boadcast domain, L2 or L3 ?

I'm going over CCNA guide after few months and now I'm confused about some basics

Is broadcast domain defined at layer 2 or 3? If the answer is at layer 2, then why would a router, a L3 device would have the ability to know about L2 activity?

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Comments

empc4000xl

a broadcast domain goes out all layer 2 devices like a arp request or something, but a router will block it. Just like a switch breaks up collision domains a router can break up broadcast domains. I'm not sure if I understand you correctly, but since you have the VLANS setup if you wanted to talk to another computer in a VLAN you need a layer 3 device to send the information to a new VLAN. Hopefully this explains it.

Netstudent

Routers segment broadcast domains and switches segment collision domains.
With the router you are segmenting 3 broadcast domains\subnets with 3
separate VLANS.

VLANS logically separate broadcast domains with the help of a router. Since a
router is a layer3 device on the OSI, broadcast domains are at layer3.

binarysoul

Umm, you two seem to differ whether broadcast domain is L2 or L3 (unles I'm misinterperting what empc4000 says:

empc4000xl wrote:

a broadcast domain goes out all layer 2 devices like a arp request or something, but a router will block it.

Netstudent wrote:

Since a router is a layer3 device on the OSI, broadcast domains are at layer3.

I tend to agree with Netstudent, but empc4000x is right too when looking at diagram above that shows broadcast domain at L2. [/quote]

empc4000xl

I wrote some of that wrong a bcst can be broken up by a router, just like a switch will break up a collision domain. So it I guess it would be a layer 3 thing, but Im dead tired. I will dig out the answer at work tomorrow morning.

bighornsheep

The broadcast shown is a layer 2 broadcast.

Netstudent wrote:

VLANS logically separate broadcast domains with the help of a router. Since a
router is a layer3 device on the OSI, broadcast domains are at layer3.

VLANs can seperate broadcast domains even without the router, the router provides inter-vlan connectivity.

Layer 3 broadcast will cross the layer 2 boundary, ie. sending something to 255.255.255.255

And the diagram is also misleading, the closest layer to the user should be the application layer, not the physical layer.

networker050184

A broadcast can be either L2 or L3.

Pash

Im with networker.

Whats the difference between FF:FF:FF:FF:FF:FF and 255.255.255.255 ?

Remember, by default a router will not forward L3 broadcasts and it should be clear why L2 broadcasts stay within the broadcast domains.

freetech

The FF:FF:FF:FF:FF:FF is used for broadcasts on L2 and 255.255.255.255 is used for broadcasting at L3.

remyforbes777

Broadcast domains are L3. Collision domains are L2.
FF:FF:FF:FF:FF:FF is a Layer 2 broadcast.
255.255.255.255 is a Layer 3 broadcast.

dtlokee

A router is oing to send a broadcast of 255.255.255.255 to FFFF.FFFF.FFFF anyhow, they are basically the same as far as the scope goes. A router will never forward a broadcast (FFFF.FFFF.FFFF or 255.255.255.255) from one interface to another interface.

A broadcast domain is a L2 component, not layer 3. It just happens that IP maps a single subnet per broadcast domain, but that doesn't indicate that a broadcast domain is L3. Can a single broadcast domain have more than one subent address? yes.

interface fa0/0
 ip address 192.168.100.1 255.255.255.0
 ip address 192.168.200.1 255.255.255.0 secondary

This is still one broadcast domain even though it has two addresses, two subnets, and two subnet broadcast addresses.

Routing is a L3 function. When you ping a directly connected neighbor are you checking L2 connectivity or L3 connectivity? If you can ping a directly connected neighbor although IP is involved it doesn't indicate that the routing protocols are working, but if you can ping remote subnets you are testing L3 and routing. Now I realize that routing is involved to tell the router to forward out the directly connected interface, but turn ip routing off on all your router, and you can still ping the directly connected neighbors, but not remote subents.