Multilayer Switch is a special L3 switch that has the capability to route packets. There are 2 methods of doing MLS which are route caching and Cisco Express Forwarding (CEF)
Route caching devices have a routing processor and a switching engine. The first packet will be handled by the routing processor while the switch engine observes how the routing processor forwards the first packet of a flow (a unidirectional stream with the same protocol). After that, the switch takes over the process of forwarding or switching these packets in the same flow.
CEF is another method of MLS and can be found on certain hardwares only. CEF is easier on a switch's CPU than route caching. The CEF is enabled by default because it is hardware based. However, in order to run it, IP routing has to be enabled first. If the IP routing is not enabled, the command of "IPv4 CEF not running" will show up when the "#show ip cef" command is run.
CEF has 2 main components which are:
1. FIB - Forwarding Information Base which contains L3 routing information such as found in routing table
2. AT - Adjecency Table which contains L2 switching information and a MAC address table of the sender and destination hop.
CEF has 2 logical planes which are:
1. Control Plane or also known as Layer3 Engine which job is to build the FIB and AT table.
2. Data Plane also known as the hardware engine or Application-Specific Integrated Circuit (ASIC) which does the work of putting data on the memory and forwarding data to the next hop.
-- 22 November 2010 --
Route caching devices have a routing processor and a switching engine. The first packet will be handled by the routing processor while the switch engine observes how the routing processor forwards the first packet of a flow (a unidirectional stream with the same protocol). After that, the switch takes over the process of forwarding or switching these packets in the same flow.
CEF is another method of MLS and can be found on certain hardwares only. CEF is easier on a switch's CPU than route caching. The CEF is enabled by default because it is hardware based. However, in order to run it, IP routing has to be enabled first. If the IP routing is not enabled, the command of "IPv4 CEF not running" will show up when the "#show ip cef" command is run.
CEF has 2 main components which are:
1. FIB - Forwarding Information Base which contains L3 routing information such as found in routing table
2. AT - Adjecency Table which contains L2 switching information and a MAC address table of the sender and destination hop.
CEF has 2 logical planes which are:
1. Control Plane or also known as Layer3 Engine which job is to build the FIB and AT table.
2. Data Plane also known as the hardware engine or Application-Specific Integrated Circuit (ASIC) which does the work of putting data on the memory and forwarding data to the next hop.
-- 22 November 2010 --
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