VLAN, trunk port and administrative vlan

[Iago77]

I've set three vlan in the company where I work. Besides Vlan1 (administrative VLAN):

Switch#show vlan
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1
2 Support active Fa0/2, Fa0/3, Fa0/4
3 Intranet active Fa0/8, Fa0/9, Fa0/10, Fa0/11,
Fa0/12
15 ACME active Fa0/5, Fa0/6, Fa0/7

If I set port 1 (Fa0/1) as a trunk port:

(extracted from sh run)

interface FastEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk

From my knowledge, this port can understand "frame tagging" between another switch and the vlan can be extended.

Port 1 belongs VLAN 1 (administrative), so can this port forward frames all VLAN? Really, is it like by default port 1 would belong each VLAN?

The switch is a 2900XL and IOS version is 12.0

 

[vipergg]

No all that means is that port is set in vlan 1 and not one of your other vlans . You can set port 1 into any vlan you want . Vlan 1 is the default vlan right out of the box on all ports so unless you change it , it will stay that way . You can trunk across any port on the switch.

 

[Iago77]

From CCNA (3rd semester) v3.0: Chapter 9.1.4

"It is important to understand that a trunk link does not belong to a specific VLAN. The responsibility of a trunk link is to act as a conduit for VLANs between switches and routers."

Many thanks!

 

[vipergg]

Your statement is correct , all it is is a link that allows multiple vlans to be transported between switches and or routers . As packets come in from each vlan the switch "tags" the frame with a vlan id so that when it reaches the other end the switch or router knows what to do with the frame and to what vlan the packet belongs .

 

[kmcferrin]

As a related question, if you have multiple VLANs and multiple trunk links (say via multiple SFPs), can you restrict a VLAN to a specific trunk link and vice-versa?

In other words, say SwitchA has VLANs 1, 2, and 3. SwitchB has VLAN 2, and SwitchC has VLAN 3. Can you have two trunk links on SwitchA, one going to each of the other two switches, and make the A/B link VLAN 2 only, while the A/C link is VLAN 3 only?

 

[vipergg]

Sure but if you were only allowing one vlan on the connection you wouldn't need a trunk link just put the connecting link into the port you want it in. But yes you can specify which vlans are allowed on the trunk links .

 

[KiscoKid]

Kmcferrin

You can add or delete VLANs that are allowed on a particular trunk port with the 'switchport trunk allowed vlan 2' command in interface mode. You'd likely combine this with VTP pruning (you can enable pruning in 'vlan database' mode on the switch).

 

[ADB100]

VTP - does anyone still use this............... In fact does anyone still have campus-wide Layer-2 VLAN's?

If you have then it might be worth realising we are in the 21st century now.........