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Chapter 7
Multi-Layer Switching (MLS)
No Cache Entry
When a qualified (destined for an MLS-RP MAC address) incoming frame is
compared against the cache and fails (no match is found), a cache entry is
made. At this point, the packet is tagged as a candidate packet.
Once the cache entry is made, the packet is forwarded to the router
(MLS-RP) for normal processing. Here the router performs the route
lookup, rewrites the layer 2 header, and sends the packet out the next-hop
interface, whichever it may be.
The state of the MLS cache is only partial at this stage. A complete flow
cache has not been established because the MLS-SE has only seen a packet
come in and be forwarded to the router. It still needs to see something that
it can tag as an enable packet come back from the router.
Identifying Enable Packets
Enable packets are the missing piece of the flow cache puzzle. Just as the
MLS-SE watched all incoming frames destined for the MLS-RP MAC
addresses, it also watches all of the packets coming from the MLS-RP.
It watches these packets hoping for a match with the candidate packet
cache entry. If it can make the match, the packet is tagged as an enable
packet and the remaining elements of the flow cache are completed in the
CAM table. Figure 7.5 depicts the occurrence of an enable packet.
F I G U R E 7 . 5
Enable packet
VLAN10
VLAN50
VLAN10
VLAN50
VLAN10
VLAN50
Switch1
FE0/0.10
FE0/0
FE0/0.50
Enable packet
RouterA
HostA
HostB
HostC
HostD
HostE
HostF
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