Auto MDI/MDI-X + conference rooms + bored == loop

Sat Mar 27 00:19:28 UTC 2010

On Mar 26, 2010, at 7:48 PM, Chuck Anderson wrote:

> If you have 2 network jacks next to each other in a conference room, 
> do they each get configured as a separate "user"?

Indeed, most of the buildings have a 'community room' like that -- but all the deployed ports (unless ordered differently) will get incrementing-vlan assignments, so indeed, they'd be different vlans back to l3 core. 

> What happens if a 
> user connects them together?

Nothing, basically, as the network from edge port towards IP edge is (or should be) loop-free. The router will hear DHCP req's on 2x ints, but the client will (should) pick the first-heard response. Depending on the DHCP client implementation, it may wedge/break, but I haven't encountered one in testing. For higher-availability from edge towards IP core, LACP/PAGP provides link-independence, and UDLD/802 OAM provide something of a decent safety-net for breakage detection in metro-spans over other providers/resellers. 

> What happens if a user plugs a desktop 
> switch into one of them, then connects two ports on *that* switch 
> together?

In my example config, bcast or mcast over 100 pps shuts the port that's receiving the bcast or mcast's down -- but, that's a configurable action. It could discard them, police them, or just report a syslog/trap to the NMS... Of course, this is all switch-vendor specific, etc.

> Would this work in a collapsed L2/L3 core (no agg, no L3 at edge)?

Oh, indeed -- and is. The UTOPIA network (http://www.utopianet.org/) in SLC, Utah, is doing basically this for it's ISP-reseller tiers. ISP's get customers on vlans or Q-stacked vlans, and do what they will with it. The ISP's I've talked with have tended to use Juni ERX for this, but there's nothing stopping one from using IOS, or another vendor that can do this trick. It just implies something to consider in the layer2 transport network (support for man l2 addrs in cam, QinQ, etc) at design-time.

> When doing 1:1 VLAN:Port mapping, can you do more than 4096 
> VLANs/ports?  Or are you doing QinQ?

Indeed -- q-stacking enables this. In most cases, I don't backhaul more than a few hundred vlans per building -- if it's over 200 to 250 ports/jacks, I generally drop local 3550/3560/3750 or cpu-based boxes on-site, routing towards the metro edge/backbone.

> Cool, but I'm not sure this will work in my non-Cisco campus 
> environment with 10,000 edge ports.

Ahh; a pickle. C and J do indeed enable this in many of the popular boxes, which is great. That's not to say other vendors don't have something like it--the concept is perhaps the most valuable bit to discuss here, imho; the vendor-particulars are less important.

-Tk