router and broadcast address

Unanswered Question
Feb 25th, 2011

Hi every one

Routers by dedault stop broadcast address.

Let say we have following set-up

Lan----f0-Router-S0--------Wan

F0( ip address)  199.199..199.17 255.255.255.240

Let say router receives a packet on s0 with destination ip 199.199.199.31

what router will do?  will it send it out of f0? if yes but  it is broadcast address,

thanks and have a great weekend.

I have this problem too.
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Peter Paluch Fri, 02/25/2011 - 21:41

Hi Sarah,

A packet sent from a remote network to our  directly connected network's broadcast IP address 199.199.199.31 is in  this case called a directed broadcast. A directed broadcast is  routed through intermediate routers just like any other packet. Only  when it arrives into the router that has the destination network  directly connected, it starts to be slightly more interesting.

What will happen now depends on whether the Fa0/0 (the destination network's interface) has the ip directed-broadcast command applied to it. If yes then the router will simply forward the  packet onto the LAN, encapsulated in a broadcast-addressed Ethernet  frame. If the command is not applied (which is the default) then the  router will drop the packet.

Best regards,

Peter

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Posted February 25, 2011 at 9:02 PM
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