 3: The Inhibition Mechanism: Does each VLAN enabled has an
> “Inhibition Timer”? Why need this timer?
>
>

I'm going to let Donald answer this one...

> Thank you very much.
> Linda Dunbar

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---

From: Donald Eastlake <[email-masked]>
Date: Tue, 10 May 2011 21:38:48 -0400

Hi Linda,

On Tue, May 10, 2011 at 7:28 PM, Radia Perlman <[email-masked]> wrote:
>>...

Those Inhibition timers (or the equivalent) are needed to be loop safe
in the case of misconfigured bridges on the link. For a very simple
example, assume that RB1 and RB2 are the only RBridges on the link,
that RB1 is higher priority to be DRB, and that they both want VLAN 1
to be the designated VLAN. But there is a bridge between them
configured so that RB1 can see all the frames sent by RB2 but none of
the frames from RB1 can get through to RB2. Both will think they are
DRB, RB1 because it is higher priority even though it sees the Hellos
from RB2 and RB2 because it doesn't see the Hellos from RB1 and thinks
it is highest priority.

Say RB1 chooses to act as appointed forwarder for VLANs 2 and 3 while
RB2 chooses to act as appointed forwarder for VLANs 3 and 4. There is
no problem with VLANs 2 and 4 but if you do not do something about it,
you could have a loop involving VLAN 3. RB1 will see the Hellos RB2
issues on VLAN 3 declaring itself AF and so RB1 will be inhibited on
VLAN 3. RB2 does not see the Hellos issued by RB1 on VLAN 3 and so RB2
is uninhibited and will handle VLAN 3 native traffic.

But this situation may change. RB2 might crash or the bridge might
crash or RB2 might be re-configured so it no longer tried to act as
appointed forwarder for VLAN 3 or ... So RB1 has to maintain a VLAN 3
inhibition timer and if it sees no Hellos from any other RBridge on
the link claiming to be AF for VLAN 3 in a long enough time, then RB1
becomes uninhibited for that VLAN on the port in question and can
handle end station traffic in VLAN 3.

>> Linda Dunbar

Thanks,
Donald
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