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So I was understanding how inter-vlan routing happens using L3 switches (using the c3650 series) and came across something I couldn't find solid information on (or I don't know how to search for it). I was trying to do inter vlan routing between 3 switches. I created two SVIs to represent hosts and assigned them to VLAN 10 and 20 respectively. Without using the ip-routing command, I was able to ping SVIs belonging to different VLANs on different switches! All the switches were configured with VLAN 10 and 20 but what I didn't understand is how they were able to contact each other without the routing functionality enabled!

When we do RoaS, we have a router that does the routing function for us. But when it comes to an L3 switch, without enabling ip routing, how is it routing between the two SVIs? They're part of two different subnets!

I had heard that this phenomenon was only because SVIs are internal to the switch and hence there's no egress ingress traffic to need routing. On enabling debugging, I came across the term "LOCAL CLUSTER" and on searching about it, couldn't exactly find the solution I was looking for but I feel it might have something to do with this.

Any help would be much appreciated! Thanks in advance.

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You need to understand the the layer-3 switches are really layer-2 switches with a virtual router inside. Traffic that comes into a layer-2 ports will still use layer-2 first when traffic is on the same VLAN. That means any ping from VLAN 10 to any address on VLAN 10 will travel via VLAN 10 to the pinged address. The same for VLAN 20. When you ping from VLAN 10 to VLAN 20, your gateway, one of the SVIs on one of the switches, it will place the packet onto VLAN 20, where it will be switched at layer-2.

The way to connect the routers with routing would be to use layer-3 interfaces. You use the no switchport command on an interface, and configure that interface like a router interface. Each layer-3 switch would have its own separate set of VLANs, and it doesn't matter if they are the same numbers, they will be separate VLANs. The switches will only be connected via layer-3 interfaces, instead of the layer-2 interface you currently use.

Just think of it like the layer-3 switches are layer-2 switches with external routers connected to them by the SVIs. That is basically how they work.

  • Hey Ron, thank you for your answer, I understand what you've explained above. But now say for example, instead of using SVIs, I use actual end hosts. Why does the routing not happen? When I use SVI's to represent hosts, I don't even need a default gateway for them! But the moment I use actual hosts instead of SVIs, I require 2 SVIs in VLAN 10 and 20 for a default gateway and the ip-routing command on that L3 switch as well. – Izy- Jun 15 '16 at 6:14
  • You don't need to set a default gateway for the switch if you have routing enabled. If you have two hosts on a switch, one connected to each VLAN, and routing is enabled, they should be able to ping each other. If you set up a routed link to a different switch, you need to share routing information between the switches, either statically, or with a routing protocol. – Ron Maupin Jun 15 '16 at 13:40
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I guess you have pinged them from the switch. It works like that because when the source IP address is not specified the device sets it to the port's IP address. When you have couple SVIs, you have a number of IP addresses to choose from. That's why you are able to ping / connect to the switches in the different VLANs (As long as you have an SVI which belongs to that subnet) You do not need a routing because you are moving around the same subnets. The source IPs are changing depending on the subnet you want to connect to.

  • Thank you. That's alright, but even when a ping one VLANs host with the source of an IP of a host that's from the other VLAN on another switch, it still works. – Izy- Jun 14 '16 at 8:51

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