I have to forward broadcast packets from a LAN (XX.XX.X1.0 255.255.255.0) to another LAN (XX.XX.X2.0 255.255.255.0) 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.
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 188.8.131.52 255.255.255.0 and a broadcast comes
in with destination address 184.108.40.206, the router will forward to the next
hop address for 220.127.116.11.
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
18.104.22.168 or 255.255.255.255, a helper address or spanning tree will be required.
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