Client Retains Lower MTU on Subsequent Transmissions to Multicast Route

When a client receives an Internet Control Message Protocol (ICMP) message that states "Destination Unreachable", along with "fragment needed and DF set" message indicating that a lower Maximum Transmission Unit (MTU) is needed for a multicast route, the client uses the lower MTU for all subsequent transmissions to that route (multicast address).

To resolve this problem, obtain the latest service pack for Windows 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

This problem was first corrected in Windows 2000 Service Pack 2.

When the client receives an ICMP "Destination unreachable" message indicating need for a lower MTU for a multicast route, it should not modify its MTU. MTU modifications should be made only for unicast routes.