Specifications

Multilayer Switching Overview
Introduction to IP Multicast MLS
10
Layer 3-Switched Multicast Packet Rewrite
When a multicast packet is Layer 3-switched from a multicast source to a destination multicast group,
the MMLS-SE performs a packet rewrite based on information learned from the MMLS-RP and stored
in the multicast MLS cache.
For example, if Server A sends a multicast packet addressed to IP multicast group G1 and members of
group G1 are on VLANs other than the source VLAN, the MMLS-SE must perform a packet rewrite
when it replicates the traffic to the other VLANs (the switch also bridges the packet in the source
VLAN).
When the MMLS-SE receives the multicast packet, it is formatted similarly to the sample shown in
Table 38.
The MMLS-SE rewrites the packet as follows:
• Changes the source MAC address in the Layer 2 frame header from the MAC address of the server
to the MAC address of the MMLS-RP (this MAC address is stored in the multicast MLS cache entry
for the flow)
• Decrements the IP header Time to Live (TTL) by one and recalculates the IP header checksum
The result is a rewritten IP multicast packet that appears to have been routed by the router.
The MMLS-SE replicates the rewritten packet onto the appropriate destination VLANs, where it is
forwarded to members of IP multicast group G1.
After the MMLS-SE performs the packet rewrite, the packet is formatted as shown in Table 39:
Partially and Completely Switched Flows
When at least one outgoing router interface for a given flow is multilayer switched, and at least one
outgoing interface is not multilayer switched, that flow is considered partially switched. When a partially
switched flow is created, all multicast traffic belonging to that flow still reaches the router and is
software forwarded on those outgoing interfaces that are not multilayer switched.
A flow might be partially switched instead of completely switched in the following situations:
• Some multicast group destinations are located across the router (not all multicast traffic is received
and sent on subinterfaces of the same trunk link).
Table 38 Layer 3-Switched Multicast Packet Header
Frame Header IP Header Payload
Destination Source Destination Source TTL Checksum Data Checksum
Group G1
MAC
Server A
MAC
Group G1 IP Server A IP n calculation1
Table 39 Layer 3-Switched Multicast Packet Header with Rewrite
Frame Header IP Header Payload
Destination Source Destination Source TTL Checksum Data Checksum
Group G1
MAC
MMLS-RP
MAC
Group G1 IP Server A IP n – 1 calculation2