Understanding Multicast Traceroute (Mtrace) - Dell S4048–ON Configuration Manual

Preventing a PIM Router from Processing a Join
To permit or deny PIM Join/Prune messages on an interface using an extended IP access list, use the following command.
NOTE: Dell Networking recommends not using the ip pim join-filter command on an interface between a source and the
RP router. Using this command in this scenario could cause problems with the PIM-SM source registration process resulting in
excessive traffic being sent to the CPU of both the RP and PIM DR of the source.
Excessive traffic generates when the join process from the RP back to the source is blocked due to a new source group being
permitted in the join-filter. This results in the new source becoming stuck in registering on the DR and the continuous generation of
user datagram protocol (UDP)-encapsulated registration messages between the DR and RP routers which are being sent to the CPU.
• Prevent the PIM SM router from creating a state based on multicast source and/or group.
ip pim join-filter
NOTE: When you configure a join filter, it is applicable for both ingress and egress flows. There is no option to specify in
or out parameters while configuring a join filter.

Understanding Multicast Traceroute (mtrace)

Multicast Traceroute (mtrace) is a multicast diagnostic facility used for tracing multicast paths.
Mtrace enables you to trace the path that a multicast packet takes from its source to the destination. When you initiate mtrace from a
source to a destination, an mtrace Query packet with IGMP type 0x1F is sent to the last-hop multicast router for the given destination. The
mtrace query packet is forwarded hop-by-hop untill it reaches the last-hop router.
NOTE: If the system initiating the mtrace is the last-hop router, then the Query message will not be initiated. Instead, the router
sends the request message to it previous router.
The last-hop router converts this query packet to a request packet by adding a response data block. This response data block contains the
last-hop router's interface address. The response data block inserted by the router also contains the following information:
• Incoming interface details
• Outgoing interface details
• Previous-hop router address
• Forwarding Code
• Query Arrival Time
• Routing Protocol details
The last-hop router calculates the path to reach the source in the reverse direction of the multicast data traffic. Based on this calculation,
the last-hop router estimates the possible next-hop neighbor that is located in the direction towards the source and forwards the request
packet to that neighbor.
Each router along the multicast path fills its response block in a similar manner. When the mtrace request reaches the first-hop router, it
sends the response (with IGMP type 0x1E) to the response destination address specified in the mtrace query.
The response may be returned before reaching the first-hop router if a fatal error condition such as "no route" is encountered along the
path.
If a multicast router along the path does not implement the mtrace feature or if there is an outage, no response is returned.
When the initiator does not get a response for a specified time interval, the system performs a hop-by-hop expanding-length search to pin
point the location in the network where the problem has occurred.
NOTE: You cannot configure the wait time. It is fixed to 3
seconds.
600 Multicast Features