VLAN

[Mohdz]

Hi Guy's,

I've just start my CCNA, and i have a question, why when we have two switch's and more than a vlan in each switch, we need a physical link for each vlan (without using a trunk ) .

Sorry if my question seems silly 

[ammahend · Accepted solution]

there is no such rule that you have to use individual link for each vlan, you can use a single link as trunk and allow multiple vlan on the trunk.

if you don't use trunk, it means you are using access port, when you use access port, the port tags the traffic coming to the port into a vlan, an access switch port can tag traffic only in 1 VLAN because the number of headers available inside an Ethernet frame as per IEEE standards allows room for only 1 VLAN information to be carried. So for each VLAN you need individual port on this case.

[Joseph W. Doherty]

". . . when you use access port, the port tags the traffic coming to the port into a vlan . . ."

Possibly, more correct to note an ingress frame and access port, on a VLAN capable switch, is assigned to the VLAN configured for that access port, i.e. we really don't know whether the switch uses an 802.1q frame tag, internally.  (It's possible some switches do, as perhaps also possible that switches use ISL tagging, internally, too.  Consider some older switches that supported .1q and ISL, such switches might, internally, keep track of VLAN using one of those frame formats for either external tagging, match internal tagging to ports using that tagging format, or keep track of frame's VLAN in a totally different manner.)

". . . an access switch port can tag traffic only in 1 VLAN because the number of headers available inside an Ethernet frame as per IEEE standards allows room for only 1 VLAN information to be carried."

Regarding an access switch port being only able to tag only 1 VLAN, consider basic access ports don't (I believe) generate/accept tagged frames, but data/voice access port can generate/accept tagged frames for one VLAN (the voice VLAN), of two (data and voice VLANs), assigned to such an access port.

[Martin L]

yes, that is true but not necessary since you can use trunk.  You could use a single access link for each your vlans; however, I never seen that set up since invention of the trunk link; trunk links will carry all vlan by default unless you modify which vlans can go thru (allow vlan list).

Regards, ML
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[Richard Burts]

Perhaps we should ask the original poster to clarify some things in the original post. Both of the responses so far have reacted to the part of the question asking about 2 switches and more than one vlan. And they agree that 2 switches with more than one vlan the optimum solution is to use a trunk. But the original question contains an important qualification for the question "whitout using a trunk". If we understood why not use a trunk we might be able to provide better answers.

And yes - if a switch has access ports and not trunk ports, then an access port can carry traffic for only one vlan. So if you have multiple vlans then you will need multiple access ports (one access port per vlan).

[MHM Cisco World]

SW1-SW2-SW3

the SW1,SW2 & SW3 have VLAN1-4
if you not run trunk then you need one link for each VLAN. 
here the Q why I need this ?
this is L2 SW if client connect to SW1 VLAN1 need to talk to client  connect to SW3 VLAN1, 
the SW2 need to allow VLAN1 traffic to pass if there is no link between SW1-SW2 and SW2-SW3 then the traffic will drop and both client can not talk to each other. 

[Joseph W. Doherty]

". . . why when we have two switch's and more than a vlan in each switch, we need a physical link for each vlan (without using a trunk ) ."

Well assuming you want to exchange traffic, within the same VLAN, on different switches, you'll need some physical link to carry that traffic (for those shared VLANs).

e.g. suppose switch 1 has VLANs 1, 2, 3, 4 and switch 2 has VLANs 3, 4, 5, 6.  Those two switches only have VLANs 3 and 4 in common, but if you only want to share VLAN 3 between those two switches, you would only need one physical link.  Ditto, if you only wanted to share VLAN 4.

However, if you wanted to share VLANs 3 and 4, then you would need two physical links (one interconnecting a VLAN 3 port on both switches while the other physical link interconnecting a VLAN 4 port on both switches).

If you wanted to extend the VLAN 2 on both switches, besides also needing a physical link dedicated for that VLAN, you would need to also define VLAN 2 on the switch where it's not currently defined, and assign a port, on that switch, to connect to a VLAN 2 port on the other switch, to VLAN 2.

BTW, what I've just described, determining which VLANs are shared between switches, by allocating such their own physical link, can also be done on a trunk link where you "allow/disallow" VLANs to use the trunk.

Also BTW, as most modern switches support a data and voice VLAN on an access port, you could also interconnect two VLANs, on a single physical link.  I.e. in my example, VLANs 3 and 4, could be shared between two switched using a data/voice access port.  (NB: for it to work correctly, both switches would need to insure the data and voice VLANs are the same VLAN number.)

Lastly, if the switches in question were L3 switches, you could route between the two switches using a single physical link.  However, although VLANs might use the same VLAN number on both switches, they would not be the "same" L2 VLAN.  I.e. just as VLANs 3 and 4 are not the same VLAN, with a L3 connection, VLANs 3 and 3 on the two switches, would be much like VLANs 3 and 4 on the same switch.

[Mohdz]

Thank's Guys for your help