IT資訊-實驗室[MIS LAB ]

Let's look layer2 switch and other what's Different!!

Layer 2 Switching

Devices that forward frames at Layer 2 involve the following functions:
• MAC addresses are learned from the source addresses of incoming frames
.
• A table of MAC addresses and their associated bridge/switch ports is b
uilt and maintained.
• Broadcast and multicast frames are flooded out to all ports.
• Frames destined to unknown locations are flooded out to all ports.
• Bridges and switches communicate with each other using the Spanning-Tr
ee Protocol to eliminate bridging loops.

A Layer 2 switch performs essentially the same function as a transparent bridge. However, a switch can have many ports and can perform hardware-based bridging. Frames are forwarded using specialized hardware called application-specific integrated circuits (ASICs). This hardware gives switching great scalability , with wire-speed performance, low latency, low cost, and high port density. As long as frames are being switched between two Layer 1 interfaces of the same media type, such as two Ethernet connections or an Ethernet connection and a Fast Ethernet connection, the Layer 2 frames do not have to be modified. However, if the two interfaces are different media, such as Ethernet and Token Ring or Ethernet and Fiber Distributed Data Interface (FDDI), the Layer 2 switch must translate the frame contents before sending out the Layer 1 interface. Layer 2 switching is used primarily for workgroup connectivity and network segmentation. Traffic between users and servers in a workgroup

Layer 3 Routing

Devices involved in Layer 3 routing perform the following functions:
• Packets are forwarded between networks, based on Layer 3 addresses.
• An optimal path is determined for a packet to take through a network t
o the next router.
• Packet forwarding involves a table lookup of the destination network,
next-hop router address, and the router’s own outbound interface.
• An optimal path can be chosen from among many possibilities.
• Routers communicate with each other using routing protocols.

By nature, routers do not forward broadcast packets and only forward multicast packets to destinations that are multicast clients. This action provides control over broadcast propagation and offers segmentation of the network into are as of common Layer 3 addressing. Logical addressing is possible on a network with routers because the Layer 3 (network layer) address uniquely identifies a device only at the network layer of the OSI model. Actual frame forwarding occurs using the Layer 2 or data link address of devices. Therefore, some method must exist to associate a device’s data link (MAC) address with its network layer (IP) address. A router must also have addresses from both layers assigned to each of its interfaces connected to a network. This assignment gives the router the functionality to support the logical network layer addresses assigned to the physical networks. In addition, a router must examine the Layer 3 header of each packet before making a routing decision. Layer 3

Layer 3 Switching

Devices involved in Layer 3 switching perform the following functions:
• Packets are forwarded at Layer 3 just as a router would do.
• Packets are switched using specialized hardware ASICs for high-speed a
nd low latency.
• Packets can be forwarded with security control and Quality of Service
(QoS) using Layer 3 address information.

Layer 3 switches are designed to examine and forward packets in high-speed LAN environments. Whereas, a router might impose a bottleneck to forwarding through put, a Layer 3 switch can be placed anywhere in the network.

Layer 4 Switching

Devices involved in Layer 4 switching perform the following functions:
• Packets are forwarded using hardware switching, based on both Layer 3
addressing and Layer 4 application information.
• Layer 3 protocol types (UDP or TCP, for example) in packet headers are
examined.
• Layer 4 segment headers are examined to determine application port num
bers.

Switching at Layer 4 allows finer control over the movement of types of inform ation. For example, traffic can be prioritized according to the source and destination port numbers, and QoS can be defined for end users. Therefore, video or voice data can be switched at a higher level of service with more bandwidth availability than file transfer or HTTP traffic. Layer 4 port numbers for source and destination can also perform traffic accounting. A Layer 4 switch must also allocate a large amount of memory to its forwarding tables. Layer 2 and Layer 3 devices have forwarding tables based on MAC and network addresses, making those tables only as large as the number of network devices. Layer 4 devices, however, must also keep track of application protocols and conversations occurring in the network. Their forwarding tables become proportional to the number of network devices multiplied by the number of applications.

Multilayer Switching (MLS)

Devices involved in MLS perform the following functions:
• Packets are forwarded in hardware that combines Layer 2, Layer 3, and
Layer 4 switching.
• Packets are forwarded at wire speed.
• The traditional Layer 3 routing function is provided as route one, swi
tch many. Routing sets up a network conversation, while hardware switches the traffic flow at high speeds.
Cisco switches perform multilayer switching at Layer 3 and Layer 4. At Layer 3 , the Catalyst family of switches will cache traffic flows based on IP addresses. At Layer 4, the traffic flows are cached based on source and destination addresses, in addition to source and destination ports. All switching is perfor med in hardware, providing equal performance at both Layer 3 and Layer 4 switc hing.