I've got a pair of C2960G-48 switches (running IOS 12.2(46)SE, if it makes a difference) with redundant trunks in place. On these switches there are a bunch of VLANs that carry quite heavy traffic at times (enough to completely saturate the trunks each). I'd like to put in a per-VLAN cross-connected cable to help carry the load and avoid saturating the common trunk. The one thing I'm not sure of is how to prioritise this link in the spanning tree so that it will always be used for the VLAN if it's available, and the switches will only fall back to using the common trunk if the VLAN-specific link falls over (cable failure, etc).

Cisco isn't my native language, and I'd prefer not to experiment too much on these switches, as a visit from the fuckup fairy would have... consequences (but a staging environment with two more of these switches for me to play around with isn't cost-effective). So, I'm hoping that someone with IOS in their fingers can point me in the right direction.

Moah infos, for concreteness:

Trunk ports are Gi0/4[78] on both switches; one of the VLANs, 104, is on Gi0/3[78] on both switches. I'm going to put Gi0/36 into VLAN 104 and then run a cable between this port on both switches.


If I understand your OP correctly, you're talking about running multiple links between switches and then specifying different VLANs per each link. This would be less than optimal. A more ideal configuration would be to configure multiple connections in an etherchannel and then run all of your VLANs across that one etherchanneled link.

Edit: Thank you for the nice comment =)

Let me go one step further to state that the configuration as posed in the OP is not only less efficient, but likely places you at severe risk of creating a loop in your network that will bury it quite nicely. It could be done, but if you don't setup bpduguard and filtering properly, you will almost assuredly suffer from severe problems related to looped layer 2 traffic.

  • Absolutely the right way to do it.
    – PEra
    Nov 10 '09 at 20:32
  • It doesn't put him at any risk of creating a loop in the network with STP running, but otherwise yeah; Etherchannel would be the best route. You can also combine more than just two links in your aggregated ethernet if necessary.
    – sclarson
    Nov 11 '09 at 4:45
  • 1
    Yeah, +1 for fixing the real problem rather than supporting my half-arsed band-aids. Now if only I could deploy Etherchannel without causing an apocalypse...
    – womble
    Nov 11 '09 at 5:48
  • Etherchannel has been rolled out, and is working (both ports have non-zero input/output rates) but I'm still overloading the links, and I'm pretty sure I won't be able to avoid that because of the limited load balancing options -- no matter how many links I put into the etherchannel, my full-gigabit traffic will use the same port as some other traffic and I'll get horrendous packet drops like I am now. Round-robin load-balancing would be nice... I guess it's back to my original idea.
    – womble
    Nov 11 '09 at 15:21
  • I checked the documentation for the 2960, and it looks like you can run up to 8 ports in that etherchannel. If you're still overloading at 8 ports, you might want to make sure you're not killing the buffers... Nov 11 '09 at 21:21

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