term paper on layer 2 switching

7/30/2019 term paper on Layer 2 Switching

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Layer 2 Switching

Layer 2 switches are similar to multiport bridges in that they learn and forward frames on each

port. The major difference is the involvement of hardware that ensures that multiple switching

paths inside the switch can be active at the same time. For example, consider Figure 1, which

details a four-port switch with stations A on port 1, B on port 2, C on port 3 and D on port 4.

Assume that A desires to communicate with B, and C desires to communicate with D. In a single

CPU bridge, this forwarding would typically be done in software, where the CPU would pick up

frames from each of the ports sequentially and forward them to appropriate output ports. This

process is highly inefficient in a scenario like the one indicated previously, where the traffic

between A and B has no relation to the traffic between C and D.

Figure : Layer 2 switch with External Router for Inter-VLAN traffic and connecting to the

Internet

Enter hardware-based Layer 2 switching. Layer 2 switches with their hardware support are able

to forward such frames in parallel so that A and B and C and D can have simultaneous

conversations. The parallel-ism has many advantages. Assume that A and B are NetBIOS

stations, while C and D are Internet Protocol (IP) stations. There may be no rea-son for the

communication between A and C and A and D. Layer 2 switching allows this coexistence

without sacrificing efficiency.

Layer 2 switching is hardware based; it uses the host's Media Access Control (MAC) address. Switches use Application Specific Integrated Circuits (ASIC) to build and maintain filter tables. Switches tend to be faster than Routers, because they don't look at the logical address (Network

layer headers), they instead use the hardware address defined at the Data Link (MAC) layer to

decide whether to forward or discard the frame.

Layer 2 switching is so efficient because it doesn't modify the data packet only the frameencapsulating the packet; this also causes it to be less error prone.
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Uses Layer 2 switching for network connectivity and network segmentation (each port is aseparate collision domain).

During planning, the planner needs to be careful how a network is segmented and ensure thattheir users spend 80% of their time on their local segment, and all the segments of a switch are

still in the same broadcast domain. Use routers to split up broadcast domains.

Benefits of Layer 2 Services

A layer 2 service might offer some or all of the following benefits:

Bandwidth---L2 service provides excellent performance for individual users by allocating

dedicated bandwidth to each switch port (for example, each network segment). This technique is

known as microsegmenting.

VLANs---LAN switches can group individual ports into logical switched workgroups calledVLANs, thereby restricting the broadcast domain to designated VLAN member ports. VLANs

are also known as switched domains and autonomous switching domains. Communication

between VLANs requires a router.

Automated packet recognition and translation---Cisco's unique Automatic Packet

Recognition and Translation (APaRT) technology recognizes and converts a variety of Ethernet

protocol formats into industry-standard CDDI/FDDI formats. With no changes needed in either

client or server end stations the Catalyst solution can provide an easy migration to 100-Mbps

server access while preserving the user's investment in existing shared 10Base-T LANs.

The 3 Functions of Layer 2 Services

1. Address learning - Layer 2 switches retain, in their filter tables, the source hardwareaddress and port interface it was received on.

2. Forward/Filter decisions - When a frame is received, the switch looks at the

destination hardware address and finds the interface it is on in the filter table. If the

address is unknown, the frame is broadcast on all interfaces except the one it was

received on.

3. Loop Avoidance - If multiple connections between switches exist for redundancy,

network loops can occur. Spanning Tree Protocol is used to stop loops while still

allowing redundancy.


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Spanning Tree Protocol

STP is a Layer 2 link management protocol that provides path redundancy while preventing

undesirable loops in the networks.The Spanning Tree Protocol (STP) is a network protocol that

ensures a loop-free topology for any bridged Ethernet local area network. The basic function of

STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning treealso allows a network design to include spare (redundant) links to provide automatic backup

paths if an active link fails, without the danger of bridge loops, or the need for manual

enabling/disabling of these backup links.

