broadcast traffic

Answered Question
Dec 25th, 2008

Hi,

Is the "broadcast traffic" 255.255.255.255?

If so, can VLAN block it?

rdgs

I have this problem too.
0 votes
Correct Answer by richard.m.gilbert about 7 years 11 months ago

As a frame comes through an ingress port, the switch does three things ... 1. where to forward it, 2. should it be forwarded, and 3. how it should be forwarded.

#2 is refering to QoS and those types of things so we are really talking about #1 and #3.

1. The switch takes the frame's destination address and looks it up in the Content Addressable Memory (CAM) table. If the address is there, all it has to do is read off the egress port and the vlan ID to figure out which port it needs to go out (that is #3). But if it can't find the address (and this is what we are interested in), the switch will then flood that frame out every switchport ... in that vlan.

So ... a switch will not forward a broadcast out every switchport ... just every switchport in a vlan. A frame containing the address FF:FF:FF:FF:FF:FF will be forwarded to all connected segments ... of that vlan.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
richard.m.gilbert Thu, 12/25/2008 - 09:36

I think the broadcast address depends on the subnetwork address. For instance ... 10.0.0.1 /8 has a broadcast address of 10.255.255.255.

VLANs are broadcast domains so switches will forward all broadcast but a router (or switch running router process) will not. So, essentially, I think the answer to your question is yes. A router will not forward any broadcast from one network segment, or vlan, to another.

Maybe one of the experts can verify.

hth,

Richard

anitachoi3 Thu, 12/25/2008 - 21:33

Hi Richard,

If the broadcast address is 10.255.255.255 and there are two VLANs in the same switch, then the 10.255.255.255 broadcast traffic should be within the same VLAN switch ports. It will not flood to another VLAN.

If the broadcast address is 255.255.255.255 and there are two VLANs in the same switch, then (I guess), the broadcat traffic "may" flood to another VLAN. I am not sure.

If so, it may have one issue. if one PC has a lot of broadcast traffic (255.255.255.255) on the segment, it is not only affect itself VLAN, but also to affect another VLAN which is in the same hardware.

how to address or work around to solve it?

rdgs

richard.m.gilbert Thu, 12/25/2008 - 22:14

A switch is unable to dissemenate traffic across vlans. It has to send traffic up to the router for that to happen. That is how they got the name router on a stick. Routers do not forward broadcasts ... they discard them. So a broadcast in one vlan will not propogate into any other vlans.

anitachoi3 Thu, 12/25/2008 - 22:53

Hi Richard,

if the broadcast MAC address is ff:ff:ff:ff:ff:ff, does it flood (or forward) the frame to all ports of switch?

rdgs

Correct Answer
richard.m.gilbert Thu, 12/25/2008 - 23:19

As a frame comes through an ingress port, the switch does three things ... 1. where to forward it, 2. should it be forwarded, and 3. how it should be forwarded.

#2 is refering to QoS and those types of things so we are really talking about #1 and #3.

1. The switch takes the frame's destination address and looks it up in the Content Addressable Memory (CAM) table. If the address is there, all it has to do is read off the egress port and the vlan ID to figure out which port it needs to go out (that is #3). But if it can't find the address (and this is what we are interested in), the switch will then flood that frame out every switchport ... in that vlan.

So ... a switch will not forward a broadcast out every switchport ... just every switchport in a vlan. A frame containing the address FF:FF:FF:FF:FF:FF will be forwarded to all connected segments ... of that vlan.

joseph.derrick Mon, 12/29/2008 - 17:37

Hi,

You cannot block a broadcast traffic within the same VLAN.

Broadcast traffic can be blocked thought for each different VLAN.

This is to say that:

WITHIN VLAN: broadcast not blocked.

INTER-VLAN: broadcast blocked.

Hope this helps.

Thanks,

k0rg

Actions

This Discussion