switches

Fast forward switching

Fast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address.

Cut through switching

In cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port to forward the data. The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. no error checking faster but more corrupt frames

Fragment-free switching

In fragment-free switching, the switch stores the first 64 bytes of the frame before forwarding.

Store-and-forward switching

In store-and-forward switching, when the switch receives the frame, it stores the data in buffers until the complete frame has been received. During the storage process, the switch analyzes the frame for information about its destination. In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame. Store-and-forward switching is required for Quality of Service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary. For example, voice over IP data streams need to have priority over web-browsing traffic.

FIB

Like a routing table / determine the best path to destination network based on the network portion of IP address. Don't have to look up route cache optimized.

Shared Memory Buffering

deposits all frames into a common memory buffer that all the ports on the switch share. The amount of buffer memory required by a port is dynamically allocated. The frames in the buffer are linked dynamically to the destination port. This allows the packet to be received on one port and then transmitted on another port, without moving it to a different queue. The switch keeps a map of frame to port links showing where a packet needs to be transmitted. The map link is cleared after the frame has been successfully transmitted.

port based memory buffering

frames are stored in queues that are linked to specific incoming and outgoing ports. A frame is transmitted to the outgoing port only when all the frames ahead of it in the queue have been successfully transmitted. It is possible for a single frame to delay the transmission of all the frames in memory because of a busy destination port. This delay occurs even if the other frames could be transmitted to open destination ports.

adjacency table

maintains layer 2 next hop address for all FIB entries The adjacency table can be built separately from the FIB table, allowing both to be built without any packets being process switched. The MAC header rewrite used to forward a packet is not stored in cache entries, so changes in a MAC header rewrite string do not require invalidation of cache entries.

process switching (router supports these packet forwarding mechanisms)

packet it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet. router does this for every packet, even if the destination is the same for a stream of packets. slow and rarely used

Fast Switching (router supports these packet forwarding mechanisms)

uses a fast-switching cache to store next-hop information packet is forwarded to the control plane where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is also stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention.

CEF

CEF decouples the usual strict interdependence between Layer 2 and Layer 3 decision making The two main components of CEF operation are the: Forwarding Information Base (FIB) Adjacency tables

CEF (router supports these packet forwarding mechanisms)

CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information. Cisco Express Forwarding is the fastest forwarding mechanism and the preferred choice on Cisco routers.