Multi-Layer Switching

Layer 2 Switching
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Decisions on three tables are made simultaneously by independent portion of switch hardware. Then packets are placed on egress queue based on Qos
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Multilayer Switching
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1st generation MLS – route caching based: route processor (RP) process the first packet to determine the route-shortcut which is stored in MLS cache, Switch Engine (SE) forward the subsequent packet of that flow based on the shortcut route. [Namely, Netflow LAN switching, flow-based or demand-based switching, as well as “route one, switch many”].

2nd generation MLS – Topology based: Route Processor pushes latest routing table into Forwarding Information Base (FIB), a dedicated portion of hardware. all 4 tables including security TCAM, Qos TCAM, Layer 2 CAM, FIB are checked simultaneously in hardware. [Namely, Cisco Express Forwarding, only 2nd generation MLS is supported on cisco IOS switch families].

Multilayer Switching Exceptions
Packets in following scenarios will be flagged by MLS for further processing, and sent or “punted” to the switch CPU for process switching.
– ARP requests and replies
– IP packets requiring a response from a router (TTL has expired, MTU is exceeded, fragmentation is needed, and so on)
– IP broadcasts that will be relayed as unicast (DHCP requests, IP helper-address functions)
– Routing protocol updates
– Cisco Discovery Protocol packets
– IPX routing protocol and service advertisements
– Packets needing encryption
– Packets triggering Network Address Translation (NAT)
– Other non-IP and non-IPX protocol packets (AppleTalk, DECnet, and so on)