how many trunks are allowed in a switch ?

[SJ K]

Hi all,

Today I have came across NIC card that is able to tag VLAN as well as the tagging of VLAN in VM environment from the guest OS nic card.

I believe sometime tagging a VLAN on the host/guest devices is necessary as oppose to configuring the VLAN information in the switch

Q1) Therefore in the example listed, i believe that the physical connectivity between the host device's ethernet adaptor port and the network switch port have to be trunked -- since the host device's ethernet adaptor might be carrying multiple VLAN frames  -- am i right ?

Q2) Assuming this particular network switch(1) is being uplink to another switch(2) and the 2 switches will also requires to exchange VLAN frames.
Can I still create a trunk from switch(1) to switch(2) ?  bearing in mind that I have create a trunk earlier from my host device to switch(1)

Q3) In the event that switch(1) receive a VLAN tagged frame from the host device (A) via the 1st trunk -> how does it knows whether or not it need to transverse up another trunk link to switch (2) ?   

Does the frame get untagged by the receiving switch (1), see the mac address of the destination -> realise it must go through the uplink trunk port --> tag back the original vlan --> and send out via another trunk port to switch (2) ?

Q4) What happen when a switch receive a vlan tagged frame via its trunk port ? Does it need to untagged the VLAN information from the frame in order to look into the frame's destination mac address to check against the mac forwarding table ? Does it tagged back the VLAN information when it has determined to send the frame to another access port of that particular VLAN ? Where does the switch get the VLAN information from then, since it is already untagged ? or it is tagged at the access port when the particular frame is send there.

Regards,
Noob

[Jon Marshall]

For points 1) and 2) yes I think that' a fair summary.

For the last paragraph it's difficult to say. Peter says that using the VLAN ID does not optimise the lookup and because of broadcasts etc. the vlan ID in the mac address table cannot be used to determine all ports membership.

So it's unclear really as to why the vlan ID is in there at the moment.

Jon

[SJ K]

Duly noted.

Thanks Jon! 

[Jon Marshall]

Peter

Sorry for going on about this, it's just it touches on one of the things I have always wanted to understand properly but as you say that sort of documentation isn't readily available.

I suspect you are right about there being some sort of internal table or control structure built not just from dynamic mac learning but perhaps also initially populated from the actual configuration to account for things like broadcasts etc.

I shouldn't have made the comment about using the mac address table so casually because it wasn't right and a bit more thought at the time would have confirmed that but it's frustrating to have to say it's all internal and leave it at that :-)

I agree, if any Cisco person who knows how all this works is on these forums, please feel free to enlighten me (us ?).

Jon