bridges and switches

8/3/2019 Bridges and Switches

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Bridges and Switches

Credits: Web resources of books on

Computer Networks by

Peterson & Davie, Forouzon andStallings



Bridges

• Link layer device

– stores and forwards Ethernet frames– examines frame header and selectively

forwards frames based on MAC destaddress

– when frame is to be forwarded onsegment, uses CSMA/CD to accesssegment

• transparent

– hosts are unaware of presence of bridges

• plug-and-play, self-learning (typically)

– bridges do not need to be configured



Bridges: traffic isolation

• Bridge installation breaks LAN into LAN segments

• bridges filter packets: – same-LAN-segment frames not usually

forwarded onto other LAN segments

– segments become separate collisiondomains

bridgecollisiondomain

collisiondomain

= hub

= host

LAN (IP network)

LAN segment LAN segment



Bridging Function

• A bridge has a bridge table

• entry in bridge table:

– (Node LAN Address, Bridge Interface,

Time Stamp)– stale entries in table dropped (TTL can be

60 min)



Filtering/Forwarding

When bridge receives a frame:

index bridge table using MAC dest address

if entry found for destinationthen{

if dest on segment from which frame arrivedthen drop the frame

else forward the frame on interfaceindicated

}

else floodforward on all but the interface on which the frame arrived



Bridge Table ConfigurationTypes

• Can be configured

–Manually (termed fixed bridging /routing) – has to be changed if

necessary by the administrator

–Self Learning



Fixed Routing

• Complex large LANs need alternative routes– Load balancing– Fault tolerance

• Bridge must decide whether to forward

frame• Bridge must decide which LAN to forward

frame on• Routing selected for each source-destination

pair of LANs– Done in configuration– Usually least hop route– Only changed when topology changes



Spanning Tree

• Bridge automatically developsbridge table

• Automatically updates in responseto changes

–Frame forwarding

–Address learning

–Loop resolution

Reference: Stallings



Frame forwarding

• Maintain forwarding database for each port– List station addresses reached through each port

• For a frame arriving on port X:– Search forwarding database to see if MAC address is

listed for any port except X– If address not found, forward to all ports except X

– If address listed for port Y, check port Y for blockingor forwarding state

• Blocking prevents port from receiving or transmitting

– If not blocked, transmit frame through port Y



Address Learning

• When frame arrives at port X, it has comeform the LAN attached to port X

• Use the source address to update forwarding

database for port X to include that address

• Timer on each entry in database (resetwhenever frame received)

• Each time frame arrives, source address

checked against forwarding database



Bridge Address Learningexample

Suppose C sends frame to D and D replies backwith frame to C.

Bridge receives frame from from C notes in bridge table that C is on interface 1

because D is not in table, bridge sends frame intointerfaces 2 and 3

frame received by D



Bridge Learning: example

D generates frame for C, sends

bridge receives frame notes in bridge table that D is on interface 2

bridge knows C is on interface 1, so selectively forwardsframe to interface 1



Flooding Can Lead to Loops• Switches sometimes need to broadcast frames

– Upon receiving a frame with an unfamiliardestination

– Upon receiving a frame sent to the broadcastaddress

• Broadcasting is implemented by flooding– Transmitting frame out every interface– … except the one where the frame arrived

• Flooding can lead to forwarding loops– E.g., if the network contains a cycle of switches

– Either accidentally, or by design for higher reliability



Solution: Spanning Trees• Ensure the topology has no loops

– Avoid using some of the links when flooding– … to avoid forming a loop

• Spanning tree– Sub-graph that covers all vertices but contains no

cycles

– Links not in the spanning tree do not forward frames



Constructing a Spanning Tree

• Need a distributed algorithm– Switches cooperate to build the spanning tree– … and adapt automatically when failures occur

• Key ingredients of the algorithm

– Switches need to elect a “root” • The switch with the smallest identifier

– Each switch identifies if its interfaceis on the shortest path from the root• If not it excludes from the tree

– Messages (Y, d, X)• From node X• Claiming Y is the root• And the distance is d

root

One hop

Three hops



Steps in Spanning TreeAlgorithm

• Initially, each switch thinks it is the root– Switch sends a message out every interface– … identifying itself as the root with distance 0 – Example: switch X announces (X, 0, X)

