Ethernet/IP and IGMP

[mhopley]

I am designing a network with a contrologix PLC. We are proposing to connect the drives and remote I/O using ethernet.

The controllogix rack will have 2 ethernet cards, one to connect to the customers network and one for the control. In total there will be about 22 nodes on the the control network.

I am worried about the amount of multicast traffic that will produced. I understand that a layer 2 switch, even though it supports IGMP snooping will not filter the multicast packets without a IGMP querier.

24 port industrial switches are not common (and expensive) and layer 3 19" rack mount switches are not cheap either.

My main question is there a layer 2 switch that has an IGMP querier and do I really need to worry about multicast if it confined to the control network?

Thanks

Mark

 

[SteveMaves]

IGMP is an inherently Layer 3 protocol, see the description here...
http://en.wikipedia.org/wiki/Internet_Group_Management_Protocol

So, no, there is no pure Layer 2 switch that has an IGMP querier.

As far as what bad things might happen if you did go with a layer 2 switch instead of a layer 3 switch, the chain is only as strong as the weakest link. I've heard of some devices that aren't hardened very well that fail in a "broadcast storm" scenario long before the A-B PLC stops working.

I think in the long run, you are better off paying for the good switch up front.

 

[Ken Roach]

There are a handful of switches aimed at the industrial market that include an IGMP Querier. The N-Tron "-A" firmware for the 500 series as well as the 7000 and 9000 series works very well. I've also used the Hirschmann RS20, Allen-Bradley Stratix 8000 and Cisco IE3000.

To my surprise, the older Cisco 2950 and 2955 do not have a querier included in them, despite a lot of Cisco user manuals that claimed they did.

It's unlikely to matter to the A-B drives and I/O devices if you do not have an IGMP querier in place because they are built from the start to filter IGMP multicast before it gets very high in the operating system. Computers and third-party devices generally have a harder time discarding multicast traffic without examining it and therefore taking up CPU resources.

But it is imperative to have a managed switch with diagnostic features. It's no guarantee that you will never have any network problems, but you stand a fighting chance of solving them if you have a managed switch. With an unmanaged switch, problems either get solved with 10x the labor, go on for years, or get solved with wholesale replacement of hardware. If you want to hear me rant like a madman, go ahead and pull up one of those "I don't see any reason to use managed switches" threads.

Most systems rely on a router to provide the IGMP Querier function. Make sure yours includes IGMP querying as one of its basic functions; many "firewall" devices do not.

 

[bikerdude]

Hi,
Since you are in the design stage, why Ethernet? I know that lots of people on this forum go on and on about how nice it works. Personally I can't justify a $1,000 dollar network switch to put I/O on Ethernet when I can do it on DeviceNet without the cost penalty.
Makes me long for the old days when RIO ruled with AB. Cheap & dirty, that's my favorite.

If you must use Ethernet I/O & drives comms. Go with a switch that can take the pressure & heat or it will cause problems sooner or later.

BD

 

[SteveMaves]

A-B IGMP Internals

It's unlikely to matter to the A-B drives and I/O devices if you do not have an IGMP querier in place because they are built from the start to filter IGMP multicast before it gets very high in the operating system. Computers and third-party devices generally have a harder time discarding multicast traffic without examining it and therefore taking up CPU resources.

I agree, the devices that I've seen that have trouble with "broadcast storm" are not Allen-Bradley devices.

 

[The Plc Kid]

I have began to rplace all our non managed switch with stratix 8000 which i believe is a layer 2 switch. Can someone explain this IGMP Querier a little better as i though what i am buying in the stratix is what i need?

Would you only need this when connecting the automation network to the corporate?

I hae heard that stratix will soon have firmware to make it layer 3 will this include the querier?

I undersatnd the IGMP snooping pretty well but the Querier is new to me?

 

[mhopley]

Thanks for all the replies.

The customer likes the idea of Ethernet, "it is the future" as we keep getting told by the vendors .
There will be a couple of Ktron feeders and some Eurotherm equipment connected as well that talk Ethernet/IP.

I agree with Bikerdude that Ethernet does not look appealing when you need an expensive switch. I wish I could find one for $1000.

Anything industrial with > 24 ports is going to be modular and expensive. That is why I was looking at commercial 19" rack mount switch but then I do not get the IGMP querier.

Ken, I agree with the benefits of a managed switch, I just wish there a way to get rid of the multicast for a reasonable price.

Mark

 

[mhopley]

Just to update, we have gone with the stratix 8000 switch which gives us 26 ports.

Thanks again to everyone for their advice.