How to forward Broadcast Packet Accross the routers?

Assuming its a Cisco router, one can forward the broadcast traffic by making use of the command " IP Helper-address X.X.X.X

For non IP broadcasts, one can use the router in a bridge mode.

Router send broadcost but not forward broadcost.

it can send broadcost packet of DHCP request form of unicost.it called DHCP Relay. 

we can enter this command on that interface where broadcost packet is drop.

#interface fast/giga (no)

#ip helper-address (enter dhcp ip)

Depends on the system. Cisco's will forward certain broadcast traffic.

Forexample DHCP, normally it would cease being broadcast when relayed, but many system can be configured to broadcast the relayed request.

ssume IPv4 broadcast traffic enters the router like this:

10.0.1.255 Gi0/0 +---------+ +---------+ Gi0/4 ---> 10.0.1.254/24 | Router1 |---{ WAN }----| Router2 | 192.0.2.254/24 UDP port 55666 +---------+ +---------+

This configuration would forward the UDP broadcast traffic on port 55666 from Router1:Gi0/0 toRouter2:Gi0/4. The ip helper-address 192.0.2.255 command forwards the broadcast to 10.0.1.255 as 192.0.2.255 to Router2:Gi0/4. The broadcast would then exit Router2:Gi0/4 as an ethernet broadcast to IP address 192.0.2.255 (the IP subnet broadcast address).

We can let router to pass broadcast packet through it by using below command on interface:

 

no ip directed-broadcast

You didn't specify a router vendor or network protocol, but let's assume it's a Cisco and IP (for the moment).

Example of forwarding IP broadcasts from 10.0.1.0/24 to 192.0.2.0/24

Assume IPv4 broadcast traffic enters the router like this:

10.0.1.255 Gi0/0 +---------+ +---------+ Gi0/4 ---> 10.0.1.254/24 | Router1 |---{ WAN }----| Router2 | 192.0.2.254/24 UDP port 55666 +---------+ +---------+

This configuration would forward the UDP broadcast traffic on port 55666 from Router1:Gi0/0 to Router2:Gi0/4. The ip helper-address 192.0.2.255 command forwards the broadcast to 10.0.1.255 as 192.0.2.255 to Router2:Gi0/4. The broadcast would then exit Router2:Gi0/4 as an ethernet broadcast to IP address 192.0.2.255 (the IP subnet broadcast address).

Router1's config:

! hostname Router1 ! interface GigabitEthernet0/0 ip address 10.0.1.254255.255.255.0 !! if an IP broadcast matches the forward-protocol, send to 192.0.2.255 ip helper-address 192.0.2.255 ! !! allow helper-addresses to forward UDP/55666 broacasts ip forward-protocol udp 55666

Router2's config:

! hostname Router2 ! interface GigabitEthernet0/4 ip address 192.0.2.254255.255.255.0 !! Allow directed broadcasts to exit as an ethernet mac-layer broadcast. !! This is the default setting, but is often turned off for security purposes ip directed-broadcast !

Forwarding non-IP broadcasts across routers...

There are a few options... for instance:

• Use a router in bridge mode
• Use something like DLSw

You didn't specify a router vendor or network protocol, but let's assume it's a Cisco. Cisco's will forward certain broadcast traffic (hint: google "ip helper-address") Other protocols have specific mechanisms to forward ("relay") their broadcast traffic -- for example DHCP, normally it would cease being broadcast when relayed, but many system can be configured to broadcast the relayed request.