1. Switching and Bridging Textbook Ch3.1 and 3.4
Prof. Athirai Irissappane
CSS432: Switching and Bridging

2. CSS432: Switching and Bridging
Scalable Networks
Switch
A mechanism that allows us to interconnect links to form a large network
A multi-input, multi-output device which transfers packets from an input to one or more outputs
Adds the star topology to the point-to-point link, bus (Ethernet), and ring (802.5 and FDDI) topologies
Connect switches (packet and circuit switched n/w)
A switch’s primary job is to receive incoming packets on one of its links and to transmit them on some other link
This function is referred as switching and forwarding
CSS432: Switching and Bridging

3. CSS432: Switching and Bridging
Scalable Networks
Why switches
Connects two or more network segments (>2500m in Ethernet)
Support large numbers of hosts (>1024 hosts in Ethernet)
Maintain performance (>two packets through a switch)
Network Switches
Packet Switch
Circuit Switch
virtual
Circuit Switch (1. Circuit Establishment: allocates certain bandwidth and establish a path 2. Data Transfer 3. Circuit Termination If circuit not available: “Busy signal” Ex: Telephone networks)
Circui
Virtua Circuit similar to Packet Switch + Circuit Switch; data sent as packets
Connectionless
Connection Oriented
(Virtual Circuit)
IP datagrams
ATM
X25
CSS432: Switching and Bridging

4. CSS432: Switching and Bridging
Circuit Switching
Dedicated connection between source and destination
No one else can use the link
Send streams
Packet Switching
No dedicated connection
Efficient use of resources. E.g., link capacity
Data sent as packets
Link can be shared by others
Virtual circuit
Establishes a dedicated connection between source and destination
Link can be shared
CSS432: Switching and Bridging

5. CSS432: Switching and Bridging
Packet Switching
How does the switch decide which output port to place each packet on?
It looks at the header of the packet for an identifier that it uses to make the decision
Datagram/connectionless switching IP
Virtual circuit/connection-oriented switching
X.25
ATM (Asynchronous Transfer Mode)
Source routing
Connection requests in virtual circuit
CSS432: Switching and Bridging

6. Datagram Switching (Internet)
Every packet contains enough information (destination address) for switch to decide how to get it to destination
To decide how to forward a packet, a switch consults a forwarding/routing table (every switch maintains a table)
Port (out)
Destination
B, G, H 1 F 2 E 3
A, C, D 1 3 2
Switch 3
Host B
Switch 2
Host A
Switch 1
Host C
Host D
Host E
Host F
Host G
Host H
Table at Switch 2
Switch can learn based on oncoming information but not sufficient to send data: use routing algorithms like shortest path to determine where to send the data and which path to follow
Broadcasting is one option before the switch learns which port to use, but it will always not give the best/shortest route

7. Datagram Switching A sends ARP request about F1 3 2
Switch 3
Host B
Switch 2
Host A
Switch 1
Host C
Host D
Host E
Host F
Host G
Host H
A sends ARP request about F
F sends MAC address to A
E sends ARP request about H
H sends MAC address to E
C sends ARP request about B
B sends MAC address to C
Switch 1
Switch 2
Switch 3
Port
Destin
Port
Destin
Port
Destin
ation
ation
ation
H, B H A
A, E, C
A, E 1
F, E F
F,E, B 1 F 1 2 A 2 E 2 H 3 C 3 A
A, C 3 B

8. Ethernet (MAC) Address
Each host on an Ethernet has a unique Address.
The (unicast) address belongs to the adaptor, not the host.
MAC Address
sequence of six numbers separated by colons
each number corresponds to 1 byte of the 6 byte (48 bit) address and is given by a pair of hexadecimal digits, one for each of the 4-bit nibbles in the byte
Leading 0s are dropped.
E.g., 8:0:2b:e4:b1:2 =
address consisting of all 1s a broadcast address.
All adaptors pass frames addressed to the broadcast address up to the host.
an address with first bit set to 1 but is not the broadcast address is called a multicast address.
host can program its adaptor to accept multicast addresses.
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9. Datagram Switching A host can send a packet anywhere at any time
No connection setup phase. When a host sends a packet, it has no way of knowing if the network is capable of delivering it or if the destination host is even up and running
Each packet forwarded independently of previous packets sent to the same destination
successive packets from host A to host B may follow completely different paths
A switch or link failure might not have any serious effect on communication if it is possible to find an alternate route around the failure and update the forwarding table accordingly

