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New Member

Simple broadcast/VLAN question

Hi all, I am having a bit of a mental blank here.

If two PCs on a switch are in two separate VLANs (and the switch is connected to a router) and one of the PCs sends a layer 3 broadcast, who gets the broadcast?

- All of the PCs on the switch regardless of which VLAN they are in, but the broadcast does not go past the router?

- All of the PCs in that VLAN (including PCs past the router)?

- All PCs in that VLAN before the router?

- None of the above??

Thanks guys.

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Simple broadcast/VLAN question

To make a router forward broadcasts between two interfaces you need to configure the two interfaces to bridge traffic between them. The router would simply forward the broadcast out the interfaces that are part of the same bridge group. It doesn't convert the broadcast into any other form with this setup.

HTH

Sundar

Re: Simple broadcast/VLAN question

Layer 3 switch is nothing but a router. Therefore, you guessed is correct it doesn't forward broadcasts. Of course, we talking about VLAN (layer 3) interfaces only and the switch (layer2) itself forwards broadcast between the switchports as any other layer 2 switch.

HTH

Sundar

10 REPLIES

Re: Simple broadcast/VLAN question

The switch would propagate the broadcast out all the ports that are on the same VLAN plus any trunk port it's connected to. Router's, by default, do not forward broadcasts. The one exception being if the router interface is doing bridging then it would forward the broadcasts out the bridged interface(s).

HTH

Sundar

New Member

Re: Simple broadcast/VLAN question

Thanks for the quick reply.

I thought because it was a layer 3 broadcast it would ignore the VLAN allocations because they are layer 2?

And according to your explanation (not that I'm saying your wrong... your a CCIE after all :P) but some PCs in the VLAN would receive the broadcast but others wont. Is this correct? If it is, that's not very good because it may provide inconsistency between the devices.

Re: Simple broadcast/VLAN question

Even a layer 3 broadcast has to stay within the VLAN because broadcasts are confined to that particular VLAN. I shall try to explain this with an example.

Let's say you have the following setup and you initiate a broadcast ping from the router. The switch would get the broadcast packet on port f0/1, that's part of vlan 40, it would forward the packet out f0/2 and f0/4. As I stated in my earlier post a switch would forward the broadcast out every port (except the port it received the traffic on) belonging to the same VLAN and any trunk port(s) it's connected to. As such it wouldn't forward the packet out f0/3 because it isn't part of vlan40. This is where VLAN identifier comes to play and VLANs separate broadcasts. Instead of a switch let's say if you had a VLAN-unaware device (like a hub) the packet would be flooded out all ports.

Router:

int f0/0

description connected to Switch

ip address 10.1.1.1 255.255.255.0

Switch:

int f0/1

description connected to Router

switchport access vlan 40

int f0/2

description connected to PC1

switchport access vlan 40

int f0/3

description connected to PC2

switchport access vlan 41

int f0/4

description connect to Switch2

switchport mode trunk

HTH

Sundar

New Member

Re: Simple broadcast/VLAN question

ahh yes of course. That is one of the main reasons VLANs were implemented, to segment the network into separate broadcast domains.

About the router issue. So if there are two switches connected on two different router ports, broadcasts would not be sent between the two switches because routers inherently block broadcast traffic. So if one wanted to allow broadcast traffic in a VLAN over a router, they would have to specifically configure the router to allow this, is this correct? If so, is it true the router turns the broadcast into multiple unicasts?

Re: Simple broadcast/VLAN question

To make a router forward broadcasts between two interfaces you need to configure the two interfaces to bridge traffic between them. The router would simply forward the broadcast out the interfaces that are part of the same bridge group. It doesn't convert the broadcast into any other form with this setup.

HTH

Sundar

New Member

Re: Simple broadcast/VLAN question

OK great, thanks for all of your help.

Re: Simple broadcast/VLAN question

No problem. I am glad that I could help clear your doubts.

Thanks for the rating :)

New Member

Re: Simple broadcast/VLAN question

No probs buddy, you desereve it :)

By the way, just another quick question. Do layer 3 switches forward broadcasts? Or do they, like routers, stop the forwarding of broadcasts?

Re: Simple broadcast/VLAN question

Layer 3 switch is nothing but a router. Therefore, you guessed is correct it doesn't forward broadcasts. Of course, we talking about VLAN (layer 3) interfaces only and the switch (layer2) itself forwards broadcast between the switchports as any other layer 2 switch.

HTH

Sundar

New Member

Re: Simple broadcast/VLAN question

Beautiful, thanks for that :)

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