3. Switching Technologies
Overview – layer 2 switch
Advantages
Hardware based (ASIC)
Wire speed
Low latency
Low cost
Disadvantages
Can not break up broadcast domain
Performance issue
Network size limitation
4. Switching Technologies
Comparison between switches & bridges
Bridge – software based, one STP per bridge, up
to 16 ports
Switch – hardware based, many STP, hundreds of
ports
6. Switching Technologies
Address learning - Layer-2 switches and bridges
remember the source hardware address of each frame
received on an interface and enter this information into a
MAC database.
Forward/filter decisions - When a frame is received on
an interface, the switch looks at the destination hardware
address and finds the exit inter-face in the MAC database.
8. Switching Technologies
Loop avoidance - If multiple connections between
switches are created for redundancy, network loops can
occur. The Spanning-Tree Protocol (STP) is used to stop
network loops and allow redundancy
Broadcast storm
Multiple frame copies
Thrashing(constantly updating the MAC filter table with same
SA)
Multiple loops(generating throughout an internetwork)
11. Switching Technologies
STP (Spanning Tree Protocol)
Root bridge – reference point, the
lowest bridge ID
Root port – the port with the lowest cost
to the root bridge
Designated port – root bridge ports and
non root bridge forwarding ports
Non root bridge
12. Switching Technologies
STP rule
One root bridge per network
subnet/VLAN
One root port per non-root bridge
One designated port per segment
All ports of root bridges are designated
ports
13. Switching Technologies
Election rule – through BPDU
Root bridge – highest priority (lowest
priority value), lowest MAC address
Root port – lowest cost to the root
bridge
Designated port – all root bridge ports,
lowest cost to the root bridge, bridge ID
16. Switching Technologies
STP port states
Blocking – not forward frames but listen to
BPDU (stable state)
Listening - only receive BPDU
Learning – learning MAC addresses, build MAC
table
Forwarding – send and receive data (stable state)
18. Switching Technologies
LAN switch types
Store and forward - the complete data frame is received on the
switch’s buffer, a CRC is run, and then the destination address
is looked up in the MAC filter table.
Cut through - the switch only waits for the destination hardware
address to be received and then looks up the destination
address in the MAC filter table.
Fragment free - the default for the Catalyst 1900 switch, it is
sometimes referred to as modified cut-through. Checks the first
64 bytes of a frame for fragmentation (because of possible
collisions) before forwarding the frame.
