Finding Non-Pruning Routers

In Dense mode multicast routing protocols, a router that fails to prune (whether it be a router that does not support pruning, a misconfigured router that sends prunes to the wrong place, or a buggy router) causes that router to act as a sink for all multicast traffic. This is, understandably, not very desirable.

There are three scripts that will assist you with finding such routers. They analyze the current multicast forwarding cache and display the percentage of traffic that is flowing towards each interface. Sample output from the SNMP version:

1: 423/423 (100%) <- possible non-pruner
3: 306/388 (78%)
7: 289/422 (68%)
8: 289/420 (68%)
10: 227/422 (53%)
11: 416/422 (98%) <- suspicious amount of traffic
12: 282/417 (67%)
15: 419/419 (100%) <- possible non-pruner
19: 19/422 (4%)
20: 283/421 (67%)
25: 74/74 (100%) <- possible non-pruner
26: 225/419 (53%)
27: 280/410 (68%)
28: 244/407 (59%)
30: 286/419 (68%)
31: 418/418 (100%) <- possible non-pruner
32: 245/418 (58%)
33: 280/413 (67%)
35: 273/418 (65%)

In this case, this is a Cisco router at a service provider. IfIndex 1 is an Ethernet, and since IOS doesn't support DVMRP pruning on multi-access links, this interface is shown as a non-pruner. In this case, the router being monitored is the non-pruning culprit.

The other interfaces shown as forwarding 100% of their traffic are likely to point towards close non-pruning routers.

Just because an interface doesn't show 100% doesn't mean that there's not a non-pruning router in that direction. If the non-pruning router has multiple paths, and this router is only being used for some sources, this analysis will not find said router.

And, non-pruners are only in this particular direction; they're not necessarily this particular neighbor.

This material is based upon work supported by the National Science Foundation under Grant No. 9729498. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).