Session 1

COMP 1161: Adv. Network and Security

Switching Technologies

OverviewAddress learningForwarding or filteringLooping avoidance – Spanning

Tree ProtocolLAN switch types

Switching Technologies

Overview – layer 2 switchAdvantages

Hardware based (ASIC)Wire speedLow latencyLow cost

DisadvantagesCan not break up broadcast domainPerformance issueNetwork size limitation

Switching Technologies

Comparison between switches & bridgesBridge – software based, one STP

per bridge, up to 16 portsSwitch – hardware based, many

STP, hundreds of ports

Switching Technologies

Switching functionsAddress learningForward/filter decisionsLoop avoidance

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.

Switching Technologies

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)

Switching Technologies

Switching Technologies

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 bridgeDesignated port – root bridge

ports and non root bridge forwarding ports

Non root bridge

Switching Technologies

STP ruleOne root bridge per network

subnet/VLANOne root port per non-root bridgeOne designated port per segmentAll ports of root bridges are

designated ports

Switching Technologies

Election rule – through BPDURoot 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

Switching Technologies

Switching Technologies

Switching Technologies

STP port statesBlocking – not forward frames but

listen to BPDU (stable state)Listening - only receive BPDULearning – learning MAC

addresses, build MAC tableForwarding – send and receive

data (stable state)

Switching Technologies

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.

Switching Technologies

Switching Technologies