08-11-2013 10:03 PM - edited 03-07-2019 02:51 PM
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?
Solved! Go to Solution.
08-11-2013 10:25 PM
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.
08-11-2013 10:25 PM
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.
08-12-2013 07:48 AM
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?
08-12-2013 07:55 AM
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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