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New Member

Forwarding broadcast through routers


I have to forward broadcast packets from a LAN (XX.XX.X1.0 to another LAN (XX.XX.X2.0 through 2 Cisco 2600 routers (router A and router B).

So I've configured IP Helper on input of router A with a directed broadcast destination address (IP HELPER XX.XX.X2.255). On the interface of router B which is directly connected to XX.XX.X2.0, I've activated directed broadcast (IP DIRECTED BROADCAST).

However when I'm testing this configuration, the router B receives the directed broadcast from router A but doesn't change it into a broadcast on the output. When I start DEBUG IP PACKET on router B, I see directed broadcast packets arriving on an interface of router B, but it doesn't send it through the LAN interface.

Here is my router B XX.XX.X2.0 interface configuration:

Interface FastEthernet 0/0

ip address XX.XX.X2.1

ip directed broadcast

The problem seems to be with the directed broadcast because when I put an unicast address on IP Helper (XX.XX.X2.2), the destination receives it well.

What can be the problem?

Thanks for your help.

New Member

Re: Forwarding broadcast through routers

Is router B just dropping the broadcasts ?

You might try putting IP forward-protocol in

To specify which protocols will be forwarded, use the following command in global configuration mode:

New Member

Re: Forwarding broadcast through routers

Yes, router B drops the broadcasts.

I make my tests with DHCP broadcasts (just for the tests) and ip forward-protocol is forwarding by default DHCP broadcasts so it should work. I tried to enter the command but nothing changed.

New Member

Re: Forwarding broadcast through routers

just as aside, what port are you really tring to forward? dhcp does forward without the forwared-protocol, but not much else .

New Member

Re: Forwarding broadcast through routers

Have you tried to disable fast switching on the inbound and outbound interfaces on router B? This could affect your debug results.

Be sure before you do that your CPU utilization is low enough so that this won't affect your users.


New Member

Re: Forwarding broadcast through routers

I've just tried but it didn't work. In fact, I am sniffing on the output of the interface with sniffer pro to be sure and I see nothing.

When I am debugging on the router and it recieves the packet, it is printed:

IP: s=XX.XX.X1.1 (FastEthernet0/1), d=XX.XX.X2.255 (FastEthernet0/0), len 328, rcvd 5

So the router receives it but doesn't forward it. I don't understand.

New Member

Re: Forwarding broadcast through routers

Heres some stuff from the old archives, I couldn't link it.

How Does Directed Broadcast Work?

Question: What is Cisco policy on passing a UDP-directed broadcast through a router to a specific port number? It seems I need the ip forward-protocol udp PORT# command in some releases of Cisco IOS 10.2 to pass this at all. Is the forwarding of directed broadcasts enabled by default?


Unless you have specifically turned off directed broadcasts, the router will

forward non-local directed broadcasts toward the next-hop found in the routing

table for the network number in the directed broadcast address. For example, if

an interface has an IP address of and a broadcast comes

in with destination address, the router will forward to the next

hop address for

If the broadcast coming in is a local or all-nets broadcast, you must either

enable helper-addresses or UDP forwarding via spanning tree to get the broadcast

propagated. With either of these methods, certain UDP ports like BOOTP, TFTP,

NetBIOS over IP, and some others will be forwarded once you apply the helper-

address or the spanning-tree/bridge-group commands. For other UDP ports, such as

111 SunRPC, you will specifically need to use the ip forward-protocol udp command

in conjunction with the helper address or spanning tree. So, using the interface

in the previous example, if a broadcast comes in with a destination address of or, a helper address or spanning tree will be required.

Last Modified: 02-DEC-96

New Member

Re: Forwarding broadcast through routers

it seems that the problem came from a directed-broadcast in DHCP which is inappropriate. I tried with another broadcast (UDP port 234 eg) and that work well. So, problem solved.

Thank you for your help.

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