[c-nsp] Maximum switched ethernet daisy chain size

[mack]

> ------------------------------
>
> Message: 3
> Date: Sat, 7 Jun 2008 17:50:59 +0200
> From: "Pavel Skovajsa" <pavel.skovajsa at gmail.com>
> Subject: [c-nsp] Maximum switched ethernet daisy chain size
> To: cisco-nsp at puck.nether.net
> Message-ID:
>         <323aca890806070850me92fb8bq53e7d8ef4e0d2841 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hello all,
> Recently I have found an interesting issue.
> I have found that on our network the packets that traverse more then
> 17 (18 and more) L2 ethernet switching hops all connected with MM
> fiber 1G links have lowered probability of arrival.
> The way I have found this is that our big ethernet ring with 30+
> switches was cut (fiber issue) behind 18th switch from L3 switch, and
> everybody hosted after 18th switch has some interesting packet loss
> issues.See the smokeping details for 18th switch on
> http://img171.imageshack.us/img171/3777/hop18kn4.jpg.
>
> This is not very logical for me, as switched ethernet works based on
> store&forward method so no old-school 5-4-3 rule applies as in shared
> collision (hub) environment. Anybody has a good explanation why adding
> more L2 hops to the path tends to kill packets?
>
> Regards,
> Pavel
>
> p.s. yes I know that building large ethernet rings is generally not a
> good idea, however sometimes due to large distances you have no other
> choice
>
>

This sounds like a spanning-tree diameter problem.
Spanning-tree limits the number of intermediate switches between the root
and each end-point.  Without tuning this value is 7.

This link describes possible issues with diameter:
http://www.cisco.com/warp/public/473/16.html#awkward

This link may help you resolve the problem:
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml

--
LR Mack McBride
Network Administrator
Alpha Red, Inc.