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Do routers broadcast,multicast
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dasaint
Junior Member
M
Registered: Sep 2002
Location:
Country: Nigeria
State:
Certifications: CCNA
Working on: CCNP,CCDP
Total Posts: 12
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 Do routers broadcast,multicast
Hello guys,
i having been trying to work out a little problem for a while Do routers forward broadcasts and multicasts?
i know by default they don't, but what happens when you feed in 255.255.255.255?which is a broadcast address
though i feel it will not forward it.
But the one i am not too sure of is if you try and forward an address like 172.16.255.255 which is a multicast to all subnets.
then what of 172.16.20.255 which is to all hosts.
please guys try and look into this
Dasaint
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 12-18-02 04:43 PM
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MadChef
A Huge Fake
Registered: Sep 2000
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Total Posts: 1426
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 Re: Do routers broadcast,multicast
quote:
Originally posted by dasaint
But the one i am not too sure of is if you try and forward an address like 172.16.255.255 which is a multicast to all subnets.
No, all that is is just a unicast packet to the 172.16.255.255 host, though it may be turned into a local directed broadcast depending on the interface configuration of that subnet.
Unless the router is directly connected to a 172.16.x.x network, the router will forward that because it has no idea if that's a valid host address or not since it has no idea what the local mask is. You may very well have a 172.16.0.0/15 network out there and have 172.16.255.255 as a perfectly valid host.
When the router is directly connected to the 172.16.0.0/16 network in this case, then it depends on whether IP directed broadcast is enabled or not. That governs whether the router will convert the layer 3 broadcast address (which 172.16.255.255 is for the 172.16.0.0/16 subnet) into the layer 2 broadcast address (an all F's MAC).
If you pull the mask out further, to say /24, then packet is not destined to all subnets like you might think, but rather specifically to the 172.16.255.0/24 subnet. Once it gets to that particular network, it again depends on the interface config as to whether or not it is translated into a broadcast.
You can extend this directly to your second example since it all depends on how your subnets are sliced up.
Clear?
MadChef
Last edited by MadChef on 12-19-02 at 10:19 AM
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12-19-02 10:16 AM
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dmaftei
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I believe MadChef clarified the broadcast issue for you. As for the multicast, it looks like you're taking directed broadcasts for multicasts...
If you really think of multicast addresses (224.0.0.0 to 239.255.255.255) then, yes, routers forward packets addressed to a multicast group if there are hosts belonging to that group attached to one of the router's interfaces and the packets are received on a different interface.
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12-19-02 02:16 PM
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