I have a cisco 3560 switch with 3 routers and 4 servers, and an uplink port connected to it, all in the same VLAN.

Until lately the uplink was a trunk port connected to another cisco switch, and configuration of the VLAN was made by the engineer of the backbone ISP.

A few days we changed backbone provider, the uplink port is now connected to another switch port via fiber, without any trunking. The other side are telling me that their switch port is learning a new mac for the port allocated to me. I believe what is happening is that all the mac addresses of my servers and routers are being forwarded plain to their port, and is worsening our network performance.

Any advice on how can a fix this? Possibly without a new router or a trunk, as I need to make changes soon. Can I forward just one mac to the outside switch?

Feedback is greatly appreciated!


This is how switching works. Why do you think it's worsening your network performance?

If you want to present only a single MAC to the outside switch, you need to use routing instead of switching. While that may have other advantages, performance usually isn't one of them. At best, you can route as efficiently as you can switch, which would make it have no effect on performance.

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  • Thank you David! So is it ok to forward 7 different mac addresses to just one switch port? I say performance is worse because I had no complains before when the trunk was on with the other provider, now I hear customers are completely happy with the service. – Ed.C Oct 22 '11 at 20:36
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    Without knowing a lot more about your network configuration and how you are connected to your provider, I can't really advise you to do or not do specific things. Generally, if you have the hardware to do it, it is preferred to route to your provider because some things are only possible that way and it's considered ugly to have a broadcast domain that spans administrative borders. – David Schwartz Oct 22 '11 at 20:39
  • I have a vlan with my backbone provider that gives me 4 ports in 4 different towns. First thing next week I am planning to co-locate a fairly decent router (not cisco) to manage my domain. The problem is that each of these outlets goes to one of my 4 cisco switches (sfp port) via some kilometers of fiber, where my other equipment is located. Latency is around 1 ms from each end to the other, not bad for me. I guess I will have to use trunking. So how many ports ports do you think I will need at the co-location point? One for the subnet vlan and one for the uplink? Or can I use just one? – Ed.C Oct 22 '11 at 20:59

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