Siemens SIMATIC NET S7-300 Manual page 147

Why Does an S7-CP Prevent Reception on Broadcast Connections?
It is often necessary for one station to send frames to a number of partner stations.
It is important that the frames are sent at the same time and arrive and practically
the same time. Sending and receiving broadcast frames is always required. A
broadcast message is received by all nodes in the network.
A typical application is the sending of broadcast frames to find a MAC address for
an IP address (ARP request).
For this reason, a communications module must accept broadcast frames and
evaluate them in its software. The major disadvantage of this is that network
performance sinks significantly if there are too many broadcast frames because
each individual module needs to process all the broadcast frames to find out
whether the frames were intended for it.
To avoid these disadvantages, S7-CPs handle broadcast as follows:
S Following reception, the broadcast frames are filtered out with high priority by all
Ethernet CPs. This means that frames that cannot be interpreted are discarded
immediately. Only frames that can be interpreted, for example an ARP request,
are passed on over the LAN controller and evaluated. This prevents a negative
influence on the other connections by broadcast frames.
S For the application, this means that the S7-CP cannot receive broadcast frames
intended for the transfer of user data. It is, however, possible for the module to
send broadcast frames within the network.
S7-CPs for Industrial Ethernet Configuring and Commissioning
Release 01/2007
C79000-G8976-C182-07
5 Configuring Communication Connections
A -147