Why switches send traffic to Router if it has a MAC ADD Table?

[tiateng1]

Excuse the terrible title. My question is if a Switch has two data VLANs (Accounting/Marketing) and it maintains a MAC Address Table, When a switch receives traffic from an end user device connected to the switch and the device its trying to send traffic to on the same switch but a different VLAN, does the switch not just send the data to the interface that receiving device belongs to? Why does it route traffic on a different network if we are dealing Mac Address not IP.

[MHM Cisco World]

If you draw follow then

host in vlan-A will send ARP ask mac of host in vlan-B

How will reply to this ARP request? 

It not host in vlan-B it the router interface in vlan-A 

So when host in vlan-A send traffic to host in vlan-B the traffic use Mac of router interface not real Mac of host vlan-B 

That why SW can't forward traffic directly but it send to router and from router the traffic re-send to host in vpan-B

MHM

[liviu.gheorghe]

Hello @tiateng1 ,

vlans are a way of separating a network in different broadcast domains. Hosts in the same broadcast domain, which also share a common IP subnet, can communicate directly using the ARP protocol to determine the MAC address of the destination host.

In case the two hosts are in a different vlan, or different broadcast domain for that matter, the broadcast ARP request of the originating host will only be received by hosts in the same vlan.

This is where the router comes into play - he is part of the two vlans and he respond to the ARP request on behalf of the destination host with it's own MAC address. Now the originating host has a destination MAC address and can send the frame to the router. The router being a member of the two vlans, receives the frame from the originating host, looks at the destination IP address, sends a broadcast ARP request in the second vlan, gets the MAC from the destination host and sends the frame directly to it.

Hope this helps.