• Switches update their view of the root– Upon receiving a message, check the root id– If the new id is smaller, start viewing that switch as

root

• Switches compute their distance from the root

– Add 1 to the distance received from a neighbor– Identify interfaces not on a shortest path to the

root– … and exclude them from the spanning tree



Example From Switch #4’s Viewpoint

• Switch #4 thinks it is the root– Sends (4, 0, 4) message to 2

and 7

• Then, switch #4 hears from #2– Receives (2, 0, 2) message from

2

– … and thinks that #2 is the root – And realizes it is just one hop

away

• Then, switch #4 hears from #7– Receives (2, 1, 7) from 7– And realizes this is a longer path– So, prefers its own one-hop path– And removes 4-7 link from the

tree

1

2

3

4

5

67



Example From Switch #4’s Viewpoint

• Switch #2 hears aboutswitch #1– Switch 2 hears (1, 1, 3)

from 3– Switch 2 starts treating 1

as root– And sends (1, 2, 2) to

neighbors

• Switch #4 hears from

switch #2– Switch 4 starts treating 1as root

– And sends (1, 3, 4) toneighbors

1

2

3

4

5

67



Robust Spanning TreeAlgorithm

• Algorithm must react to failures– Failure of the root node

• Need to elect a new root, with the next lowest identifier

– Failure of other switches and links• Need to recompute the spanning tree

• Root switch continues sending messages– Periodically reannouncing itself as the root (1, 0, 1)– Other switches continue forwarding messages

• Detecting failures through timeout (soft state!)

– Switch waits to hear from others– Eventually times out and claims to be the root



Another Example:

A

C

E

D

B

K

F

H

J

G

I

B3

B7

B4

B2

B5

B1

B6

A

C

E

D

B

K

F

H

J

G

I

B5

B2

B3

B7

B4

B1

B6



Example: working

–B1 is the root bridge

–B3 and B5 are both connected to LANA, but B5 is the designated port since

it's closer to root–B5 and B7 are both connected to LAN

B, but B5 is the designated port dueto smaller ID (equal distance).



BRIDGE FEATURES ANDROUTERS



Some bridge features

• Isolates collision domains resulting in highertotal max throughput

• Can connect different Ethernet types

• Transparent (“plug-and-play”): no

configuration necessary



Bridges vs. Routers

• both store-and-forward devices

– routers: network layer devices (examine network layerheaders)

– bridges are link layer devices

• routers maintain routing tables, implement

routing algorithms

• bridges maintain bridge tables, implement

filtering, learning and spanning tree algorithms



Routers vs. Bridges

Routers + and -

+ arbitrary topologies can be supported, cycling

is limited by TTL counters (and good routing

protocols)+ provide protection against broadcast storms

- require IP address configuration (not plug andplay)

- require higher packet processing

• bridges do well in small (few hundred hosts)while routers used in large networks (thousandsof hosts)



Ethernet Switches

• Essentially a multi-interfacebridge

• layer 2 (frame) forwarding,filtering using LAN addresses

• Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

• large number of interfaces

• often: individual hosts, star-connected into switch

– Ethernet, but no collisions!



Ethernet Switch Types

• Store and Forward Switches:

receives, buffers and sends

• cut-through switching: frameforwarded from input to outputport without awaiting for assemblyof entire frame (as soon as it sees

destination address in header)– slight reduction in latency



OVERALL PICTURE



Message, Segment, Packet, and Frame

HTTP

TCP

IP

Ethernetinterface

HTTP

TCP

IP

Ethernetinterface

IP IP

Ethernetinterface

Ethernetinterface

SONETinterface

SONETinterface

host host

router router

HTTP message

TCP segment

IP packet IP packetIP packet

Ethernet frame Ethernet frameSONET frame



Shuttling Data at Different Layers

• Different devices switch different things– Network layer: packets (routers)– Link layer: frames (bridges and switches)– Physical layer: electrical signals (repeaters

and hubs)

Application gateway

Transport gateway

Router

Bridge, switch

Repeater, hub

Frame

header

Packet

header

TCP

header

User

data



LAYER 2 AND 3 DEVICES

Add ons: from William Stallings



Layer 2 and Layer 3 Switches

• Now many types of devices forinterconnecting LANs

• Beyond bridges and routers

• Layer 2 switches

• Layer 3 switches



Physical Layer: Repeaters• Distance limitation in local-area

networks– Electrical signal becomes weaker as it travels– Imposes a limit on the length of a LAN

• Repeaters join LANs together– Analog electronic device– Continuously monitors electrical signals on

each LAN– Transmits a reconstructed copy

Repeater



Hubs

• Active central element of star layout• Joins multiple input lines electrically• Each station connected to hub by two lines

– Transmit and receive

• Hub acts as a repeater

• When single station transmits, hub repeats signal onoutgoing line to each station• Line consists of two unshielded twisted pairs• Limited to about 100 m

– High data rate and poor transmission qualities of UTP

• Optical fiber may be used– Max about 500 m

• Physically star, logically bus• Transmission from any station received by all other stations• If two stations transmit at the same time, collision



Hub Layouts

• Multiple levels of hubs cascaded

• Each hub may have a mixture of stations andother hubs attached to from below

• Fits well with building wiring practices

– Wiring closet on each floor

– Hub can be placed in each one

– Each hub services stations on its floor



Two Level Star Topology



Buses and Hubs

• Bus configuration– All stations share capacity of bus (e.g.

