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Community Member

Routing between VLANS

Hello,

I am working towards getting my CCNA and I have some confusion about VLAN routing.

Given this setup

  Switch1 ----- Router ------ Switch2 ------ Switch3

   |   |                      |    |         |    |

  PC1  PC2                   PC3  PC4       PC5  PC6

Can I put PC1, PC3, and PC5 on Vlan 2

and PC2, PC,4, and PC6 on Vlan 3

If so, how would the logical addressing for this work?

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Routing between VLANS

Normally we associate a Layer 2 VLAN at a given site with a unique Layer 3 network. Layer 2 VLANs do not pass through a router (without some advanced techniques). So without them being contiguous (all on one switch or set of directly connected switches) you won't get connectivity to work in the diagram above.

To make it work with two VLANs on each side (assuming each side is a single physical interface) you'd need to make the switch-switch and switch-router connections trunks and put logical subinterfaces on the router physical interface. You would then have VLAN1 and 2 on switch 1 and VLANs 3 and 4 on switches 2 and 3. That's more of a CCNP level task on the router as far as certification goes.

3 REPLIES
Hall of Fame Super Silver

Routing between VLANS

Normally we associate a Layer 2 VLAN at a given site with a unique Layer 3 network. Layer 2 VLANs do not pass through a router (without some advanced techniques). So without them being contiguous (all on one switch or set of directly connected switches) you won't get connectivity to work in the diagram above.

To make it work with two VLANs on each side (assuming each side is a single physical interface) you'd need to make the switch-switch and switch-router connections trunks and put logical subinterfaces on the router physical interface. You would then have VLAN1 and 2 on switch 1 and VLANs 3 and 4 on switches 2 and 3. That's more of a CCNP level task on the router as far as certification goes.

Community Member

Routing between VLANS

A further concept question:

If I had to make 4 VLANS

Vlan 1: PC1

Vlan2: PC2

Vlan3: PC3, PC5

Vlan4: PC4, PC6

Is there any way at all to make the router forward broadcast traffic from Vlan3 to Vlan1 on the other router sub-interface?

Hall of Fame Super Silver

Routing between VLANS

Not really. One of the definitions of a VLAN is a "layer 2 broadcast domain".

You can convert certain broadcasts into unicast traffic - that's what "ip dhcp helper" does. But, generally speaking, the goal of creating VLANs is to reduce broadcast domains by not forwarding them outside the local VLAN.

You can certainly allow VLAN 1 traffic to communicate with host in VLAN 3 - but it's via routing (layer 3) not via VLAN broadcasts (layer 2). Remember, the most common broadcast traffic in a VLAN is a host trying to find the MAC address of another host (ARP). For hosts in a remote VLAN, it ARPs for its default gateway (the router) and then sends the traffic via it.

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