Layer 2 Switching
- Layer 2 switching is hardware based, it uses the host's Media Access Control
- 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
- 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
- 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
- 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
- Address learning - Layer 2 switches retain, in their filter tables,
the source hardware address and port interface it was received on.
- 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.
- 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
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
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
- 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
- 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.