10Mbps)

– Only one station transmitting at a time

• Hub uses star wiring to attach stations tohub– Transmission from any station received by hub

and retransmitted on all outgoing lines

– Only one station can transmit at a time– Total capacity of LAN is 10 Mbps

• Improve performance with layer 2 switch



Shared Medium Bus and Hub



Shared Medium Hub andLayer 2 Switch



Layer 2 Switches

• Central hub acts as switch

• Incoming frame from particularstation switched to appropriateoutput line

• Unused lines can switch other traffic

• More than one station transmitting ata time

• Multiplying capacity of LAN



Types of Layer 2 Switch

• Store-and-forward switch– Accepts frame on input line

– Buffers it briefly,

– Then routes it to appropriate output line

– Delay between sender and receiver– Boosts integrity of network

• Cut-through switch– Takes advantage of destination address appearing at

beginning of frame

– Switch begins repeating frame onto output line as soonas it recognizes destination address

– Highest possible throughput

– Risk of propagating bad frames

• Switch unable to check CRC prior to retransmission



Layer 2 Switch v Bridge

• Layer 2 switch can be viewed as full-duplex hub• Can incorporate logic to function as multiport bridge• Bridge frame handling done in software• Switch performs address recognition and frame

forwarding in hardware

• Bridge only analyzes and forwards one frame at atime

• Switch has multiple parallel data paths– Can handle multiple frames at a time

• Bridge uses forward operation

• Switch can have cut-through operation• Bridge suffered commercially

– New installations typically include layer 2 switches withbridge functionality rather than bridges

bl h



Problems with Layer 2Switches (1)

• As number of devices in building grows, layer 2switches reveal some inadequacies

• Broadcast overload

• Lack of multiple links

• Set of devices and LANs connected by layer 2switches have flat address space– Allusers share common MAC broadcast address

– If any device issues broadcast frame, that frame isdelivered to all devices attached to network connected

by layer 2 switches and/or bridges– In large network, broadcast frames can create big

overhead

– Malfunctioning device can create broadcast storm

• Numerous broadcast frames clog network

bl h 2



Problems with Layer 2Switches (2)

• Current standards for bridge protocols dictate noclosed loops– Only one path between any two devices

– Impossible in standards-based implementation to providemultiple paths through multiple switches between devices

• Limits both performance and reliability.

• Solution: break up network into subnetworksconnected by routers

• MAC broadcast frame limited to devices and switchescontained in single subnetwork

• IP-based routers employ sophisticated routingalgorithms– Allow use of multiple paths between subnetworks going

through different routers



Problems with Routers

• Routers do all IP-level processing insoftware– High-speed LANs and high-performance

layer 2 switches pump millions of packetsper second

– Software-based router only able to handlewell under a million packets per second

• Solution: layer 3 switches– Implementpacket-forwarding logic of router

in hardware• Two categories

– Packet by packet– Flow based

P k b P k



Packet by Packet orFlow Based

• Operates insame way as traditional router

• Order of magnitude increase in performancecompared to software-based router

• Flow-based switch tries to enhanceperformance by identifying flows of IPpackets– Same source and destination

– Done by observing ongoing traffic or using aspecial flow label in packet header (IPv6)

– Once flow is identified, predefined route can beestablished



Typical Large LAN Organization

• Thousands to tens of thousands of devices• Desktop systems links 10 Mbps to 100 Mbps

– Into layer 2 switch

• Wireless LAN connectivity available for mobile users• Layer 3 switches at local network's core

– Form local backbone– Interconnected at 1 Gbps– Connect to layer 2 switches at 100 Mbps to 1 Gbps

• Servers connect directly to layer 2 or layer 3 switchesat 1 Gbps

• Lower-cost software-based router provides WANconnection• Circles in diagram identify separate LAN subnetworks• MAC broadcast frame limited to own subnetwork



TypicalLarge

LANOrganizat

ion

Diagram

bridges and switches

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