Subject: [rbridge] trill-adj

From: Donald Eastlake <[email-masked]>
Date: Sun, 6 Feb 2011 21:56:40 -0500

Response to some comments by Les Ginsberg:

> Preventing Multiple DRBs on a LAN
> ---------------------------------

> It is not clear to me how the use of multiple announcing VLANs
> achieves the stated goal of one and only one DRB in the presence of
> inconsistent connectivity / configuration.

Sending Hellos on multiple VLANs doesn't guarantee one DRB on a link
in the presence of arbitrary port and bridge configuration, which is
why there are two additional loop safety mechanisms in TRILL. But it
does a lot better than sending Hellos on only one VLAN.

> Consider the following
> simple example where VLANs(1,2,3) are in the enabled set on all RBs:

> RB Name/     DVLAN   Announcing VLANs   VLAN Connectivity
> Priority
> --------------------------------------------------------
> RB1/50         1       1,2              1,2,3
> RB2/60         1       1                1
> RB3/70         3       2                2,3

Configuring Announcing VLANs to be anything less than the default of
all enabled VLANs is to deliberately accept an increased risk of loops
in return for fewer Hellos. If you leave Announcing VLANs at its
default value, there is no problem in this case.

In the TRILL working group, the two main opinions were that RBridges
should always send Hellos on all VLANs and that they should send
Hellos only on the Designated VLAN. The TRILL base protocol standard
defaults for sending Hellos are a compromise that provides reasonable
safety for plausible configurations. The ability to configure the
Announcing VLANs set is a facility primarily for those who insist that
their networks are fully configured with nothing funny in between
RBridges so that it is safe to reduce the number of Hellos.

Configuring the RBridges on a link with inconsistent desired
Designated VLANs and a reduced number of Hellos via a non-default
Announcing VLANs set is risky, as you demonstrate.

> Given the above the following would ensue:

> RB Name     Neighbors       DRB/
>              Detected       DVLAN
> --------------------------------------------------------
> RB1            2,3          3/2
> RB2            1            2/1
> RB3            2,3          3/2

> Now, given the above, both RB3 and RB2 will think they are DRB and
> potentially assign multiple AFs for each of the VLANs. Keeping in
> mind [TRILL] Section 4.4.3

> "Great care should be taken in configuring an RBridge
>   to not send TRILL Hellos on any VLAN where that RBridge is appointed
>   forwarder as, under some circumstances, this can lead to loops."

I think the "this" towards the end of that sentence should probably be
replaced by "failure to send such Hellos", for clarity. I'll make a
note of this as an editorial clarification.

> it becomes apparent that at a minimum at least one RB MUST send IIHs
> on each enabled VLAN in order to implement loop prevention. But this
> does not scale well.

The DRB does send such Hellos unless you configure things so that it
doesn't.

If you are willing to bet that all your RBridges on a link have
connectivity to each other over the Designated VLAN for that link,
then Hellos just on that VLAN are adequate and you can get that by
configuring the Announcing VLANs set on all the ports to be the null
set. This would lead to N Hellos being sent if there are N RBridges on
the link.

If you are not willing to bet on that, then the TRILL default for N
RBridges where k VLANs are enabled would be N+k-1 Hellos. If the DRB
assigns lots of VLANs to other RBridges, this would likely tend toward
2*k Hellos. But all this scales much better than the N*k Hellos some
people were insisting on.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-333-2270 (cell)
 155 Beaver Street
 Milford, MA 01757 USA
 [email-masked]

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