Spanning Tree Protocol (STP) is standardized as IEEE 802.1D. As the name suggests, it creates

a spanning tree within a networkof connected layer-2 bridges (typically Ethernet switches), and

disables those links that are not part of the spanning tree, leaving a single active path between

any two network nodes. Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges

and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure

that you do not create loops when you have redundant paths in your network. Loops are deadlyto a network.

STP is based on an algorithm that was invented by Radia Perlman while she was working

forDigital Equipment Corporation.

The Catalyst series switches use STP (IEEE 802.1D bridge protocol) on all Ethernet virtual

LANS (VLANs). When you create fault-tolerant internetworks, you must have a loop-free path

between all nodes in a network. In STP, an algorithm calculates the best loop-free path

throughout a Catalyst-switched network. The switches send and receive spanning-tree packets at

regular intervals (2 seconds). The switches do not forward the packets, but use the packets to

identify a loop-free path. The default configuration has STP enabled for all VLANs.

Multiple active paths between stations cause loops in the network. If a loop exists in the

network, you might receive duplicate messages. When loops occur, some switches see stations

on both sides of the switch. This condition confuses the forwarding algorithm and allows

duplicate frames to be forwarded.

To provide path redundancy, STP defines a tree that spans all switches in an extended

network. STP forces certain redundant data paths into a standby (blocked) state. If one network

segment in the STP becomes unreachable, or if STP costs change, the spanning-tree algorithm

reconfigures the spanning-tree topology and reestablishes the link by activating the standby path.
http://en.wikipedia.org/wiki/Network_protocolhttp://en.wikipedia.org/wiki/Network_topologyhttp://en.wikipedia.org/wiki/Bridging_(networking)http://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/Local_area_networkhttp://en.wikipedia.org/wiki/Bridge_loophttp://en.wikipedia.org/wiki/Broadcast_radiationhttp://en.wikipedia.org/wiki/Network_planning_and_designhttp://en.wikipedia.org/wiki/IEEE_802.1Dhttp://en.wikipedia.org/wiki/Spanning_tree_(mathematics)http://en.wikipedia.org/wiki/Networkhttp://en.wikipedia.org/wiki/Network_bridgehttp://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/Network_switchhttp://en.wikipedia.org/wiki/Algorithmhttp://en.wikipedia.org/wiki/Radia_Perlmanhttp://en.wikipedia.org/wiki/Digital_Equipment_Corporationhttp://en.wikipedia.org/wiki/Digital_Equipment_Corporationhttp://en.wikipedia.org/wiki/Radia_Perlmanhttp://en.wikipedia.org/wiki/Algorithmhttp://en.wikipedia.org/wiki/Network_switchhttp://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/Network_bridgehttp://en.wikipedia.org/wiki/Networkhttp://en.wikipedia.org/wiki/Spanning_tree_(mathematics)http://en.wikipedia.org/wiki/IEEE_802.1Dhttp://en.wikipedia.org/wiki/Network_planning_and_designhttp://en.wikipedia.org/wiki/Broadcast_radiationhttp://en.wikipedia.org/wiki/Bridge_loophttp://en.wikipedia.org/wiki/Local_area_networkhttp://en.wikipedia.org/wiki/Ethernethttp://en.wikipedia.org/wiki/Bridging_(networking)http://en.wikipedia.org/wiki/Network_topologyhttp://en.wikipedia.org/wiki/Network_protocol


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Spanning Tree Protocol Port States

Blocking - doesn't forward any frames, but still listens to BPDUs. Ports default toblocking when the switch powers on. Used to prevent network loops. If a blocked port is

to become the designated port, it will first enter listening state to ensure that it won't

create a loop once it goes into forwarding state.

Listening - listens to BPDUs to ensure no loops occur on the network before passing dataframes.

Learning - learns MAC addresses and builds filter table, doesn't forward frames. Forwarding - sends and receives all data on the bridge ports. A forwarding port has

been determined to have the lowest cost to the root bridge.

term paper on layer 2 switching

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