VLAN Routing - Will every L3 switch in the traffics path route if enabled?

[Steve21]

Good Morning All,

 

Slightly badly worded title, but if you have multiple L3 switches setup with VLAN routing will they always route traffic that passes through them, or only if it's set to actually be the Default Gateway etc?

 

One of my schools has their ISP router still doing their VLAN routing, which is causing a bottleneck for things like WiFi traffic getting shunted back and forth across buildings to the ISP router so I'm looking to move it onto their own switches, but was wondering if I setup L3 routing on a "core" switch in each main building, would it just be able to route it within said building without changing every clients DG to a specific one etc (effectively with their current ISP one as the DG still set on each device)

 

I know it'd mean managing a few for new VLANs/IP ranges etc, but at least for the short term that'd remove the bottleneck and mean they'd easily route the traffic on their own devices

 

Many thanks,

Steve

[danielghares]

A L3 switch will only route traffic if it's set as default gateway. You can have multiple layer 3 switches in the same subnet, but the only one that will route traffic is whichever is set as the default gateway.

 

What needs to be done depends on how your VLANs and subnets are setup already. If clients across all your buildings are in the same subnet (i.e they're all in the same vlan), then adding a L3 switch at each 'main building' wouldn't do anything as that client subnet would still need to go across all buildings. Instead you'd need to create a new subnet for each 'main building' so that traffic is isolated to it, and then the core will be able to route that traffic.

[FN-GM]

It really depends on the design on your network. You can make it do this but there probably isn't much benefit for a network the size of a school. If the network isn't designed correctly this could make it worse.

 

You're probably best off keeping your Layer 3 switches in the core and then not have subnet spanning across multiple switches. That way routing isn't hopping back and forward between switches. Basic routing will always be better on switches than routers (Ironically!) as it's done in the hardware ASIC, routers use software.