10. Datagram Switching (Cont’d)
No connection setup
Pros 1: A source can send data as soon as it is ready.
No way of knowing if a packet is delivered.
Cons 1: A source must estimate network congestion or disconnection.
Each packet may take a different route.
Pros 2: No signle point of failure happens.
Cons 2: The order of packets at destination is different from that at source.
CSS432: Switching and Bridging

11. Virtual Circuit Switching
Widely used technique for packet switching
Uses the concept of virtual circuit (VC)
connection-oriented model: set up a virtual connection from the source host to the destination host and then send the data
Explicit connection setup (and tear-down) phase
Subsequent packets follow same circuit
Each switch maintains a VC table
is a means of transporting data over a packet switched computer network in such a way that it appears as though there is a dedicated physical layer link between the source and destination end systems of this data
CSS432: Switching and Bridging

12. Virtual Circuit Switching
Connection setup
Establish connection state (entry in the “VC table”) in each of the switches between the source and destination hosts
One entry in the VC table on a single switch contains
A virtual circuit identifier (VCI) that uniquely identifies the connection at this switch and that will be carried inside the header of the packets that belong to this connection
An incoming interface on which packets arrive at the switch
An outgoing interface in which packets leave the switch
A potentially different VCI that will be used for outgoing packets
Entry?
If a packet arrives on the incoming interface and that packet contains the designated VCI value in its header, then the packet should be sent out the specified outgoing interface with the specified outgoing VCI value first having been placed in its header
CSS432: Switching and Bridging

13. Virtual Circuit Switching (Cont’d)
Global address information (host) is replaced with local VCI (specific to switch).
Each switch has local but not global information.
To set up connection:
switch needs a global view of network configuration to forward a connection request message to destination
VCI + interface unique identifier for a virtual connection 1 3 2
Switch 3
Host B
Switch 2
Host A
Switch 1
Host C
Host D
Host E
Host F
Host G
Host H
VCI=5
VCI=11
VCI=7
VCI=4
VCI=6
VCI=12
VCI=8
VCI=4
Switch 1
Port (in)
VCI
Port (out) 2 5 1 11 6 12 3 7 8 4
Switch 2
Switch 3

14. Virtual Circuit Switching (Cont’d)
VCI for each host/switch+port is unique
Switch picks unused VCI for incoming connection request 1 3 2
Switch 3
Host B
Switch 2
Host A
Switch 1
Host C
Host D
Host E
Host F
Host G
Host H
Switch 1
VCI=6
VCI=12
VCI=8
VCI=4
Port (in)
VCI
Port (out)
VCI 2 6 1 12
Switch 2
Port (in)
VCI
Port (out)
VCI 3 12 8
Switch 3
Port (in)
VCI
Port (out)
VCI 8 2 4

15. Virtual Circuit Model (Cont’d)
Establishing connection state
Network Administrator will configure the state
The virtual circuit is permanent (PVC), administrator can delete this
A host can send messages to the network for the state to be established
This is referred as signaling and called switched virtual circuit (SVC)
A host may set up and delete such a VC dynamically without administrator
In real n/ws, signaling is always used (PVC, signal initiated by admin)
Signaling: (Host A source, Host B destination)
Host A sends a setup message (with destination address) to appropriate switch
At switch, entry is created and send the request to next switch
To complete the connection, an acknowledgement is sent by the downstream neighbor with its VCI starting with Host B
When a host (A) no longer wants to send data to host (B), it tears down the connection by sending a teardown message to the relevant switch. Switch removes corresponding entry and forwards the tear down message to the other switches in the path
In real networks, the burden of configuring VC tables correctly in a large number of switches would quickly become excessive

