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Ran into something I've never seen before in a production network I'm new to supporting and wanted to get the communities take on it.

I have an EIGRP network that interconnects about 60 sites across a Metro-E network. The sites use a /24 network and use VLAN 95 for communication. The EIGRP endpoints are mix of routers with sub-interfaces and switches with SVI's.

What I noticed is that some of the sites with switches have spanning-tree turned off for VLAN 95 (EIGRP VLAN) and some sites do. Further, I found an old 6000 series switch that has elected itself as the root bridge for VLAN 95.

What are the ramifications for this being off? I've never worked on any network where spanning-tree has been turned off. As far as I'm aware, spanning-tree being off causes MAC and broadcast instability but again, never seen this in production.

Should I turn it back on everywhere since it's essentially a connected Layer 2 VLAN EIGRP is using for L3 EIGRP transport or turn it off everywhere. Scratching my head here...

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switch that has elected itself

You should never leave the root bridge to chance. If the current root switch goes offline or is rebooted your spanning tree needs to reconverge, causing temporary connectivity failure. Also, the root bridge is the one that traffic always flows over between branches.

Select the most central (likely core) switch as root and set its bridge priority to 0. Disallow any other switch to be set to 0 and turn on root guard on all ports.

On all other switches you control turn on root guard on all ports but the one(s) leading to the root switch.

What are the ramifications for this being off?

STP has no impact on EIGRP. However, those switches with STP turned off do not participate in redundant link detection. If they are part of a loop, there's a broadcast storm and the network drowns.

If you don't control the other switches you should talk to the other admins and make RSTP/MSTP mandatory. You should also agree on which switch is root (yours) and how to deal with topology change notifications (TCNs).

Additionally, you should activate broadcast limiting (to perhaps 10%) on all ports towards 'alien' switches for damage control - better safe than sorry.

With network-savvy peers and decent switches you should consider using MSTP with multiple instances. That way, you could run a common spanning tree (CST) among the peer switches, and each peer could run a tree of their own inside their LAN.

In case your running PVST+, the spanning tree is only valid for the trunked VLAN(s) anyway.

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  • Thank you for the incredibly detailed answer Zac67. Thankfully I control all the switches and can make the changes you suggested :)
    – user68992
    Aug 9 '21 at 19:19
  • @user68992 Don't forget to vote for a useful answer and consider accepting it for your question. ;-)
    – Zac67
    Aug 9 '21 at 19:33
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Spanning-tree is a loop detection / prevention mechanism. As such, it has nothing to do with EIGRP. Unless you have loops, or parallel layer-2 links in the network, STP isn't doing anything for you. If there are more than 7 hops between any two points, STP won't detect loops. (if you tweak the timers, it can go out to ~14, but, don't.)

As Zac points out, root selection should not be left to chance as it can lead to poor path selections -- where there are loops. Without a diagram of your network, it's hard to say if STP is necessary, actually doing anything, etc. In general, there's few reasons to ever turn it off.

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