InterVLAN Routing

[lagapidis (Lazarus Agapidis)]

Hello Brian

Please allow me to step in and participate, as this is an issue that I had trouble in visualising and understanding. I hope I can be of some help. For the most part, you’ve got it, maybe I can make things a little bit clearer for you.

@wilder7bc

so four switches all four are in same subnet, and lets just say they are in VLAN 10.

I think I have been coming at this all wrong. when I thought of SVI I thought about Default Gateway when we gave it an IP but that does not seem to be the case???

It is possible to have SVIs on multiple switches be in the same subnet, and depending on how you have set up your network, you can make any one of those SVIs a default gateway for use by the hosts on the 192.168.1.0/24 subnet. In the same way, you can place multiple routers on the same subnet and have them function as multiple possible default gateways. The concept is the same.

@wilder7bc

so if I had four different PC each one could if manually configured have each have a different default gateway. which makes sense when you think about what a Default Gateways job is in regards to a specific device.

Yes, that is correct.

Keep in mind that:

• An SVI can be thought of as a (virtual) layer three interface that resides on a VLAN.
• It can be used for several purposes, one of which is to function as a default gateway for inter-VLAN routing, so devices on that specific VLAN will be able to communicate with other subnets, either on or off of the specific layer three switch.
• It can also used as an interface to configure the switch itself, either via telnet/ssh or via http.

@wilder7bc

so if I have an SVI on a switch with the 192.168.1.0 subnet for VLAN 10 and I have on that switch also VLAN 20, 30, and 40 and they all have an SVI interface then that switch will allow all that traffic from those VLANs and their different subnets to communicate.

Yes, that is absolutely right!

@wilder7bc

However, an SVI does not mean its a Default Gateway it only means it could be its real purpose is just inter vlan routing.

Keep in mind that inter-VLAN routing is still routing, and as such, it still requires a default gateway, so in this sense, an SVI will function as a default gateway.

I hope this has been helpful!

Laz