Subject: [rbridge] loops in trill networks

From: kreeger at cisco.com (Larry Kreeger (kreeger))
Date: Tue, 11 Oct 2005 15:21:48 -0700

Radia,

I agree that loops happen within 802.1d bridged networks due to
circumstances such as misconfiguration, HW bugs, SW bugs etc, and that
these tend to cause the network to melt down.  This is why a TTL is a
good thing, and I have no objection to it.

However, I'd like to separate out meltdown behavior (which is driven by
events which happen less often), and temporary loops which can be caused
by much more commonplace events (link up/down).

In the case of simple link up/down events, 802.1d networks don't
duplicate unicast frames.  If we are calculating the RBridge
bcast/mcast/unknown spanning tree using IS-IS, then unknown unicasts
will get duplicated in some topologies.  If this is deemed acceptable
behavior for an RBridge network, it should be documented as a known
behavior.  If this causes problems for some protocols, users will be
able to choose to either fix the protocol or not use RBridges.  If it is
not deemed acceptable behavior, then it should be fixed.

I'm just trying to point out what I see as difference between the two
technologies as defined.

 - Larry 

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
Behalf Of Radia Perlman
Sent: Tuesday, October 11, 2005 2:48 PM
To: Developing a hybrid router/bridge.
Subject: Re: [rbridge] loops in trill networks

There was never a "guarantee" in STP that there not be loops. STP
attempted to avoid them by waiting before turning ports on. RSTP,
instead of waiting before turning on ports, attempts to avoid loops by
using local information to turn some links on while turning others off.

Although it's certainly good to try to avoid temporary loops, it's never
possible to *guarantee* no temporary loops. These can be caused by
misconfiguration (someone claiming a port will only have endnodes,
whereas it actually does get connected to a switch), by bringing up a
repeater, or having enough consecutive BPDUs lost for whatever reason
(congestion on the link causing packets to not reach a bridge, or the
bridge not being able to keep up with worst case wire speed).

The thing that got me interested in this again is finding out that
bridge meltdowns (due to temporary loops) do occur, and apparently with
increasing frequency with new features that have been added to bridges
that involve configuration that can be disastrous if set incorrectly, or
with bridges deployed that can't keep up with wire speeds.

So the idea is that although it's certainly good to try to avoid
temporary loops, we want to make it not a disaster if they do occur.

Radia

Larry Kreeger (kreeger) wrote:

>Joe,
>
>As far as I know, all variations of STP guaranteed that bridged 
>networks never have temporary loops, so existing bridges never have
this problem.
>If they did, then I'd agree that it is no different. 
>
> - Larry
>
>
>  
>

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