MAC-address — what is this?

In any packet transmitted over Ethernet there are two MAC-addresses present: one is a source MAC-address and another is a destination MAC-address. When forwarding data, Ethernet-switches use this important data. So what is a MAC-address? What are the differences between unicast, multicast and broadcast destination MAC-addresses? What destination MAC-addresses are common for IEC 61850 standard protocols? Let's have a more detailed look on this.

MAC-address consists of 48 bits represented in hexadecimal format. Every 8 bits are separated either by a colon (:) or a hyphen (-). Let’s come up with MAC-address example: 00:26:57:00:1f:02.

Each product, possessing communication interface, has the unique MAC-address. IEEE is the organization that provides manufacturers with source MAC-addresses range to be used in their products: for this, first three octets of a specific value are fixed for each manufacturer. These three octets are called OUI (ORGANIZATIONALLY UNIQUE IDENTIFIER). So, as you already may see, one can easily identify a device manufacturer by its MAC-address. You can do this online by yourself – check for IEEE online database (Be attentive! OUI should be entered with a hyphen!). For the MAC-address given earlier as an example one can define that it belongs to Russian relay protection manufacturer EKRA. The remaining three octets are set by the manufacturer and should never repeat each other in combination with the first three octets of the same value. Since these last three octets identify the network controller of a device, they are usually referred as NIC Specific (Network Interface Card Specific).

OUI
Identification of the vendor by OUI at IEEE website

Typically, MAC-address of the device can be determined by looking at IED labels – often network interface panel has the label, indicating IED’s MAC-address.

SchneiderMAC
Label, showing Schneider Electric communication card MAC-address
AlstomMAC
Label, indicating MAC-address for Alstom Grid IED

Let’s see what values destination MAC-address can take.

When it comes to transferring data from one device to another, then destination MAC-address field has the value of the recipient – that is, the address which we have discussed above. In this case, destination MAC-address is a unicast (unicast) MAC-address. In substation environment, specifically in IEC 61850 context, destination unicast MAC-address is there in MMS packets.

MMSUnicast
Unicast destination MAC in MMS packet

But what if the same data packet needs to be transmitted to multiple network devices? For this purpose there is a destination multicast (multicast) MAC-address. These MAC-addresses are easily identifiable – least significant bit of the first octet of the destination MAC-address is set to 1, for example, 01-0C-CD-00-00-01. If such a packet is received by Ethernet switch, then the latter forwards it to all other ports – in case no recipient groups are specified at the switch. The possibility of organizing data transfer for the group of devices is one of the differences between using the destination multicast MAC-address and destination broadcast (broadcast) MAC-address. In substation environment, specifically in IEC 61850 context, destination multicast MAC-address is there in GOOSE and Sampled Values messages. For example, one can find a GOOSE with destination MAC-address 01:0C:CD:01:01:21.

MulticastGOOSE
Многоадресная посылка GOOSE

Speaking about the destination multicast MAC-addresses it is worth noting another interesting point – IEEE not only assigns MAC-addresses’ bands to device manufacturers, but also to standardization bodies, to uniquely identify the traffic transmitted according to the standards. For this, again, IEEE reserves the values of first three octets. For example, for IEC TC 57 the following three octets are reserved – 01-0C-CD-xx-xx-xx. Then standardization body decides the rules of MAC-address assignment to specific protocols by itself. For GOOSE messages TC 57 WG 10 then determined the fourth octect to be equal to 01 and the range of addresses as: 01-0C-CD-01 -00-00 to 01-0C-CD-01-01-FF; for Sampled Values ​​- the fourth octet equal to 04 and range of addresses as: 01-0C-CD-04 -00-00 to 01-0C-CD-04-01-FF. For example, for PTP and RSTP protocols traffic specific destination MAC-addresses are determined.

Broadcast MAC-address – is always the same: ff: ff: ff: ff: ff: ff. When switch receives such a packet it forwards it to all its ports. Broadcast destination MAC-address is used only by some service protocols such as ARP (Address Resolution Protocol), which we will have a look at next time and which plays an important role in information exchange process in Layer 2 networks.

Summary

  1. If you are IED developer and your IED is to have communication interface, make sure IEEE reserves MAC-addresses band for your products. Company will have to pay for this.
  2. For protection and control system designers of the future and for the commissioning guys there may be another recommendation – never set the same destination multicast MAC-address for different GOOSE and Sampled Value messages (there are examples of the configuration software not restricting you from doing this). Otherwise, you risk to lose the capability of using multicast filtering function on switches. Keep in mind that there are many IEDs on the market which do not support this multicast filtering functionality. If you are not capable of activating this functionality on the switch then all network devices, including IEDs, will listen to all the messages, even those that they do not really need and this as we have already mentioned, places an impact on GOOSE transfer time. It will increase with the increased loa

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