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Layer 2 Switching

  • 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 frame encapsulating the packet; this also causes it to be less error prone.
  • Uses Layer 2 switching for network connectivity and network segmentation (each port is a separate collision domain).
  • Be careful how you segment your network, ensure that the 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 LAN Switches (Layer 2 Services)

An individual Layer 2 switch might offer some or all of the following benefits:

  • Bandwidth---LAN switches provide 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 called VLANs, 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.

Three functions of layer 2 switching

  1. Address learning - Layer 2 switches retain, in their filter tables, the source hardware address 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.

Spanning Tree Protocol

STP is a Layer 2 link management protocol that provides path redundancy while preventing undesirable loops in the network.  For an Ethernet network to function properly, only one active path must exist at Layer 2 between two stations.  STP operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched LAN of multiple segments.

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.

  • Defined as IEEE 802.1d
  • It first elects a root bridge (only 1 per network), root bridge ports are called designated ports which operate as forwarding-state ports.  Forwarding-state ports can send and receive traffic.  Other switches in your network are nonroot bridges.
  • The nonroot bridge's port with the fastest link to the root bridge is called the root port, and it sends and receives traffic.
  • Ports that have the lowest cost to the root bridge are called designated ports.  The other ports on the bridge are considered non designated and will not send or receive traffic, (blocking mode).
  • Switches or bridges running STP, exchange information with what are called Bridge Protocol Data Units (BPDU).  BPDUs send configuration information using multicast frames, BPDUs are also used to send the bridge ID of each device to other devices.  The bridge ID is used to determine the root bridge in the network and to determine the root port.  The Bridge ID is 8 bytes long, includes priority and MAC address.  The default priority of devices using IEEE STP is 32,768 (215).
  • To determine the root bridge the priority and the MAC addresses are combined, if priority is the same, the MAC address is used to determine the who has the lowest ID, which determines who will be the root bridge.
  • Path Cost is used to determine which ports will be used to communicate with the root bridge (designated ports).  STP cost is the total accumulated path cost based on the bandwidth of the links.  The slower the link the higher the cost.

Spanning Tree Protocol Port States

  • Blocking - doesn't forward any frames, but still listens to BPDUs.  Ports default to blocking 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 data frames.
  • 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.

LAN Switching Modes

  • Store and Forward - the entire frame is copied into its buffer and computes the Cyclic Redundancy Check (CRC).  Since it copies the entire frame, latency varies with frame length.  If the frame has a CRC error, is too short (<64 bytes), or is too long (>1518 bytes) it is discarded.  If no error, the destination address (MAC) is looked up in the filter table and is sent to the appropriate interface. Is the default state for 5000 series switches.
  • Cut Through - fastest switching mode as only the destination address is copied.  It will then look up the address in its filter table and send the frame to the appropriate interface.
  • Fragment Free - modified form of Cut Through switching.  The switch waits for the first 64 bytes to pass before forwarding the frame.  If the packet has an error, it usually occurs in the first 64 bytes of the frame. Default mode for 1900 switches.
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