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New Member

Question regarding broadcasts/traffic flow using a layer 3 switch

I will be installing a layer 3 switch to breakup one large broadcast domain. I will be creating trunks from each data closet access switch, to the new layer 3 switch. This will be a collapsed core setup. My question is, I have 10 closets to move over, if I do one at a time, with the new vlans being on their own subnet, I know that inter-vlan routing can be used to talk to other vlans ending on that switch, but how will it allow traffic to talk to the rest of the flat network. Would I create a routed interface on the switch using the no switchport command, and then assign it an ip/secondary address that falls within the ip range being used. 192.168.x.x with a b mask. Also, in this scenario, will broadcasts be forwarded. I am looking for the new dist/core switch to be an endpoing for all traffic, and then static route to either the firewall or remote sites from there. Any insight would be greatly appreciated.

Hall of Fame Super Blue

Re: Question regarding broadcasts/traffic flow using a layer 3 s


If you have one big broadcast domain, what is the default gateway set on the clients at the moment.

Are all the switchports on your existing switches in the same vlan at the moment ?, presumably if you haven't done anything to the switches they are in vlan 1 by default.

Easiest thing to do would be to ensure all client ports are in the same vlan. Create a layer 3 SVI for that vlan on your new L3 switch with the ip address of the existing default gateway of your clients.

As you say make all the uplinks trunk links.

Then create your other vlans and layer 3 SVI's for those vlans and then migrate the clients across.

**Edit - forgot about 2nd part. Broadcasts will not be forwarded between router interfaces unless you configure them explicitly to do so. If you need to forward DHCP requests add "ip helper-address x.x.x.x" commands to your L3 SVI's **



New Member

Re: Question regarding broadcasts/traffic flow using a layer 3 s

Default gateway is a pix.

Yes, we have hp pro curve switches in the closets. All devices are in vlan1.

My plan was to create a new vlan on each access switch, and assign ip/gateway info via dhcp. The gateway would be the ip address of the svi on the new layer 3 switch. To get to the pix, which currently has a address, I was going to add an ip address in the same range as the pix, and add a default route on the switch to the pix. To get to remote sites on different subnets, or other devices on the flat network not in the new vlan ip range, i.e email/file servers, etc.. I was going to create a routed port on the layer 3 switch, and add primary and secondary addresses in the range of those networks, and then static route to each location. If that is not optimal, what are other options. After the vlans hit the new layer 3 switch, they will either go to the pix for internet access, or through ppp links to remote sites, or connect to resources on the non-vlanned switches.

Thanks for the broadcast info. It sounds like if my routed port is connected to a layer 2 switch, no broadcasts will be forwared. It will, however, talk to devices on the other segment if the routed port on the L3 switch has an ip address in the correct range.

Thank you so much for your insight. Very much appreciated.

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