[c-nsp] gigabit ethernet ring for metro ethernet deployment

[Michael K. Smith - Adhost]

Hello Affan:

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-
> bounces at puck.nether.net] On Behalf Of Affan Basalamah
> Sent: Thursday, January 04, 2007 8:58 AM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] gigabit ethernet ring for metro ethernet deployment
> 
> Hi all,
> 
> I just want to ask for your opinion in deploying gigabit ethernet ring
> for metro ethernet MAN deployment. The information I got is that
> gigabit ethernet ring has drawbacks in ring capacity bandwidth for L2
> metro deployment. I know, gigeth doesn't been designed to allow ring
> topology, so creating p2p link with layer 2 will cause some
> limitation, thanks to the spanning tree protocol.
> 
> I want to know, is it possible to deploy L3 point-to-point link to
> create gigeth ring that can overcome the limitation above ?
> 
> The goals that I was trying to achieve is to design and deploy metro
> ethernet ring for MAN backbone with the smallest budget possible, and
> I find that metro gigabit ethernet switch is quite cheap these days,
> compared to the RPR and sonet/sdh-based solution.
> 
> Thank you all for your information,
> 
> Regards,
> 
> -affan

You can create a ring with Ethernet, you just need to be aware of the
number of maximum hops between spanning tree members.  I'm not sure if
there is a "real" limit to the number of hops, but I've always heard 7
is safe.  If you wanted to go above that you might want to consider
cross-linking disparate spanning tree domains that feed multiple rings.

We had a ring of 3550's over a WAN of about 2000 fiber miles and it
worked great, most of the time.  We did get hit with bugs a few times
that caused cascading failures, and these become very hard to mitigate
in a ring because, by the time you fix one switch the issue has moved on
to the next, and so on until you're back to the first switch having
issues.  I would strongly recommend having some sort of Out of Band
management for your devices should you go for this topology.

Regards,

Mike