Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

%MCAST-2-IGMP_FALLBACK: IGMP: Running in FALL BACK mode

Read Cisco's explanation of this error. Can anyone translate to practical terms? Had a 4 hour outage last week I think attributed to this error.

1 REPLY

Re: %MCAST-2-IGMP_FALLBACK: IGMP: Running in FALL BACK mode

Have you read this link?

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008014f550.shtml#subtopic1g

The message is occuring because you are using a group address taht maps to a L2 mcast control packet, remember there is a 32-1 L3 group address-to-L2 mcast mac-address. This 32-to-1 mapping is important when we consider that for a given "reserved" IP multicast address (for example, 224.0.0.1), there are 31 other IP multicast addresses (say, 237.0.0.1, or 239.128.0.1) that map to the same Layer 2 multicast MAC address (01-00-5e-00-00-01).

Note: In PFC2 systems, IP addresses, not MAC addresses, are used to identify specific control traffic and therefore aliasing is not an issue. However, problems will occur if excessive traffic (e.g., a multicast data stream) is sent to a reserved IP address.

Please rate helpful posts.

327
Views
0
Helpful
1
Replies
CreatePlease login to create content