Does VLAN Interface on Router forward broadcast?

[shengkangjin]

Here's just a simple question that I don't have an answer to.

Suppose that we have the following topology switch A - Router B - switch C

all 4 ports connecting to each other are access VLAN 5

If switch A sends out an arp request on VLAN 5 towards Router B, will switch C get the arp request?

What happens if Router B is replaced with Layer 3 Inter-VLAN switch B2? Because Router B would use sub-interface for VLAN tagging, yet a Layer 3 switch will simply just attach the same virtual VLAN interface onto the behind of 2 connecting switchports.

Thanks for answering.

[Kasun Bandara]

layer 3 devices not passing layer 2 boradcast. 

https://networklessons.com/cisco/ccna-routing-switching-icnd1-100-105/broadcast-domain

[Flavio Miranda]

Where do you want to get? 

Start with you main question:

"Does VLAN Interface on Router forward broadcast? "

Nop.  A layer 3 interface does not forward broadcast unless you instruct them to using the command 'ip helper address'

"If switch A sends out an arp request on VLAN 5 towards Router B, will switch C get the arp request?"

Imagining that on the Switch A you have an interface vlan with IP address and on the Router B you have a interface on the same network. then, on the Switch C you also have a interface vlan with another ip address and on the router also a interface then the answer is Yes. Confuse, right?

Let me try to examplify.

                                        192.168.1.1 --  Router B --192.16.2.1

                             =                                                            =

Swich A - 192.168.1.2                                                                   Switch C - 192.168.2.2

Switch A and Switch C is connected to Router C. Each one  with its respective network.  What you did ask is if Switch A pinging Switch C, the ARP will get to Switch C, right? As broadcast nature.

  Well, the answer is no. The router will broke the broadcast domain. When the ARP request gets to the router interface, it replies to the switch A, with  the mac address of Switch C and then, Switch A can send the packet direct to switch C as a unicast ip packet.

What happens if Router B is replaced with Layer 3 Inter-VLAN switch B2?

Nothing change. Switch with inter-vlan feature is actually a router.

Because Router B would use sub-interface for VLAN tagging, yet a Layer 3 switch will simply just attach the same virtual VLAN interface onto the behind of 2 connecting switchports.

Now you are mixing vlan tagging and ARP with are different things. 

[shengkangjin]

Thanks for the answer.

I'd just need to clarify on my very last bit.

From a router's point of view, 2 different local interfaces cannot have ip addresses belong to the same subnet unless bounded together.

But a Layer 3 switch can

what if I have on switch A = 192.168.1.1/24, switch C 192.168.1.2/24, and in the middle, Layer 3 Switch B with the following config:

interface ethernet Fa0/1

description switch A

switchport access vlan 5

switchport mode access

interface ethernet Fa0/2

description switch C

switchport access vlan 5

switchport mode access

interface vlan 5

ip address 192.168.1.3 255.255.255.0

In this case, will switch A and switch C be considered in the same broadcast domain?

[Flavio Miranda]

"From a router's point of view, 2 different local interfaces cannot have ip addresses belong to the same subnet unless bounded together."

that´s right. And that´s why router break broadcast domain.

"In this case, will switch A and switch C be considered in the same broadcast domain?"

That´s correct. They are on the same Layer 2 domain which is the vlan 5 and they are on the same network which is 192.168.1.0/24. So, when  an ARP request were sent, it will travel along all ports on the same vlan.

[Richard Burts]

In your description of using a switch in the middle between switch A and switch B clearly there is a single vlan and any broadcast originated on switch A on vlan 5 would be forwarded to switch C on vlan 5.

I am not clear what you were asking in the original question with a router in the middle. How would the switches be connected to the router? Am I correct in assuming that switch A would connect to one router interface and switch B would connect to a second router interface? If these are treated as regular router interfaces then each interface would be in separate IP subnets and no broadcast would be forwarded. If the router supports Concurrent Routing and Bridging  then the router interfaces do not configure with an IP address but participate in bridging, there is a virtual interface on the router with an IP address in the subnet, and in that case yes a broadcast from the first switch would be forwarded to the second switch.