Why MLS?
The first Cisco switches did not have any Layer 3 routing
capability. They merely switched frames at Layer 2 between hosts on the same
VLAN. All inter-VLAN switching was forwarded to the router. The switch was many
times faster in switching packets than the router. A router had greater latency
when it had to forward packets at Layer 3. To get around this problem, as
mentioned in earlier chapters, VLANs were extended throughout the LAN campus,
minimizing the role of routers in the LAN network.
The actual connection between the switch and the router came in
two forms. The first was one physical link per VLAN that was configured on the
switch. The second method involved one physical cable between the switch and
router with trunking enabled. The former was widely deployed because most LAN
designs did not require trunking between a router and switch. After the packet
arrived on a router's interface, the router did the Media Access Control (MAC)
rewrite and software switched the packet out of one of its other interfaces. A
popular router at that time, 4000 router, could only fast switch packets at a
rate of 14,000 packets per second (pps). The enterprise router, 7500 family with
an RSP2 card, could only do 150,000 pps. A Catalyst 5000 switch can switch
packets at millions of pps (Mpps).
The multilayer switching mechanism enabled the use of Catalyst
5000 hardware to switch packets between different subnets in hardware, which
translated into higher performance of packet handling on the network.
Newer hardware for Catalyst 6500 with Cisco Express Forwarding
(CEF) implementation can switch packets at very high rates. For example,
Supervisor II with a Switch Fabric Module (SFM) card can switch packets at rates
as high as 210 Mpps. The Supervisor 720 can go as high as 400 Mpps. The
discussion at the moment is with MLS and its role in Cisco switches.
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