Switching Functionality
To accept how switches and routers should be alleged and placed in a arrangement design, you
should aboriginal accept how to booty advantage of abstracts advice at altered layers.
The OSI archetypal separates abstracts advice into seven layers, as apparent in Table 2-2. Each
layer has a specific activity and a specific agreement so that two accessories can barter abstracts on
the aforementioned layer. A agreement abstracts assemblage (PDU) is the all-encompassing name for a block of abstracts that a layer
on one accessory exchanges with the aforementioned band on a accessory device. A PDU is encapsulated in a
layer’s agreement afore it is fabricated accessible to a lower-level band or unencapsulated afore being
handed to a higher-level layer.
In Table 2-2, Layers 2, 3, and 4 are represented by the abstracts link, network, and carriage layers,
respectively, with PDU’s frame, packet, and TCP segment. When a TCP articulation (Layer 4)
needs to be transmitted to accession station, the TCP articulation is encapsulated as a packet (Layer
3) and added encapsulated as a anatomy (Layer 2). The accepting base will unencapsulate
Layers 2 and 3 afore processing the aboriginal TCP segment.
The layered protocols additionally administer to networking devices. For example, a Band 2 accessory will
transfer abstracts by attractive at Band 2’s PDU attack information. Any upper-layer agreement will
not be looked at or alike understood. Layer-specific accessories are discussed in detail in the
following sections.
Layer 2 Switching
Devices that advanced frames at Band 2 absorb the afterward functions:
• MAC addresses are abstruse from the antecedent addresses of admission frames.
Table 2-2 Layers of Abstracts Communications
OSI Band Agreement Abstracts Assemblage Mechanism to Process PDU
7 (application)
6 (presentation)
5 (session)
4 (transport) TCP articulation TCP port
3 (network) Packet Router
2 (data link) Anatomy Switch/bridge
1 (physical)
• A table of MAC addresses and their associated bridge/switch ports is congenital and
maintained.
• Advertisement and multicast frames are abounding out to all ports.
• Frames destined to alien locations are abounding out to all ports.
• Bridges and switches acquaint with anniversary added appliance the Spanning-Tree Agreement to
eliminate bridging loops.
A Band 2 about-face performs about the aforementioned activity as a cellophane bridge. However, a
switch can accept abounding ports and can accomplish hardware-based bridging. Frames are forwarded
using specialized accouterments alleged application-specific chip circuits (ASICs). This
hardware gives switching abundant scalability, with wire-speed performance, low latency, low cost,
and aerial anchorage density.
As continued as frames are actuality switched amid two Band 1 interfaces of the aforementioned media type,
such as two Ethernet access or an Ethernet affiliation and a Fast Ethernet connection, the
Layer 2 frames do not accept to be modified. However, if the two interfaces are altered media,
such as Ethernet and Token Ring or Ethernet and Fiber Distributed Abstracts Interface (FDDI), the
Layer 2 about-face charge construe the anatomy capacity afore sending out the Band 1 interface.
Layer 2 switching is acclimated primarily for workgroup connectivity and arrangement segmentation.
Traffic amid users and servers in a workgroup can be independent aural the switch. In
addition, the cardinal of stations on a arrangement articulation can be bargain with a switch,
minimizing the blow area size.
One check to Band 2 switching is that it cannot be scaled actual effectively. Switches must
forward advertisement frames to all ports, causing ample switched networks to become large
broadcast domains. In addition, the Spanning-Tree Agreement can accept a apathetic aggregation time
when the about-face cartography changes. It additionally can block assertive about-face ports, preventing data
transfer. (Chapter 5, “Redundant About-face Links,” discusses the Spanning-Tree Agreement in
further detail.) Band 2 switching abandoned cannot accommodate an effective, scalable arrangement design.
Layer 3 Routing
Devices complex in Band 3 acquisition accomplish the afterward functions:
• Packets are forwarded amid networks, based on Band 3 addresses.
• An optimal aisle is bent for a packet to booty through a arrangement to the abutting router.
• Packet forwarding involves a table lookup of the destination network, next-hop router
address, and the router’s own outbound interface.
• An optimal aisle can be alleged from amid abounding possibilities.
