Subject: [rbridge] RBridge issue with selecting designated vlan

From: Abhinav Bhatia <[email-masked]>
Date: Tue, 17 May 2011 22:10:59 +0530

Hi Donald,

As part the initial trill study I am facing following critical issue:

In trill protocol the designated router bridge(drb) selects the designated
vlan(dvlan) on which all the router bridges will
communicate. This designated vlan should be enabled on all the vlans.

Now the next thing is to make sure that this designated vlan is enabled vlan
on each of the neighbors,
since we dont have separate tlv for enabled vlans, we need to keep track of
vlans on which hellos are received from each of the neighbors.

But Doing this does not guarantee the proper working, problems with doing
that:

1) If we make announcing vlan set as null, then hellos will only be sent on
designated vlan by non-drb. But drb can not
    select designated vlan other than the one on which hello was sent by non
drb. If this designated vlan gets disabled on non-drb
    then it may be totally isolated (new designated vlan selected by drb may
not be enabled on the non-drb router bridge).

2) Even if the announcing vlan is not null, still the sending of hellos on
different vlans depends upon the set of appointed
    forwarders and if drb makes the non-drb appointed forwarder for limited
set of vlans then again hellos wont be sent on all
    vlans leading to the incomplete knowledge of enabled vlans.

There can be be two approaches to that:

a) Fix the designated vlan and make sure that is enabled on all router
bridges, not giving the router bridges an option to change
    designated vlan. Again using this approach an router bridge will be
isolated if designated vlan gets disabled.

b) Send an extra tlv containg enabled vlans, i think using this should
resolve the above mentioned issues.

It will be great if you can help us with this issue.

Thanks,
Abhinav
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From: Donald Eastlake <[email-masked]>
Date: Wed, 18 May 2011 08:20:00 -0400

Hi Abhinav,

> communicate. This designated vlan should be enabled on all the vlans.

Pretty much right, assuming your last word above was intended to be
"Rbridges" or "Rbridge ports on the link" or the like.

However, I don't think your word "selects" corresponds very well with
the TRILL base protocol standard. In the standard, each RBridge port
on the link is configured with a desired Designated VLAN and whichever
port wins the DRB elections dictates that that VLAN be the Designated
VLAN on the link. There is no "selection" process for the Designated
VLAN except to the extent that there is a selection process, the DRB
election, for the DRB.

> vlans on which hellos are received from each of the neighbors.

Actually, there is the Enabled VLANs sub-TLV, as documented in Section
2.2.2 of draft-ietf-is-is-trill-05, which can be included in the MT
Port Capabilities TLV; however, sending it is optional. There is no
requirement to keep track of all the VLANs on which you receive Hellos
from neighbors. See draft-ietf-trill-adj-07.

The way to make sure the Designated VLAN is enabled on all the
neighbor RBridge ports on the link is to properly configure the
neighbor RBridges so this is true.

> not be enabled on the non-drb router bridge).

The ability to configuring the announcing set to be a smaller set
(null in the limit) is intended for network operators who want fewer
Hellos, are confident that their networks are properly configured, and
are willing to accept the higher chance of loops or orphaned
(isolated) RBridge ports if they turn out to be wrong and their
networks are not properly configured.

On a properly configured link, all the RBridge ports are configured
with the same desired Designated VLAN and that VLAN is enabled on all
RBridge ports. If that is not true, the link is not properly
configured. And if you such an improperly configured link and then
configure the announcing VLAN set to be null at one or more RBridge
port on the link, you are making the configuration even worse and
possibly no longer loop-safe.

> isolated if designated vlan gets disabled.

I'm not sure exactly what you mean in (a) above, but configuring all
the RBridge ports on the link so the same VLAN is desired Designated
VLAN and enabled on them all is proper configuration. TRILL tries hard
to be safe for improperly configured links but does not worry about
efficient operation for improperly configured links.

> resolve the above mentioned issues.

How to select the priorities of different RBridge ports to be DRB, how
to select the desired  Designated VLAN to configure in the ports on a
link (if it is not he default VLAN 1), how to determine which RBridges
to appoint forwarder for which VLANs, when to send an Enabled VLANs
sub-TLV, etc., are all beyond the scope of the TRILL base protocol
standard.

In summary, things will work well for a properly configured link as
described above. Things will not work so well if desired Designated
VLANs and/or enabled VLANs are inconsistent -- and there are no
provisions in the TRILL base protocol standard to try to automatically
fix improper configurations. And if you decrease the membership of the
announcing set below its default membership, you accepting an
increased chance of unsafe operation in exchange for fewer Hellos.

Thanks,
Donald
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