Layer 3 switching Help!

Hello

My hair is turning grey from learning layer 3 switching.. I really can't figure out how to make all vlans on all switches able to ping each other through a layer 3 switch..

So ive got a layer 3 switch in the center of my network. In the layer 3 switch ive got two L2 switches connected with trunk lines. All three switches has Vlan 10, Vlan 20, Vlan 30 and Vlan 99 (Management) configured..

My Layer3 switch has all the default gateways of my vlans configured:

Vlan 10:
10.145.10.1
Vlan 20:
10.145.20.1
Vlan 30
10.145.30.1
Vlan 99
10.145.99.1

So what i dont understand is what IP Addresses i give out to my L2 switches in order for all vlans to be able to ping eachother. As far as i understand, all i need to do is give my layer 2 switches a ip address in the management vlan aswell as a default gateway?

So i have configured my first layer2 switch with an ip address of 10.145.99.10 and my second switch with 10.145.99.20 aswell as an ip default-gateway of 10.145.99.1
I can ping sucessfully till and from the layer 3 switch on vlan 99, but none of my other vlans (10,20,30) are able to reach the Layer 3 switch.
It makes sence, but at the same time it dosen't.. Cause how is a pc on vlan 10 supposed to know its default gateway when the switch it is connected to has a default gateway of 99.1 which is another subnet. I could change my switchs default gateway to 10.145.10.1 so that my PC on vlan 10 would be able to reach the layer 3 switch, but what about the other pc's in vlan 20 and 30? I can't have multiple gateways configured..

I know i make it really confusing, but i dont know how to explain my problem. Overall my issue is that i dont understand how to configure a layer 2 switch with multiple vlans, and make them all able to reach a layer 3 switch.

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Comments

Staunchy

Have you enabled IP routing on the L3 switch? Your pc's connected to the L2 switch must have the default gateway of the VLAN switch virtual IP interface.

Default gateway on a L2 switch is only for remote management of the switch. When PC on VLAN10 on the L2 switch sends a ARP broadcast the L2 switch will then add a tag to that broadcast frame to indicate that it belongs to VLAN10 and forward that broadcast out on the trunk link to the L3 switch, the L3 switch will receive the ARP request and see that the traffic is destined for it's VLAN10 virtual IP then because it is an ARP request it will respond with it's mac address to the client pc.

Magic Johnson

As far as I was aware you just created an SVI for each VLAN, gave it an IP on that particular subnet and then did no shutdown to enable it. After that any frames received on that VLAN interface are routed appropriately by the L3 switch. Obviously as Staunchy says ip routing must be enabled. Each port must be configured to be a trunk too.

I think that's right.

So if vlan 1 on l2 switch connected to a trunk port on l3 switch then you just give that VLAN interface on the l3 switch an IP on the vlan subnet configured on the l2 switch. because it is tagged the switch will forward the frame out of whatever interface it knows vlan 2 say was connected, then the l2 switch would store and forward it to whatever interface was intended to receive the frame.

I appreciate that is about as clear as mud.

(and possibly not correct, but I am learning like you so am interested to hear feedback!)

nb-

Thanks! All i needed to do was to execute ip routing on the Layer3 switch. Everything works now.

f0rgiv3n

nb- wrote: »

what IP Addresses i give out to my L2 switches in order for all vlans to be able to ping eachother.

Just to clarify on here. Technically the IP addresses on the L2 switches have nothing to do with connectivity between the vlans. You could leave the L2 switches with no IP and only manage them by console. As long as you have the trunks and VLANs configured correctly to get to the gateway (L3 switch) then your routing would work.

The IP addresses on the L2 switches is literally only for management of the switches. I just wanted to make sure you knew that