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Chapter 7
Multi-Layer Switching (MLS)
You can clearly see that the MFSC has been assigned the XTAG value of 1.
The MFSC receives the assignment because the MFSC acts as the MLS-RP.
In this example, only one MAC address is associated with XTAG 1. How-
ever, there are two VLANs associated with it.
MLS Cache
Once MLS-SEs have established CAM entries for MLS-RPs, the switch is
ready to start scanning packets and creating cache entries. This was
described previously as identification of candidate and enable packets.
The cache entries are made in order to maintain flow data. Flow data
allows the MLS-SE to rewrite the packets with the new source and destina-
tion MAC address and then forward the packets. All this is done without
sending the packets to the router for a route lookup and to be rewritten.
Cache entries happen in two steps:
Candidate packet entries
Enable packet entries
After these entries have been made in the MLS-SE, subsequent packets are
matched against existing flow entries and dealt with accordingly.
Identifying Candidate Packets
The process of identifying candidate packets is quite simple. As has already
been established, the MLS-SE has MAC address entries for any and all inter-
faces that come from the MLS-RP. Using this information, the MLS-SE starts
watching for incoming frames destined for any MLS-RP-related MAC
addresses.
An incoming frame will match one of the following three criteria:
Not destined for an MLS-RP MAC address
Destined for an MLS-RP MAC address, but no cache entry exists for
this flow
Destined for an MLS-RP MAC address, but a cache entry already
exists for this flow
Different actions will be taken by the MLS-SE, depending on which cri-
teria match. We will discuss the first one right now. The others will be
addressed in the following sections.
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