Help with routing through two layer 3 switches

[BigTex]

Hi there,

    I'm trying to solve this issue and I feel like I'm close, but I think I've hit the limit of my current knowledge and I don't even know what to Google anymore. I have 4 PCs in SITE A and another 4 in SITE B. Each site also has two subnets, one on VLAN 20 and another on VLAN 30.

SITE A:

VLAN 20 - 10.0.20.1 255.255.255.0

VLAN 30 - 10.0.30.1 255.255.255.0

SITE B:

VLAN 20 - 10.1.20.1 255.255.255.0

VLAN 30 - 10.1.30.1 255.255.255.0

Right now I can ping from any PC in SITE A to any other and the same thing for SITE B, however I can not ping from anywhere on SITE A to SITE B. I know this has to be possible, I think I'm just missing something. I'll attach a diagram as well as my Packet Tracer file. If anyone could help me I would really appreciate it!

Thank you!

[Harold Ritter · Accepted solution]

Hi @BigTex ,

Do you want to build a L2 or L3 network?

If L2, you need the same subnet for VLAN 20 in both site A and Site B. The same goes for VLAN 30.

If L3, you need to make the inter site link a L3 rather than a L2 link. You also need to configure routing between the two sites. 

I am attaching an example of both the L2 and L3 approach.

[BigTex]

Hi there, I’m trying to do a Layer 3 link. How do I configure that? I have
a trunk port on both sides connecting the two switches which are set to
allow their respective VLANs 20 and 30, but how do I tell the switch to
accept date from the VLANs on the other site?

[BigTex]

Hey there,

    So I was able to figure it out by using your L3 example. Thank you so much! If it's not too much of a bother, can I ask you about the IP route 0.0.0.0 0.0.0.0 192.168.12.2 line in the show run output that I see on switch 0? I understand that it's a route to the IP address of the interface on the other switch that Switch 0 is connected to, but is it really that simple? Again, really appreciate you taking the time to help me!

Also, I see that interface fa0/3 is no longer set to be a trunk port, how would that work if I wanted to only pass data from one VLAN but not the other? For example if I wanted to only pass VLAN 20, but not VLAN 30?

[Harold Ritter]

Hi @BigTex ,

> can I ask you about the IP route 0.0.0.0 0.0.0.0 192.168.12.2 line in the show run output

These static ip routes are needed to tell Switch0 and Switch1 where to forward ip packets to destination that are not directly connected. I used the default route (0.0.0.0/0) on both sides, but you could use more specific routes such as "ip route 10.1.0.0 255.255.0.0 192.168.12.2" on Switch0 and "ip route 10.0.0.0 255.255.0.0 192.168.12.1" on Switch1.

> Also, I see that interface fa0/3 is no longer set to be a trunk port, 

Bear in mind that in a L3 network, VLAN20 in Site A and VLAN 20 in Site B are completely different and packets are not forwarded at layer 2 between them, but rather at layer 3. This is the reason why the inter site link is configured as a L3 interface and not as a L2 interface (trunk).

> how would that work if I wanted to only pass data from one VLAN > but not the other? For example if I wanted to only pass VLAN    > 20, but not VLAN 30?

You could configure more specific static routes. For instance 10.1.20.0/24 on Switch0 and 10.0.20.0/24 on Switch 1, which would only allow bidirectional traffic between these two subnets. You could also use access control lists (ACLs) to be more granular about what you allow and what you deny between the two sites.