Kevin Dorrell, CCIE #20765

06 Mar 2008

NMC Lab 10 – Layer 2 challenge

Filed under: Bridging on routers, LAN Switching — dorreke @ 23:16

Today I did NMC Lab 10.  This lab is a layer-2 blockbuster.  The most difficult part consists of two Spanning Trees comprising a mix of switches and IRB bridges.

I am seeing patterns in the style of the NMC labs.  Very often the problem will consist using a feature for something it was never intended for in the first place, and then using the configuration tools to get round the problems.  In this case it constructed two Spanning-Tree networks, each consisting of four routers acting as bridges, and VLANs on the four switches.  In this case, the biggest issue was that some of the bridges were multipoint Frame Relay subinterfaces in a hub-and-spoke arrangement.  Of course, the issue is that you cannot broadcast from spoke to spoke, and Spanning Tree assumes that you can.  You therefore have to fiddle the Spanning Tree so that only one spoke is in forwarding, and the others are in blocking state.

They then specify multiple constraints about which switch or bridge is root, and the paths that the traffic should take.  What you are left with is a sort of Sudoku puzzle where you have to work out the link costs on the links you are allowed to touch by deriving them from the links you are not allowed to touch.  You have to consider five sections of the lab, 10.1, 10.2, 10.3, 10.4 and 10.10 simultaneously.

At one point, they ask you to minimise the convergence timers.  I did so, and one of the Spanning Trees became unstable.  Strange … I used the same timers as in the SHOWiT.  May be something to do with the fact that I use 2950s for CAT3 and CAT4 instead of 3560s.  Wrote a note on the DISCUSSiT about it.

I think I would have failed this one.  It took me the best part of four hours to get the layer-2 working and the full inter-network connectivity, and a further hour and a bit to get the full IP connectivity.  The BGP took some time too.  Not so much of a challenge, but just big.

I like Ethan’s comments on this lab.


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