trunking and broadcasting

liven

So is this correct

a trunk between two switches does not create a broadcast domain?

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dtlokee

The VLAN creates the additional broadcast domain, the trunk carries frames for all active VLANS so it will extend the broadcast domain between switches. So if you had vlans 10,20, and 30 created on 2 switches connected via a trunk link you would ave 3 broadcast domains within the switch (well 4 since VLAN 1 is there by default and can't be removed)

Pash

Vlans are in essence seperate networks:

The best way to remember where your broadcast domains are gonna be is to imagine what will see a layer 2 broadcast from a host...on which ever network (in this case). So a host ARP's for example, which host's are gonna see that FF:FF:FF:FF:FF:FF?

liven

dtlokee wrote:

The VLAN creates the additional broadcast domain, the trunk carries frames for all active VLANS so it will extend the broadcast domain between switches. So if you had vlans 10,20, and 30 created on 2 switches connected via a trunk link you would ave 3 broadcast domains within the switch (well 4 since VLAN 1 is there by default and can't be removed)

\

First let me say thanks, but if I may can I pick your brain a little further?

Cool, ok I got the fact that each vlan is a broadcast domain.

So we have two switches, connected via trunk. VTP is running, so we have vlans 10,20,30 on both switches, and the trunk connecting the switches.

Do we have 3 or 4 broadcast domains? I know 10,20, 30 are the same broadcast domain on each switch, and lets say nothing is connect to vlan 1 right now. So does the trunk link between the two switches create vlan 4 or is it basically lumped in with the 3 broadcast domains since it carrires traffic for all the other trunks?

APA

all the vlan traffic is carried over the trunk link, trunk link belongs to the native VLAN (1 in most cases) - depends on whether you have configured pruning etc.... though.....

So you have 4 broadcast domains as even if nothing is on VLAN1 it is still considered it's own broadcast domain!!!!

Pash

liven wrote:

dtlokee wrote:

The VLAN creates the additional broadcast domain, the trunk carries frames for all active VLANS so it will extend the broadcast domain between switches. So if you had vlans 10,20, and 30 created on 2 switches connected via a trunk link you would ave 3 broadcast domains within the switch (well 4 since VLAN 1 is there by default and can't be removed)

\

First let me say thanks, but if I may can I pick your brain a little further?

Cool, ok I got the fact that each vlan is a broadcast domain.

So we have two switches, connected via trunk. VTP is running, so we have vlans 10,20,30 on both switches, and the trunk connecting the switches.

Do we have 3 or 4 broadcast domains? I know 10,20, 30 are the same broadcast domain on each switch, and lets say nothing is connect to vlan 1 right now. So does the trunk link between the two switches create vlan 4 or is it basically lumped in with the 3 broadcast domains since it carrires traffic for all the other trunks?

Just to reiterate what I said mate. Vlans are exactly what they say on the tin, virtual lans....each one creates its own broadcast domain, just remember the all F's broadcast, who will see it? All the trunk does is carry the traffic from switch to switch, you do not create any broadcast domain by doing this.

dtlokee

The trunk links do not create an additional broadcast domain they just extend the same one between switches. If a device connected to VLAN 10 on one switch sends a broadcast, the switch will forward it to all ports on the same switch that are part of VLAN 10, this includes the trunk port. The trunk port will tag it as part of VLAN 10 send it out, the other switch will receive it, strip the tag and send it to all interfaces on that switch that are part of VLAN 10 including any other trunk ports.

SW1#sh interfaces fa0/16 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/16      auto         n-isl          trunking      1

Port        Vlans allowed on trunk
Fa0/16      1-4094

Port        Vlans allowed and active in management domain
Fa0/16      1-5,10-13,22,33,44,50,56,113,117,300,500

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/16      1-5,10-13,22,33,44,50,56,113,117,300,500
SW1#

SW1#sh interfaces fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1

Port        Vlans allowed on trunk
Fa0/13      1-4094

Port        Vlans allowed and active in management domain
Fa0/13      1-5,10-13,22,33,44,50,56,113,117,300,500

Port        Vlans in spanning tree forwarding state and not pruned
Fa0/13      none

The first output shows the interface is a trunk and notice the output of "Vlans allowed and active in the management domain" then the output of the "Vlans in spanning-tree forwarding state and not pruned" So what the output shows for interface fa0/16 is the trunk is going to forward traffic for all those VLANs across the trunk (this includes traffic allowed by the mac-address-table, Broadcast and unknow unicast). Interface Fa0/13 has been blocked by STP so it shows that "none" of the vlans will be forwarded across the link.

You can basically think of a trunk link as an access port that belongs to all of the VLANs listed under "Vlans in spanning-tree forwarding state and not pruned", since they will forward traffic for all of those VLANS