Download - Computer Networks Chapter 3: Internetworking
Computer Networks
Chapter 3: Internetworking
CN_3.2
Problems In Chapter 2 we talked about
how to connect one node to another, or to an existing network.
How do we interconnect different types of networks to build a large global network?
CN_3.3
Chapter Outline 3.1 Switching and Bridging 3.2 Basic Internetworking
(IP) 3.3 Routing
CN_3.4
Chapter Goal Understanding the functions of
switches, bridges and routers Discussing Internet Protocol (IP)
for interconnecting networks Understanding the concept of
routing
CN_3.5
3.1 Switching and Forwarding
Store-and-Forward Switches Bridges and Extended LANs Cell Switching Segmentation and Reassembly
CN_3.6
Switching and Forwarding
SwitchA 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
CN_3.7
Switching and Forwarding Adds the star topology to the point-
to-point link,bus (Ethernet), and ring (802.5 and
FDDI)topologies
Switch
CN_3.8
Switching and Forwarding Properties of this star topology
Even though a switch has a fixed number of inputs and outputs, large networks can be built by interconnecting a number of switches
We can connect switches to each other and to hosts using point-to-point links
Adding a new host to the switch does not mean that the hosts already connected will get worse performance from the network
CN_3.9
Switching and Forwarding The last claim cannot be made for the
shared media network (discussed in Chapter 2) Two hosts on the same Ethernet can
not transmit continuously at 10Mbps because they share the same transmission medium
Every host on a switched network has its own link to the switch Many hosts are allowed to transmit at
the full link speed (bandwidth) provided that the switch is designed with enough aggregate capacity
CN_3.10
Switching and Forwarding
A switch is connected to a set of links and for each of these links, runs the appropriate data link protocol (MAC) to communicate with that node
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
CN_3.11
Switching and Forwarding 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
Two common approachesDatagram or Connectionless approach
Virtual circuit or Connection-oriented approach
A third approach source routing is less common
CN_3.12
Switching and Forwarding
AssumptionsEach host has a globally unique
addressThere is some way to identify
the input and output ports of each switchWe can use numbers or names
CN_3.13
Switching and Forwarding
DatagramsKey Idea
Every packet contains enough information to enable any switch to decide how to get it to destination–Every packet contains the complete destination address
CN_3.14
An example network
To decide how to forward a packet, a switch consults a forwarding table (sometimes called a routing table)
Switching and Forwarding
CN_3.15
Switching and Forwarding
Destination Port--------------------------------
---------------A 3B 0C 3D 3E 2F 1G 0H 0
Forwarding Table for Switch 2
CN_3.16
Switching and Forwarding Characteristics of Connectionless
(Datagram) Network A host can send a packet anywhere at any time,
since any packet that turns up at the switch can be immediately forwarded (assuming a correctly populated forwarding table)
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 is forwarded independently of previous packets that might have been sent to the same destination. Thus two 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
CN_3.17
Switching and Forwarding
Virtual Circuit Switching Widely used technique for packet
switching Uses the concept of virtual circuit
(VC) Also called a connection-oriented
model First set up a virtual connection
from the source host to the destination host and then send the data
CN_3.18
Switching and Forwarding Host A wants to send packets to host
B
CN_3.19
Switching and Forwarding Two-stage process
Connection setup Data Transfer
Connection setup Establish “connection state” in each of
the switches between the source and destination hosts
The connection state for a single connection consists of an entry in the “VC table” in each switch through which the connection passes
CN_3.20
Switching and Forwarding 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 for this VC arrive at the switch
An outgoing interface in which packets for this VC leave the switch
A potentially different VCI that will be used for outgoing packetsIncoming
InterfaceIncoming VC Outgoing
InterfaceOutgoing VC
2 5 1 11
CN_3.21
Switching and Forwarding
The semantics for one such entry is If a packet arrives on the
designated 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
CN_3.22
Switching and ForwardingNote:
The combination of the VCI and the interface uniquely identifies the virtual connection
There may be many virtual connections established in the switch at one time
Incoming and outgoing VCI values are not generally the same VCI is not a globally significant identifier for the
connection; rather it has significance only on a given link
Whenever a new connection is created, we need to assign a new VCI for that connection on each link that the connection will traverse
CN_3.23
Switching and Forwarding Two approaches to establishing
connection state Network Administrator configures the
state The virtual circuit is permanent (PVC) The network administrator can delete it Can be thought of as a long-lived or
administratively configured VC A host can send messages into the
network to establish the state This is referred as signalling and the resulting
virtual circuit is said to be switched (SVC) A host may set up and delete such a VC
dynamically without the involvement of a network administrator
CN_3.24
Assume that a network administrator wants to manually create a new virtual connection from host A to host B First the administrator identifies a path
through the network from A to B
Switching and Forwarding
CN_3.25
Switching and Forwarding The administrator then picks a VCI
value that is currently unused on each link for the connection For our example,
Suppose the VCI value 5 is chosen for the link from host A to switch 1
11 is chosen for the link from switch 1 to switch 2
So the switch 1 will have an entry in the VC table
Incoming Interface
Incoming VC Outgoing Interface
Outgoing VC
2 5 1 11Switch 1
CN_3.26
Switching and ForwardingSimilarly, suppose
VCI of 7 is chosen to identify this connection on the link from switch 2 to switch 3
VCI of 4 is chosen for the link from switch 3 to host B
Switches 2 and 3 are configured with the following VC table
Incoming Interface
Incoming VC Outgoing Interface
Outgoing VC
3 11 2 7
Incoming Interface
Incoming VC Outgoing Interface
Outgoing VC
0 7 1 4
Switch 2
Switch 3
CN_3.27
Switching and Forwarding For any packet that A wants to send to B, A
puts the VCI value 5 in the header of the packet and sends it to switch 1
Switch 1 receives any such packet on interface 2, and it uses the combination of the interface and the VCI in the packet header to find the appropriate VC table entry.
