On 03/12/2012 3:36 pm, Victor Hunt wrote:
I am seeing error message TCP26A2 appear in the qsysopr message queue with
some frequency. It tells me that a "Source specific multicast error
occurred". I then goes on to say that "The network will be operating in
compatibility mode, which prevents source filtering."

While I understand the concept of multicast, this error message doesn't
give me much to work on. A search on Google returns a couple of pages
mostly related to PTFs describing conditions when this error does not
display. A search of IBM's software and hardware pages returns no hits at
all for TCP26A2.

Does anyone have any insight on this error? Does the system stay in
compatibility mode until TCP is restarted? Is this a problem?

If you're seeing TCP26A2 frequently, PTF MF54194 for 710 may be helpful.

IBM i 7.1 supports source specific multicast socket APIs that allow an application to join a multicast group and receive only packets from a particular sender. When these APIs are used, TCP/IP must send IGMP report messages to the local router indicating both the multicast group being joined and the sender (source) address so that the router can filter the multicast traffic that it will forward to the local link. If the router is running an older version of IGMP, for compatibility, the report message sent to the router can only include the multicast address and no source filtering is done by the router.

The system will stay in compatibility mode until TCP/IP is restarted as long as the router keeps sending older version IGMP queries (which it should).

It's not a problem -- multicast packets can still be sent and received. The system will filter received multicast packets if an application has set source filters. If you did have any applications using source specific multicast however, there could be increased multicast traffic on the local link since the router is not providing source filtering of the multicast traffic. The wording of TCP26A2 is more alarming than the actual situation.

Brian Jongekryg


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