Layer 3 Switches

Unanswered Question
Oct 7th, 2008

Do layer 3 switches forward broadcasts out like switches, or do they act more like routers? I'm assuming this depends if you have ip routing enabled or not.



I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (3 ratings)
w.siewert Tue, 10/07/2008 - 07:37

it depends on configuration, if configured as routed port broadcasts are terminated, if configured as switchport broadcasts are forwarded!


John Blakley Tue, 10/07/2008 - 07:49

I have a 3750 that's in routing mode. How can I tell what the switchport's mode is? I ran sh int switc mod 1, and I only get back if it forwards unknown multicast or unicast packets, but nothing about broadcast or what mode the port itself is in.


Jon Marshall Tue, 10/07/2008 - 08:41


Routers do not forward broadcast unless configured to do so eg. "ip directed-broadcast"

Switches do forward broadcasts out of all ports in the same vlan that the broadcast was initiated on, and out of all trunk ports assuming that the vlan the broadcast was initiated on is allowed on that trunk.

So a 3750 with ip routing enabled will exhibit both these behaviours. If the port is configured as a switchport then it will behave as a layer 2 port. If the port is configured as a routed port "no switchport" then it will not be part of a vlan or a trunk.

A 3750 without ip routing enabled acts as purely as a L2 switch.


John Blakley Tue, 10/07/2008 - 08:51

So, if I have my router connected to port 5 on the 3750, and it's trunked, how would I keep the switch from broadcasting traffic on vlan 1 to the router, or can i?

Jon Marshall Tue, 10/07/2008 - 08:57

If the vlan is allowed on the trunk then broadcasts will be sent down the trunk. Be aware though that the broadcast will still only stay within the vlan so unless the corresponding subinterface on the router is forwarding broadcasts the packets will not cross vlans.

You can manually clear vlans from a trunk

switchport trunk allowed


John Blakley Tue, 10/07/2008 - 09:02

I shouldn't remove VLAN 1 from the trunk if it's native, should I? All of my traffic is going out of it. There's not a subinterface on the router, just multiple IP addresses on the primary interface. This is part of the whole segmentation project I've been working on. By the time I'm done, I will have VLAN1 and VLAN125 on the L3 switch, but like I said, the router doesn't have subinterfaces configured. Should I redo that in order to be able to support this configuration? In the end, after this changeover is completed smoothly, I'll have about five more VLANs. I'm thinking subinterfaces would be better.


Jon Marshall Tue, 10/07/2008 - 11:14

No you shouldn't if that is how all your traffic is being switched. Bear in mind you can use any vlan as the native vlan - Cisco recommend using a non-routed vlan for the native vlan.

You talk about 5 more vlans, will these vlans exist on the switch ? If so, given the choice between subinterfaces on a router and using L3 vlan interfaces on your L3 switch i would go for L3 vlan interfaces on L3 switch.

With subinterfaces on a router you are limited by the total bandwidth of the router interface eg. 100Mbs or 1Gbps whereas with a switch you are limited by the switch fabric.


John Blakley Tue, 10/07/2008 - 12:06

They will be on the switch. Currently we have 5 addresses on our router to support different internal subnets. We're trying to switch one of these subnets off into it's own vlan, and it will later expand into multiple vlans on the switch. From what I understand, I would assume that since the routing is done through our core, it would be safe to say that I could remove all but one address from the router, and allow the L3 switch to do all of the routing?

Currently, there are no subinterfaces on our core router at all; they're configured as secondary addresses.

Thanks Jon


Jon Marshall Tue, 10/07/2008 - 12:11


"it would be safe to say that I could remove all but one address from the router, and allow the L3 switch to do all of the routing? "

Exactly, this is what you want to be trying to do. What you want to aim for is to have all the inter-vlan routing on the L3 switch and then use either static routes or preferably a dynamic routing protocol to exchange routes between your router and the L3 switch.


lamav Tue, 10/07/2008 - 13:54

Jon, I love your explanations.

I gotta rate this thread...



This Discussion