16. Virtual Circuit Model (Cont’d)
Connection setup required
Pros 1: An opportunity to reserve resources (QoS)
Cons 1: Wait for a full RTT before sending first data packet.
Cons 2: Full address for destination still required for connection.
Packets sent along the same route
Pros 2: Each data packet contains only a VCI.
Pros 3: Flow control possible along the entire connection
Cons 3: If the connection is broken, a new one needs to be established.
Switches set aside the resources they need to meet this guarantee For example, a percentage of each outgoing link’s bandwidth Delay tolerance on each switch;
Each circuit can have different QOS, e..g, switches packets belonging to a particular circuit not delayed for more than a amount of time Switches
Flow control:
Buffers are allocated to each virtual circuit when the circuit is initialized
The sliding window protocol is run between each pair of nodes along the virtual circuit, and this protocol is augmented with the flow control to keep the sending node from overrunning the buffers allocated at the receiving node
The circuit is rejected by a given node if not enough buffers are available at that node when the connection request message is processed
CSS432: Switching and Bridging

17. Switch Implementation
Using a workstation
Flexible control
Performance problem
Using a custom hardware
Shared/share memory-based switch
Crossbar switch
Self-routing switch (Batch Banyan switch)
CSS432: Switching and Bridging

18. Workstation Used as a Switch
CPU
LAN A
NIC 1
LAN B
I/O
ctlr
NIC 2
LAN C
NIC 3
I/O Bus
Main memory
Workstation (with CPU), 3 Network Interfaces
Packet travels from NIC 1 to NIC 2
NIC1 to Memory: Direct Memory Access: allows I/O device to send receive data directly to memory bypassing CPU (Faster)
CPU examines packet header determines packet to be sent to NIC 2
Memory to NIC 2 (DMA)
CSS432: Switching and Bridging

19. Workstation Used as a Switch
Advantage: flexible because a workstation has a CPU
Disadvantage:
Each packet crosses I/O bus twice and read and written onto memory once (upper bound on throughput is half main memory bandwidth or half I/O bus bandwidth, whichever is smaller)
Processing small packets reduces throughput
Example
33MHz 32bit (33*32 = 1056 Mbps) I/O bus
1Gbps Memory bandwidth
Lower Bandwidth 1 Gbps < 1056 Mbps; Bandwidth of host = 1Gbps
Throughput = 1Gbps/ 2 = 500Mbps for a round trip between NIC and main memory
If each NIC is 100Mbps, how many NICs can be supported by the host?
500/100 = 5 NICs
What if a packet is very small like 64bytes
The workstation has to process 500,000 packets per second (pps).
Throughput: 500,000 x 64 x 8 = 256Mbps
If it is a 5-port switch then this bandwidth should be shared, i.e., 256/5 ~ 5Mbps data rate on each port (below average range that the users demand)
Better design for switch to avoid the above drawbacks
CSS432: Switching and Bridging

20. Shared Bus/Memory-Based Switch
Control
processor
DMA from port to port
Input Port
Output Port
Input Port
Fabric
Output Port
Input Port
Output Port
Shared memory
Control processor in charge of whole switch, communicates to the ports via bus or directly
Ports communicate with outside world
Fabric: when presented with a packet deliver it to the right output port
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21. CSS432: Switching and Bridging
Switch Fabric Types
Switch Fabric
Transfer packet from input to output with minimal delay and meeting the throughput of switch
Types
Shared Bus switch
Shared across the input and output port
Bandwidth of bus determines switch throughput
Shared Memory switch
Packets are written into memory by input port and read by output port
Memory bandwidth determines switch throughput
Uses high speed memory bus instead of I/O bus (as in shared Bus)
Crossbar switch
Matrix of pathways which connect any input port to any output port
Collision: Each output port needs to accept packets from all input port at once
Banyan Switch
Self routing
CSS432: Switching and Bridging

22. Crossbar Switch (4 * 4 matrix)
Without a collision, all inputs delivered to each output
All inputs may go to the same output which causes a collision in the output buffer.
Use Knockout Switch
CSS432: Switching and Bridging

23. CSS432: Switching and Fowarding
Banyan Switch
Self Routing switch
Destination information in packet
Input port decides what is the output port and attaches a header to the packet
2 x 2 switching elements interconnected in regular patterns (2 inputs and 2 outputs)
Collision avoidance by proper arrangement of switching elements
First column switches deal with the most significant bit. If 0 route packets to the top, if 1 route packets to bottom
Second column deal with the middle bit
Third column deals with least significant bit
Packets should be present in ascending order
CSS432: Switching and Fowarding

