Cisco Switching Fundamentals
The performance characteristics of a switch are determined to a large extent by how packets
are processed. The newer switches now use ASICs (Application Specific Integrated Circuits)
for high speed hardware switching of packets. This results in much faster performance than
packets processed in software on the route processor with multilayer switches and routers.
While most of the packet forwarding occurs in ASICs, there is some network traffic such as
encrypted packets that must be processed in software.
The switch data plane is where forwarding of packets occur. The forwarding of packets
is done after a decision has been made at the Supervisor Engine PFC (Policy Feature Card)
or line card DFC (Distributed Forwarding Card). The PFC and DFC modules forward packets
in hardware ASICs. The PFC forwarding engine on the Supervisor Engine does Layer 2 and
Layer 3 lookups before deciding how to forward the packet. Line cards with a DFC installed
make Layer 2 and Layer 3 forwarding decisions at the line card and send the packet across
the switch fabric to the destination line card. From there, the packet is forwarded out the
specific switch egress port. Note that when describing the "processing and forwarding of packets"
what that refers to is the switch or router examining Layer 2 and Layer 3 packet header
information (addressing etc.) and rewriting the packet before forwarding it.
The switch has a control plane where specific network control packets are processed for
managing network activity. The switch Supervisor Engine is comprised of a route processor on
the MSFC (Multilayer Switch Feature Card) where the routing table is built. The route
processor must handle certain control packets such as routing advertisements, keepalives,
ICMP, ARP requests and packets destined to the local IP addresses of the router. In addition
there is a switch processor that builds a CAM table for Layer 2 packet processing. The switch
processor manages control packets such as BPDU, CDP, VTP, IGMP, PAgP, LACP and UDLD.
The processing of control packets causes increased switch Supervisor Engine (CPU) utilization.
The management plane is where all network management traffic is processed. This includes
packets from management protocols such as Telnet, SSH, SNMP, NTP and TFTP. The route processor
also manages some of this traffic along with some control plane traffic. In addition the management
plane manages the switch passwords and coordinates traffic between the management, control
plane and data plane.
Traditional Fast Switching (Non-CEF [Cisco Express Forwarding])
Network switches without a PFC (Policy Feature Card) module for Layer 2 and Layer 3 hardware
switching of packets still use the traditional fast switching of packets. The switch Layer 2 CAM
(Content Addressable Memory) table is a list of all connected device MAC addresses with port
assignment and VLAN membership. The switch processor derives the CAM table for Layer 2 lookups
by flooding broadcast advertisements to all locally connected devices. Multilayer switches and
routers have a route processor for Layer 3 lookups and building a routing table for packet routing
decisions. The route processor as well builds an ARP table that is derived from the ARP protocol
for resolving the MAC addresses of remote servers with their assigned IP address for Layer 3 forwarding.