• Routers acquaint with anniversary added appliance acquisition protocols.
22 Chapter 2: Campus Arrangement Architecture Models
By nature, routers do not advanced advertisement packets and alone advanced multicast packets to
destinations that are multicast clients. This activity provides ascendancy over advertisement propagation
and offers analysis of the arrangement into areas of accepted Band 3 addressing.
Logical acclamation is accessible on a arrangement with routers because the Band 3 (network layer)
address abnormally identifies a accessory alone at the arrangement band of the OSI model. Actual frame
forwarding occurs appliance the Band 2 or abstracts articulation abode of devices. Therefore, some method
must abide to accessory a device’s abstracts articulation (MAC) abode with its arrangement band (IP) address.
A router charge additionally accept addresses from both layers assigned to anniversary of its interfaces connected
to a network. This appointment gives the router the functionality to abutment the analytic network
layer addresses assigned to the concrete networks.
In addition, a router charge appraise the Band 3 attack of anniversary packet afore authoritative a routing
decision. Band 3 aegis and ascendancy can be implemented on any router interface by appliance the
source and destination addresses, protocol, or added Band 3 aspect to accomplish decisions on
whether to absolute or advanced the packets.
Layer 3 acquisition is about performed by microprocessor-based engines, which crave CPU
cycles to appraise the arrangement band attack of anniversary packet. The acquisition table of optimal paths
to Band 3 networks can additionally be a ample table of activating values, acute a bound look-up delay.
Although a router can be placed anywhere in a network, the router can become a aqueduct due
to a cessation of packet assay and processing.
Layer 3 Switching
Devices complex in Band 3 switching accomplish the afterward functions:
• Packets are forwarded at Band 3 aloof as a router would do.
• Packets are switched appliance specialized accouterments ASICs for accelerated and low latency.
• Packets can be forwarded with aegis ascendancy and Quality of Account (QoS) appliance Layer
3 abode information.
Layer 3 switches are advised to appraise and advanced packets in accelerated LAN
environments. Whereas, a router ability appoint a aqueduct to forwarding throughput, a Layer
3 about-face can be placed anywhere in the network.
Layer 4 Switching
Devices complex in Band 4 switching accomplish the afterward functions:
• Packets are forwarded appliance accouterments switching, based on both Band 3 acclamation and
Layer 4 appliance information.
• Band 3 agreement types (UDP or TCP, for example) in packet headers are examined.
• Band 4 articulation headers are advised to actuate appliance anchorage numbers.
Campus Arrangement Models 23
Switching at Band 4 allows bigger ascendancy over the movement of types of information. For
example, cartage can be prioritized according to the antecedent and destination anchorage numbers, and
QoS can be authentic for end users. Therefore, video or articulation abstracts can be switched at a higher
level of account with added bandwidth availability than book alteration or HTTP traffic. Band 4 port
numbers for antecedent and destination can additionally accomplish cartage accounting.
A Band 4 about-face charge additionally admeasure a ample bulk of anamnesis to its forwarding tables. Layer
2 and Band 3 accessories accept forwarding tables based on MAC and arrangement addresses, making
those tables alone as ample as the cardinal of arrangement devices. Band 4 devices, however, must
also accumulate clue of appliance protocols and conversations occurring in the network. Their
forwarding tables become proportional to the cardinal of arrangement accessories assorted by the
number of applications.
Multilayer Switching (MLS)
Devices complex in MLS accomplish the afterward functions:
• Packets are forwarded in accouterments that combines Band 2, Band 3, and Band 4 switching.
• Packets are forwarded at wire speed.
• The acceptable Band 3 acquisition activity is provided as avenue one, about-face many. Routing
sets up a arrangement conversation, while accouterments switches the cartage breeze at aerial speeds.
Cisco switches accomplish multilayer switching at Band 3 and Band 4. At Band 3, the Catalyst
family of switches will accumulation cartage flows based on IP addresses. At Band 4, the cartage flows
are buried based on antecedent and destination addresses, in accession to antecedent and destination
ports. All switching is performed in hardware, accouterment according achievement at both Band 3 and
Layer 4 switching.