24. CSS432: Switching and Bridging
Banyan Switch
001
001
000
011 1
000
001
001
110
111 1 1
011
111
110
110
110
111
111
2 x 2 switching elements interconnected in regular patterns.
Collisions occur if packets are not presented in ascending order
CSS432: Switching and Bridging

25. Bridges and Extended LANs
Connecting two or more LANs
Repeater / Hub
L1: Physical Layer
Limitations: <= 2500m
To detect collision transmit atleast 512 bits/ limit the time to detect collision to maximum of 5.12us, hence limit length as 2500 m
Bridge (LAN switch)
L2: Datalink Layer
Fowarding frames using MAC address
Connect accessible networks
Static configuration + partial dynamic configuration (Spanning Tree Protocol)
Router
L3 – Network Layer
Routing IP packets using IP address
Dynamic configuration
Connect hosts of any networks
Start from repeater/bridge/router of LAN on the left hand side; Need to transmit packets to destination
Repeater: forwards bits
Bridge: forwards frames
Router: forwards packets; contain routing table, stop when destination is reached
repeater and Hub are layer 1 devices. Repeater address the issue of attenuation. Attenuation is the loss of signal over distance. Repeater rebuilt the electrical signal that comes in and send it out to other side. Hub is a multi-port repeater. Electrical signal came into any one hub port will be repeated on all other ports.The layer 1 devices are dumb, they have no decision making abilities. Hubs do not read any of the data passing through them, and they are not aware of the source or destination of the frame. All the devices attached to a hub are belong to one collision domain, which means if two hosts try to send data at the same time, a collision will occur. All the devices attached to a hub are also belong to one braoadcast domain, that is, broadcast frame sent by one host will be received by all other hosts in the network.
Bridge and Switch are layer 2 devices. They can make decisions about to which port the frames will go, based on MAC Addresses. Bridges and switches help avoid frame collision by breaking down one collision domain to two or more smaller collision domains, then buffer and forward frames between them. Nowadays, a switch can be configured to allow each connected host to have its own individual collision domain. As a result, all the hosts can trasnmit data simultaneously without collision, because they no longer share the bandwidth with each other. It is important to notice hosts attached to bridge or switch still belong to the same broadcast domain.
Router is layer 3 device. The purpose of router is to route packets from one broadcast domain to another. Router maintains a routing table. The routing table contains ip addresses associated with interfaces, out of which the packet will be forward to.
Collision Domains – A collision domain is defined as a network segment that shares bandwidth with all other devices on the same network segment. Generally speaking, A Collision Domain includes all of the Ethernet segments between a pair of bridges or other layer 2 devices. When two hosts on the same network segment transmit at the same time, the resulting digital signals will fragment or collide, hence the term collision domain.
Broadcast Domain – A broadcast domain is defined as all devices on a network segment that hear broadcasts sent on that segment.
All devices plugged into a hub are in the same collision domain and the same broadcast domain.
All devices plugged into a switch are in separate collision domains but the same broadcast domain. Although, you can buy special hardware to break up broadcast domains in a switch, or use a switch capable of creating VLANs. VLANs breakup broadcast domains.
Hubs and Repeaters extend collision and broadcast domains.
Switches, Bridges and Routers break up collision domains.
Bridge: same as switch with less number of ports (2)
CSS432: Switching and Bridging

26. Bridges and Extended LANs
Bridges: Accept frames on their input and forwards to output
Learning Bridges: no need to forward all the frames that a bridge receives
When a frame from host A that is addressed to host B arrives on port 1, there is no need for the bridge to forward the frame out over port 2.
Maintain forwarding table
Determine if destination is on same side or opposite side of bridge
Host
Port A 1 B C X 2 Y Z
Def 1: Bridge: same as switch with less number of ports (2)
Def 2: Switch connects hosts, bridges connect LANs

