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L3 Routing question

L3 Routing question

L3 Routing question

Hi all,

I'm a little confused on how to correctly configure a L3 switch

I have a couple of 2950 switches as access switches, I have my clients connected to them. I have access port configured for different Vlans, so VLAN 10, VLAN 20 etc. I need some of the hosts to be able to communication with hosts on different VLANs ( simply enough requirement)

So from memory I have to create the VLANs then configure trunk ports from the access switch to the L3 switch and then give the VLAN's an ip address on the L3 switch - > something like....

config t
int vlan 10
ip address
no shut

of course enable ip routing on the switch.

Can something explain the routing process to me. What I understand ( please correct if I am wrong) but assigning an ip address to the vlan allows the L3 switch to route between VLANs ( this is called a SVI interface ) so I dont have to do anything else? I just have to make sure that I have trunks from my access switches to the L3 switch.

My question how does the accesss switch know how to send the frames to the L3 switch. Don't I need some sort of statement to tell the switch to pass the frames up the trunk to the L3 switch to deal with? something like gateway of last resort on a router?


RE: L3 Routing question

You are correct about the routing.

So long as VLAN 10 and VLAN 20 are both trunked to tyhe layer3 switch, and you create a VLAN interface on the Layer3 switch for each VLAN, then (do a "show ip route" to see this) the Layer3 switch will be "connected to both networks and will therefore route between them.

How do the frames go to the Layer3 switch?
Let's summarise:
You have clients on VLAN 10. Those clients have the router address for their VLAN configured as their default gateway.
When a client on VLAN 10 attempts to send a packet to a client on VLAN 20, the first thing the client does is compare its subnet with the destination subnet (that's what the subnet mask is for). As the two don't match, the client does not send out an ARP in order to resolve the destination IP to a MAC address, instead it addresses the frame with the default gateway as the destination MAC address. The router receives the frame, breaks it open, throws away the header, works out where the destination IP address is, puts a new frame header on it and forwards it.

RE: L3 Routing question

Thank you VinceWhirlwind. Excellent description!

RE: L3 Routing question

what vince said and as long as you have the command :

ip routing

enabled on your global config....

We must go always forward, not backward
always up, not down and always twirling twirling towards infinity.

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