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Understanding How Layer 3 Switching Works
After the switch rewrites an IP multicast packet, it is (conceptually) formatted as follows:
Layer 2 Frame Header
Destination
Group G1 MAC
Understanding MLS
Supervisor Engine 1, PFC, and MSFC or MSFC2 can only do MLS internally with the MSFC or MSFC2
Note
in the same chassis; an external MLS-RP cannot be used in place of the internal MLS-RP.
Supervisor Engine 1, PFC, and MSFC or MSFC2 provide Layer 3 switching with MLS. Layer 3
switching with MLS identifies flows on the switch after the first packet has been routed by the MSFC
and transfers the process of forwarding the remaining traffic in the flow to the switch, which reduces the
load on the MSFC.
These sections describe MLS:
•
•
•
•
•
Understanding MLS Flows
Layer 3 protocols, such as IP and IPX, are connectionless—they deliver every packet independently of
every other packet. However, actual network traffic consists of many end-to-end conversations, or flows,
between users or applications.
MLS supports unicast and multicast flows:
•
•
For example, communication from a client to a server and from the server to the client are separate flows.
The Telnet traffic that is transferred from a particular source to a particular destination comprises a
separate flow from File Transfer Protocol (FTP) packets between the same source and destination.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
14-4
Layer 3 IP Header
Source
Destination
MSFC MAC
Group G1 IP
Understanding MLS Flows, page 14-4
Understanding the MLS Cache, page 14-5
Understanding Flow Masks, page 14-6
Partially and Completely Switched Multicast Flows, page 14-10
MLS Examples, page 14-11
A unicast flow can be any of the following:
All traffic to a particular destination
–
–
All traffic from a particular source to a particular destination
All traffic from a particular source to a particular destination that shares the same protocol and
–
transport-layer information
A multicast flow is all traffic with the same protocol and transport-layer information from a
particular source to the members of a particular destination multicast group.
Source
TTL
Source A IP
n–1
Chapter 14
Configuring MLS
Data FCS
Checksum
calculation2
OL-8978-04
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