Hi David Yes I've got all that and I have read Marko's article before. He is taking on the argument that the diameter parameter somehow limits the BPDU propagation, which it does not. It merely feeds into a formula to calculate the max-age and forward time. That is not the question I am asking. My question is this. If in the CCIE lab you are told to "optimize BPDU timing for the size of your network", or however they choose to word such a thing, basically "we want you to run the spanning-tree vlan x root pri diameter y" command, what do you set y to? Say you have the following topology: Cat 1 -> Cat 2 Cat 1 -> Cat 3 Cat 1 -> Cat 4. Here, you'd set diameter to be 3, as the max number of switch hops is 4 to 1 to 2 (or 3). That is fine. What if you have: Cat 1 -> Cat 2 Cat 1 -> Cat 3 Cat 1 -> Cat 4 Cat 2 -> Cat 3 Cat 2 -> Cat 4 Cat 3 -> Cat 4 All switches are fully meshed. Say that Cat 1 is the root. Here one port on the final 3 links will block. This again leaves diameter of 3. But say Paddy comes along with his JCB, and selectively cuts cables. Let's say you are left with: Cat 1 to 2 to 3 to 4. You now have a diameter of 4! Should you set the diameter at 4, even if all links are working; or should you set it at 3, because that is how it should be in the full working topology? In the real world, 4 is the safer answer, but we're talking about the CCIE lab here! George.
Date: Thu, 8 Aug 2013 21:32:36 +0200 Subject: Re: [OSL | CCIE_RS] STP Diameter Parameter From: [email protected] To: [email protected] CC: [email protected] Hello George,diameter is just a parameter for the macro that is triggered by the command 'spanning-tree vlan x root primary diameter n'. This parameter will make some adjustments to the STP timers. Marko has a good post about this topic: http://blog.ipexpert.com/2011/01/24/old-ccie-myths-spanning-tree-diameter/ Regards. On Thu, Aug 8, 2013 at 8:59 PM, George Leslie <[email protected]> wrote: Hi List! Another one that's caused me some confusion over the years! In the macro: spanning-tree vlan x root primary diameter y what exactly is the diameter? I've got my copy of Clark/Hamilton here, and they define it as: "the maximum number of bridges that any two end work stations have to go through to communicate". That is all well and good, but on what basis? Is that on the basis of a network that is fully functional, where STP is stable and all links are functional? Or do you take a conservative, worst case scenario, in which there have been link failures and the longest possible string of switches exist? E.G in the proctor labs set up, with all inter-switch links configured, you'd have a diameter of 2 as every switch is directly connected to the root switch. OR....do you say that if there are link failures, a frame may have to go through 4 switches to reach its destination? Or say that only the outside links are up (the diagonals of the square are shut). That would give a diameter of 3 if taking steady state into account, or 4 if you are catering for link failure. In an exam, which do you choose? George. _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
