Downside of making a 10/100 port a trunk

[vzvonarov1]

I have a Cisco switch at work that has 3 VLANs configured. What I want is an administrative machine plugged into the switch with a single ethernet cable that will be part of all 3 VLANs (one interface per VLAN). Normally, I would do this by making one of the switch's gigabit ports a trunk and configuring the machine to use VLANs, but both of the switch's gigabit ports are taken. My question is this: can making a 10/100 port on the switch into a trunk that carries all 3 VLANs have any negative effects? What's the worst that can happen? Can it impact performance on the VLANs for other clients? Will the port start dropping packets if, say, I tried to copy files at 100Mb/s on each of the 3 VLANs at the same time, and will this effect anything (other than speed), or will TCP recover from this?

Oh and yes, I know that this is something that can be done with a router, but I have reasons for needing it done this way.

[Peter Paluch · Accepted solution]

Hello Vitaliy,

If I understand you correctly then you do not want the administrative machine to perform inter-VLAN routing but simply to be a member of 3 VLANs at once.

This setup using a FastEthernet interface is not going to have any ill impacts on clients in those 3 VLANs. Regarding the administrative station itself and the possible bottleneck there, the situation is identical to a situation where many other clients start sending their data to a single target. They can possibly exceed the capacity of the link to the administrative station, in which case the switch will start buffering the frames and sending them at the available rate to the administrative station. If the buffer of the switch overfills, some frames will be lost. TCP will take care of that, including slowing down the transfer. UDP will ignore it. Note that this can happen anytime, even between stations in the same VLAN.

The only other thing I can think of is that the administrative station will be receiving broadcasts/multicasts from all three VLANs. Whether that is significant or not depends on the amount of multicast/broadcast in these VLANs.

Best regards,

Peter

[vzvonarov1]

Exactly what I wanted to know. Thank you.