Henry's Points

Layer 3 device will terminate Broadcast traffic. However, sometimes we need to forward those broadcast traffic to somewhere bypass several layer 3 devices. The first example is one host send DHCPDISCOVERY broadcast traffic to DHCP server on the other broadcast domain. The other example is some UDP broadcast application talk to server or client on remote site. Both cases should use some ways to carry those broadcast traffic. Cisco provides two ways to carry those traffic.

• By Unicast: ip helper-address <unicast-addr>
• By Multicast: ingress: ip helper-map <mcast-addr> <ACL>; egress: ip helper-map <mcast-addr> <destination-unicast-addr> <ACL>

The diagram demonstrate two ways network and the configuration should follow steps below:

Step 1: enable forward protocol globally.

ip forward-protocol udp <port # | protocol name>

Step 2: On the ingress interface, enable helper address or helper map. Note, it doesn’t need to enable ip directed-broadcast under ingress interface.

ip helper-addr <destination-unicast-addr>
ip helper-map <multicast-addr-group> <ACL>

Step 3: On the egress interface, we only care about multicast carrier since unicast carry broadcast directly to the destination.

ip helper-map <multicast-addr-group> <destination-unicast-addr> <ACL>
ip directed-broadcast

Note: Enable directed broadcasts on the interface connected to the receiving segment using the command ip directed-broadcast. This is needed to successfully send broadcasts out of this segment.

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