question about STP

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Hi !

In your IT team we have a different point of vue of how we should manage STP on some of our sites.

Let explain me :

on your remotes sites a lot of them have more than one switches, the WAN link connection is always connected on 1 port on the first sites. If we have redundant link for one site it is connected on the 1 first port on the second switchs.

On all remote setup if WAN connection are'nt functionnal the office in question are not able to works on there computers, all servers are in central office.

On each site the first switch is elected as a root for Spanning tree topologies, but no second root is choice. In case of where the primary root failed we don't know which switch will be elected as root.

On our sites we have between 1 switch and 18 switches. All sites are connected in loop topology to make a redundant path for other switches in case of link or switchs failed.

Base on my read for BCMSN Certification exam I was understand we have advantage to make our second switch on each site secondary root switch to make sure if the primary root failed this switch will become the new root for the network.

Someone in our IT team think this : force the second switch of each site make STP more difficult to trouble shoot. And in case in where the current root failed this switch will have to be replaced in next 48 hours calender days. We have to provide 24h 7 days a week IT support on our network.

I have this problem too.
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The easiest way I think - with basic STP configuration is VLAN priorities.

Basically in sw1:-

spanning-tree vlan 1 2 3 (or your vlan's) priority 24576

In sw2

You can leave the STP as it's currently configured or:-

spanning-tree vlan 1 2 3 (or your vlan's) priority 32768

With the above configuration - sw1 has a higher priority for the vlans in STP - than sw2, so sw1 is ROOT. IF sw1 goes down or is off-line, then sw2 is ROOT in the STP, until sw1 is back on-line.

Also - if you have 2 "core" switches at a site, you can load balance via the switches:-

Basically in sw1:-

spanning-tree vlan 1 3 5 (or your vlan's) priority 24576

Basically in sw2:-

spanning-tree vlan 2 4 6 (or your vlan's) priority 24576

sw1 would be the ROOT for VLANS's 1 3 5 etc, switch 2 would be ROOT for VLAN's 2 4 6 etc?


P.S commands differ per version of IOS and hardware platform.

bvsnarayana03 Mon, 05/12/2008 - 05:15

Completely agree with andrew. Thats the best you can configure your LAN topology.

Considering your topology is redundant with 2 core switches, load-balancing the stp for vlans across the 2 switches is a good option.

oups I maybe miss something in my question.... we don't want to load balance trafic between the WAN connection... the redundant WAN link as lower banwith of the primary link (T1 connection for redundant WAN link where the primary link is a LAN-X 10Mbps).

And all of our sites are flat VLAN. And it is not plan to change this setup at this time !

thanks a lot !!


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