AO Subsystem Network Configuration

A discussion of network topology can be found in TelNetworkLayout.

Due to implementation limitations, the BCU network interfaces are very sensitive to the kind of traffic on the ethernet network. More specifically, the BCU have a short buffer for ethernet packets (8 packets), if the buffer fills up, the BCU goes into a recovery loop and becomes unresponsive for about 0.5 sec.

This is not a problem with standard communication, because most of it is performed in a synchronous way, but becomes a sensible problem with respect to broadcast traffic.

On usual LAN there is a potentially large amount of broadcasts for various reasons, including ther ARP protocol. We have noticed that a BCU placed in a standard LAN cannot operate due to the normal broadcast traffic.

In some cases the ARP generated broadcasts on an insulated LAN including both the Adaptive Secondary and the WFS is enough to cause networking problems to the BCU (there are six of them in the Secondary and one or more in the WFS).

How to cope with all this

In order to avoid the problems discussed above, we need two fullfill two conditions:

  1. Keep the LAN used to communicate with the BCU's as small as possible to avoid that other ethernet based device can produce broadcasts. This may be accomplished by using VLAN capabilities in the network switches. The AO dedicated VLAN should only include the WFS and the six BCU's of the Adaptive Secondary
  2. Use static arp tables in the AO Workstation. In this way the ethernet address of destination interface is preloaded in the workstation ARP table and there is no need to broadcast the resolution request. This is usually accomplished in Unix/Linux with the command arp -s.


Point 1 above must be remembered whenever maintenence is performed on networking devices at the telescope: if the VLAN is not properly configured the AO susbsyem may show a funny behaviour even though network communication seems to be OK.

Point 2 must be remembered whenever some piece of hardware in the AO VLAN is substituted: the static entries in the ARP tables of the AO workstation must be edited accordingly.
Topic revision: r4 - 01 Jun 2011, LucaFini
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