broadcasts

[carl_townshend]

do broadcasts only happen at layer 2 ?

[m.sir]

Yes

Because broadcast frames are flooded out all ports on a switch the devices connected to the switch are in a broadcast domain.

Routers, which operate at Layer 3 of the OSI model, provide broadcast domain segmentation for each interface. Router by default drops all broadcasts

M.

Hope that helps rate if it does

[carl_townshend]

so can i send broadcasts to another subnet or can i only do them on that subnet ?

[cprice2k7]

What type of broadcast are you thinking of? For a dhcp broadcast you would configure an ip helper address in interface configuration mode to route that type of broadcast traffic to the dhcp server.

[carl_townshend]

just in general really, as i thought routers dont forward broadcasts, they onlt happen on a lan ?

[scottmac]

Just a minor correction / clarification:

Flooding is not the same as broadcasting.

Broadcasting implies a destination MAC of (by convention) all ones.

Flooding (like what happens when a switch doesn't have the destination MAC in the forwarding table) means that the un-altered frame is transmitted out all ports (except the port from which the frame arrived).

Short story:

Broadcast: all-ones destination MAC, transmitted out all connected ports (because the destination address matches all hosts)

Flooding: all-port transmission of the unaltered frame. No change in the destination MAC.

FWIW

Scott

[bdantzig]

Generaly, routers only send broadcasts for their own use, such as arp, just like any node on a network. However, the router may support "directed broadcasts" also called "subnet broadcasts"

Example:

Lets say I am on network 10.1.1.0/24 and can reach network 10.1.2.0/24 accross one or more routers. I can send to the IP address 10.1.2.255. This would be the IP broadcast address for the 10.1.2.0/24 network. When the router for that network receives the IP directed-broadcast packet, it will forward it out that network using layer 2 broadcast so that all nodes on that net will receive it.

This behavior can be turned on and of with the "ip directed-broadcast" IOS command in interface config mode.