Hi, we have some serious trouble to get multicast networking to work on our AMD Opteron machines. The problem seems to be that for some reason the amd64 machines are sending IGMP V3 messages with a wrong checksum whereas our Intel boxes are sending IGMP V2 messages. Following is a packet dump from one of our Opteron boxes as we start xntpd in multicast mode (IP addresses changed to protect the innocent): "uname -a" output: Linux monster540 2.4.21-201-smp #1 SMP Wed Feb 18 19:17:53 UTC 2004 x86_64 x86_64 x86_64 GNU/Linux "tethereal -V igmp" output: Capturing on eth0 Frame 1 (54 bytes on wire, 54 bytes captured) Arrival Time: Feb 25, 2004 14:40:01.975282000 Time delta from previous packet: 0.000000000 seconds Time relative to first packet: 0.000000000 seconds Frame Number: 1 Packet Length: 54 bytes Capture Length: 54 bytes Ethernet II, Src: 00:09:3d:00:09:fe, Dst: 01:00:5e:00:00:16 Destination: 01:00:5e:00:00:16 (01:00:5e:00:00:16) Source: 00:09:3d:00:09:fe (00:09:3d:00:09:fe) Type: IP (0x0800) Internet Protocol, Src Addr: some.ip.add.ress (some.ip.add.ress), Dst Addr: 224.0.0.22 (224.0.0.22) Version: 4 Header length: 24 bytes Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00) 1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30) .... ..0. = ECN-Capable Transport (ECT): 0 .... ...0 = ECN-CE: 0 Total Length: 40 Identification: 0xa360 (41824) Flags: 0x04 .1.. = Don't fragment: Set ..0. = More fragments: Not set Fragment offset: 0 Time to live: 1 Protocol: IGMP (0x02) Header checksum: 0x1ff5 (correct) Source: some.ip.add.ress (some.ip.add.ress) Destination: 224.0.0.22 (224.0.0.22) Options: (4 bytes) Router Alert: Every router examines packet Internet Group Management Protocol IGMP Version: 3 Type: Membership Report (0x22) Header checksum: 0xf9fe (incorrect, should be 0xf8fc) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this looks suspicious Num Group Records: 1 Group Record : 224.0.1.1 Change To Exclude Mode Record Type: Change To Exclude Mode (4) Aux Data Len: 0 Num Src: 0 Multicast Address: 224.0.1.1 (224.0.1.1) If I do the same thing on a i686 machine it looks like this: "uname -a": Linux greyhound 2.4.21-192-default #1 Wed Feb 18 19:26:28 UTC 2004 i686 i686 i386 GNU/Linux "tethereal -V igmp": Frame 2 (46 bytes on wire, 46 bytes captured) Arrival Time: Feb 25, 2004 14:41:16.268523000 Time delta from previous packet: 1.944834000 seconds Time relative to first packet: 1.944834000 seconds Frame Number: 2 Packet Length: 46 bytes Capture Length: 46 bytes Ethernet II, Src: 00:06:5b:76:d0:03, Dst: 01:00:5e:00:01:01 Destination: 01:00:5e:00:01:01 (01:00:5e:00:01:01) Source: 00:06:5b:76:d0:03 (00:06:5b:76:d0:03) Type: IP (0x0800) Internet Protocol, Src Addr: some.other.ip.addr (some.other.ip.addr), Dst Addr: 224.0.1.1 (224.0.1.1) Version: 4 Header length: 24 bytes Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00) 1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30) .... ..0. = ECN-Capable Transport (ECT): 0 .... ...0 = ECN-CE: 0 Total Length: 32 Identification: 0x0bfd (3069) Flags: 0x04 .1.. = Don't fragment: Set ..0. = More fragments: Not set Fragment offset: 0 Time to live: 1 Protocol: IGMP (0x02) Header checksum: 0xc6c3 (correct) Source: some.other.ip.addr (some.other.ip.addr) Destination: 224.0.1.1 (224.0.1.1) Options: (4 bytes) Router Alert: Every router examines packet Internet Group Management Protocol IGMP Version: 2 Type: Membership Report (0x16) Max Response Time: 0.0 sec (0x00) Header checksum: 0x08fe (correct) Multicast Address: 224.0.1.1 (224.0.1.1) Does anybody know why these two platforms behave differently and why the x86_64 machines uses IGMP V3 and not V2? Karsten.