Can't get ARP reply with Supplied MAC address

[lou6366]

Hi guys, I'm trying to control a cyber power PDU through SNMP. When I use the supplied MAC address that came with the Arduino ethernet shield 2, using Wireshark I see the PDU doesn't reply to the ARP request, but when I change the first byte to 0x61 on the Mac address, the PDU replies to the ARP request and everything is fine, why is this?

Below is the code, and Wireshark screenshots:

#include <Ethernet.h>
#include <EthernetUdp.h>

    byte mac[] = { 0x61, 0x61, 0x0A, 0xAE, 0xAB, 0x10 }; //MAC address that does get a reply
 //byte mac[] = { 0xA8, 0x61, 0x0A, 0xAE, 0xAB, 0x10 }; //Mac address that doesn't get an ARP reply


byte bufferOn[51] = { 0x30, 0x31, 0x02, 0x01, 0x00, 0x04, 0x06, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, 0xa3, 0x24, 0x02, 0x04, 0x17, 0xe6, 0x67, 0xfe, 0x02,
        0x01, 0x00, 0x02, 0x01, 0x00, 0x30, 0x16, 0x30, 0x14, 0x06, 0x0f, 0x2b, 0x06, 0x01, 0x04, 0x01,
        0x9d, 0x60, 0x01, 0x01, 0x06, 0x03, 0x05, 0x01, 0x04, 0x01, 0x02, 0x01, 0x01};

byte bufferOff[51] = { 0x30, 0x31, 0x02, 0x01, 0x00, 0x04, 0x06, 0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, 0xa3, 0x24, 0x02, 0x04, 0x17, 0xe6, 0x68, 0x04, 0x02,
        0x01, 0x00, 0x02, 0x01, 0x00, 0x30, 0x16, 0x30, 0x14, 0x06, 0x0f, 0x2b, 0x06, 0x01, 0x04, 0x01,
        0x9d, 0x60, 0x01, 0x01, 0x06, 0x03, 0x05, 0x01, 0x04, 0x01, 0x02, 0x01, 0x02};
                
IPAddress ip(192, 168, 20, 2);

unsigned int localPort = 8888;      // local port to listen on

EthernetUDP Udp;

void setup() {
 
    Ethernet.init(10);  // Most Arduino shields

    Ethernet.begin(mac, ip);

    Udp.begin(localPort);
    
    
    Udp.beginPacket(IPAddress(192, 168, 20, 180), 161);
    Udp.write(bufferOn,51);
    Udp.endPacket();
}

void loop() {
 
}

[pylon]

Maybe another device on the network is already using that MAC?

But it's also possible that these devices have bug that doesn't allow the highest bit of the MAC address to be set. Try with 0x81 instead of 0x61 for the first byte. If the PDU answers I would assume the former, if it doesn't answer I would assume the later.