Tuesday, May 17, 2016

Layer 3 Switch/Multilayer Switch Basic Operations Explained


A layer 3 switchELITEGLOBE: Layer 3 switches offer a lot more functionalities than layer 2 switches. Some of these services which make a layer 3 switch better than a layer 2 Switch are advanced CAM table, FIB
table, next hop IP address, next hop MAC address etc. 
If we take a closer look at the operation of the layer 3 switches, then we find that layer 3 switches not only perform the functions of the layer 2 switches but also, they are responsible for some extra operations based on layer 3 and layer 4 information.Unlike a layer 2 switch, a layer 3 switch maintains two tables:
  • CAM Table and,
  • FIB table

CAM Table:

CAM table works as layer 2 forwarding table. It typically stores information about MAC addresses and destination ports. So, when a packet is received , a Switch takes a reference for the destination MAC address of the incoming frame in the MAC table and forwards the frames to the destination ports specified in the table.
The CAM tables contains typically three important information:
  • MAC address
  • Egress Port and,
  • VLAN

Layer 3 Switch FIB Table:

The FIB table is also known as ‘Forwarding Information Base’ table. A FIB table works as a layer 3 forwarding table and it typically contains the following information:
  • IP address,
  • Next hop IP address,
  • Next hop MAC address and,
  • Egress port
If, at layer 2, the associated look-up table is called CAM table, then the associated table for layer 3 lookups is called a FIB table. Besides, containing the above information, the FIB table also contains MAC rewrite information.
Also, layer 3 switching provides additional support for the ACL (Access Control List) and QoS (Quality of Service) prioritization just like layer 2 switching.
For example, a layer 2 switch might only be able to apply rate-limiting to frames based on their source or destination MAC address, whereas a multilayer switch generally supports rate-limiting frames on IP/MAC addresses.
We can see everything this way:
The CAM Table separates the switch from the hub and similarly, the FIB table separates the ethernet switch from the layer 3 switch.

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