This document describes the Multicast Leaf Recycle Elimination (mLRE) feature on the IOS-XE router.
If hosts are connected on multiple interfaces and request multicast traffic on a router. The router has to make a copy of multicast traffic and has to send it on all the interfaces which request the multicast traffic of that particular multicast group. If the packets are processed serially, which means one packet after another, then it helps the router to improve performance. However, it creates an unfair delay in different nodes due to its nature. This serial processing of multicast traffic is called as LRE in the multicast routers and is by default enable on routers that depend upon their IOS version and model.
Even though processing packets serially causes a difference of 4-12 microseconds between to adjacently processed packets. It could create a significant delay in time-critical environments like Trading company if there are a large number of leaf nodes that request multicast traffic.
This image shows the topology to understand this better.
As you can see that we have 4 hosts connected to the LHR and they are requesting traffic for multicast group 18.104.22.168.
If packet tracer runs on LHR then it is seen that the packet received from the source is silently consumed by LHR and then it created 4 similar packets and forwarded them to each interface connected to the host.
LHR#show platform packet-trace summary
Pkt Input Output State Reason
0 Gi2 <none> CONS Packet Consumed Silently <<< recieved packet from sender
1 Gi2 Gi6 FWD <<< first replicated packet sent to int gig6
2 Gi2 Gi5 FWD <<< first replicated packet sent to int gig5
3 Gi2 Gi4 FWD <<< first replicated packet sent to int gig4
4 Gi2 Gi3 FWD <<< first replicated packet sent to int gig3
If the details of captured packets are opened, you can see the start time and end time for each packet.
If the stop time of packet 1 (02:43:56.203804) and packet 4 (02:43:56.203821) are compared, you can see that there is a time difference of 17 micro-seconds. In some time-critical organizations this delay may not be acceptable and hence needs to be reduced.
In order to avoid this time delay, disable the LRE feature on the router.
If the LRE feature is disabled, the packet processing for the different interface for the replication of multicast traffic is independent of each other and are processed parallelly.
In order to disable the mLRE feature, use this command: outer(config)# platform multicast lre off