27. CSS432: Switching and Bridging
Learning Bridges
Learn table entries based on source address (full network not known)
E.g. An entry for A is registered upon receiving a frame from A
E.g. When receiving a frame from B, don’t forward to Port 2
If no entry, forward to all ports
When a bridge first boots, this table is empty
Entries are added over time
A timeout is associated with each entry
The bridge discards the entry after a specified period of time
Linux brctl command: makes a logical bridge with max age = 4sec
The above learning does not work when loops are present
Spanning Tree Algorithm to learn forwarding table when loops are present
CSS432: Switching and Bridging

28. STP: Spanning Tree Protocol
Loops
Example:
B1 receives a frame from Node X on LAN H to Node Y on LAN C.
B1 registers an entry for Node X but not yet Node Y.
B1 forward this frame to all ports except to LAN H.
B7 receives the frame and forwards it to LAN B.
B5 forwards it to LAN A and D.
B1 receives again this frame and registers an entry for X.
B1 forwards it to all ports except to LAN H and D.
Problem:
Node Y eventually receives a frame.
Duplicated frames are forwarded along loops.
Spanning Tree Algorithm
Inactivate bridge ports so that no cycle exists in extended LAN
IEEE Specification B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I Y X
CSS432: Switching and Bridging

29. Spanning Tree Protocol
How to avoid loops ?
Block unnecessary ports
Bridges select ports to forward frame using STP
Some ports should not be used to avoid cycles
Root bridge is selected
Priority
MAC address / ID
Port number
Root forwards frames on all ports
Based on shortest distance from root, select bridge for Every LAN to forward frames to it
Use Config Messages
CSS432: Switching and Bridging

30. CSS432: Switching and Bridging
STP Details
Bridges exchange configuration messages (Y, d, X)
Y: the id of root to be
d: #hops from X to Y
X: the sending bridge id
Initially each bridge thinks it is the root, so it sends a configuration message on each of its ports identifying itself as the root and giving a distance to the root of 0
Upon receiving a configuration message over a particular port, the bridge checks to see if the new message is better than the current best configuration message recorded for that port. The new configuration is better than the currently recorded information if
It identifies a root with a smaller id or
It identifies a root with an equal id but with a shorter distance or
The root id and distance are equal, but the sending bridge has a smaller id
CSS432: Switching and Bridging

31. CSS432: Switching and Bridging
STP Details
If the new message is better than the currently recorded one,
The bridge discards the old information and saves the new information
It first adds 1 to the distance-to-root field
When a bridge receives a configuration message indicating that it is not the root bridge (that is, a message from a bridge with smaller id)
The bridge stops generating configuration messages on its own
Only forwards configuration messages from other bridges after 1 adding to the distance field
When bridge receives a configuration message that indicates it is not the designated bridge for that port (a message from a bridge that is closer to the root or equally far from the root but with a smaller id)
The bridge stops sending/forwarding configuration messages over that port
When the system stabilizes,
Only the root bridge is still generating configuration messages.
Other bridges are forwarding these messages only over ports for which they are the designated bridge
CSS432: Switching and Bridging

32. CSS432: Switching and Fowarding
STP Details
B3 receives (B2, 0, B2) (Y -root, d, X -sender)
Since 2 < 3, B3 accepts B2 as root
B3 adds 1 to the distance advertised by B2 and sends (B2, 1, B3) to B5
Meanwhile B2 accepts B1 as root because it has the lower id and it sends (B1, 1, B2) toward B3
B5 accepts B1 as root and sends (B1, 1, B5) to B3
B3 accepts B1 as root and it notes that both B2 and B5 are closer to the root than it is.
Thus B3 stops forwarding messages on both its interfaces
This leaves B3 with both ports not selected
B1, 1, B5
B2, 1, B3
B1, 1, B2
B2, 0, B2
B1, 0, B1
B1, 0, B1
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33. CSS432: Switching and Bridging
Reviews
Datagram switching
Virtual Circuit switching
Bridges: STP and limitations
Switches: workstation-based, shared bus/memory-based, crossbar, and banyan
Exercises in Chapter 3
Ex. 1 (vc sw)
Ex. 4 (datagram sw)
Ex. 13 (STP)
Ex. 26, 27 (Switch implementation)
CSS432: Switching and Bridging