VLAN's basic working

The_guroo_2 · ‎09-02-2008

gusy i m bit new and i cant get my head around to a v simple thing.....suppose we have three switches A , B , C. a server is connected to a C switch and the port is gig1/5 (layer 2 switch 4948). Switch B is another layer 2 switch and connected with switch C (trunk port all vlans allowed) now swicth A is a distribution switch and all Vlans interfaces are configured on that.......so my question is if a server wants to talk to another server which is in diff vlan on same swicth C. How the communication works. how come switch c knows where to send the traffic and how come switch B knows where to send the traffic. ideally the traffic shd hit swicth A and it will do the inter vlan routing........do we need any gateway on swicth C and B to reach A....i m v confused just like my question all i wann aknow is how this whole communication gonna work step by step.....now the strange thing why in sh mac-address table all vlans mac comes with ffff.ffff what does that mean and why its like that.....thanks guys in advance

merryllem · ‎09-02-2008

No you don't need a gateway on sw B and C... but you do need a gateway (VLAN Interface) on sw A.

It would be helpful to if you uploaded sw configs if possible.

Jon Marshall · ‎09-02-2008

An example may help.

server1 = 192.168.5.10 255.255.255.0 vlan 10

server2 = 192.168.6.10 255.255.255.0 vlan 11

both servers are on switch C. As you say there is a trunk port to switch A allowing all vlans and the L3 SVI's for vlan 10 & vlan 11 are on switch A ie.

Switch A

int vlan 10

ip address 192.168.5.1 255.255.255.0

int vlan 11

ip address 192.168.6.1 255.255.255.0

So server1 has it's default-gateway set 192.168.5.1 & server2 has it's default-gateway set to 192.168.6.1.

server1 wants to send a packet to server2. server1 compares it's network address and subnet mask so

192.168.5.10 255.255.255.0 - from this it knows it's network 192.168.5.0

server1 then compares the destination address of 192.168.6.1 (server2) with it's own subnet mask (note server1 only knows about it's own subnet mask) so

192.168.6.10 255.255.255.0 - from this it knows server2's network is 192.168.6.0

So server1 knows that server2 is on a different subnet. So it looks in it's routing table and finds it's

default-gateway (servers generally only have default-gateways and not full routing tables)

server1 then arps out for it's default-gateway address. Switch A responds with the mac-address for

int vlan 10. server1 then sends the packet to switchA down the trunk link. The packet is carried with a vlan tag of 10 to switch A.

Switch A then inspects the destination IP address, sees it is for 192.168.6.10. It arps out for server2's mac-address and when it gets a response it then routes the packet onto vlan 11 and sends it back down the trunk link with a vlan tag of 11. Server2 receieves and basically does what server1 did ie. compares network with subnet mask, sends to default-gateway etc.

Hope this makes sense

Jon