The table entry on switch 1 tells the switch to forward the packet out of interface 1 and to put the VCI value 11 in the header
CN_3.28
Switching and Forwarding Packet will arrive at switch 2 on interface 3
bearing VCI 11 Switch 2 looks up interface 3 and VCI 11 in
its VC table and sends the packet on to switch 3 after updating the VCI value 7
This process continues until it arrives at host B with the VCI value of 4 in the packet
To host B, this identifies the packet as having come from host A
CN_3.29
Switching and Forwarding In real networks of reasonable size,
the burden of configuring VC tables correctly in a large number of switches would quickly become excessive Thus, some sort of signalling is almost
always used, even when setting up “permanent” VCs
In case of PVCs, signalling is initiated by the network administrator
SVCs are usually set up using signalling by one of the hosts
CN_3.30
Switching and Forwarding How does the signalling work ?
To start the signalling process, host A sends a setup message into the network (i.e. to switch 1) The setup message contains the
complete destination address of B. The setup message needs to get all
the way to B to create the connection state in every switch along the way
It is like sending a datagram to B where every switch knows which output to send the setup message so that it eventually reaches B
Assume that every switch knows the topology to figure out how to do that
CN_3.31
Switching and Forwarding How does the signalling work ?
When switch 1 receives the connection request, in addition to sending it on to switch 2, it creates a new entry in its VC table for this new connection The entry is exactly the same shown
before Switch 1 picks the value 5 for this
connection When switch 2 receives the setup
message, it performs the similar process and it picks the value 11 as the incoming VCI
CN_3.32
Switching and Forwarding How does the signalling work ?
Similarly switch 3 picks 7 as the value for its incoming VCI
Finally the setup message arrives at host B.
Assuming that B is healthy and willing to accept a connection from host A, it allocates an incoming VCI value, in this case 4. This VCI value can be used by B to
identify all packets coming from A
CN_3.33
Switching and Forwarding Now to complete the connection, everyone
needs to be told what their downstream neighbor is using as the VCI for this connection Host B sends an acknowledgement of the
connection setup to switch 3 and includes in that message the VCI value that it used (4)
Switch 3 completes the VC table entry for this connection and sends the acknowledgement on to switch 2 specifying the VCI of 7
Switch 2 completes the VC table entry for this connection and sends acknowledgement on to switch 1 specifying the VCI of 11
Finally switch 1 passes the acknowledgement on to host A telling it to use the VCI value of 5 for this connection
CN_3.34
Switching and Forwarding Tear Down When host A no longer wants to send
data to host B, it tears down the connection by sending a teardown message to switch 1
The switch 1 removes the relevant entry from its table and forwards the message on to the other switches in the path which similarly delete the appropriate table entries
At this point, if host A sends a packet with a VCI of 5 to switch 1, it would be dropped.
CN_3.35
Switching and Forwarding Characteristics of VC
Since host A has to wait for the connection request to reach the far side of the network and return before it can send its first data packet, there is at least one RTT of delay before data is sent
While the connection request contains the full address for host B (which might be quite large, being a global identifier on the network), each data packet contains only a small identifier, which is only unique on one link. Thus the per-packet overhead caused by the
header is reduced relative to the datagram model
CN_3.36
Switching and Forwarding Characteristics of VC
If a switch or a link in a connection fails, the connection is broken and a new one will need to be established. Also the old one needs to be torn down to free
up table storage space in the switches The issue of how a switch decides which
link to forward the connection request on has similarities with the function of a routing algorithm
CN_3.37
Switching and Forwarding Good Properties of VC
By the time the host gets the go-ahead to send data, it knows quite a lot about the network- For example, that there is really a route to the
receiver and that the receiver is willing to receive data
It is also possible to allocate resources to the virtual circuit at the time it is established Bandwidth > kMbps Delay < limitation bound Buffer size
CN_3.38
Switching and Forwarding For example, an X.25 network – a
packet-switched network that uses the connection-oriented model – employs the following three-part strategy 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
CN_3.39
Switching and Forwarding Comparison with the Datagram
Model Datagram network has no connection
establishment phase and each switch processes each packet independently
Each arriving packet competes with all other packets for buffer space
If there are no buffers, the incoming packet must be dropped
CN_3.40
Switching and Forwarding In VC, we could imagine providing
each circuit with a different quality of service (QoS) The network gives the user some kind of
performance related guarantee 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
Most popular examples of VC technologies are Frame Relay and ATM One of the applications of Frame Relay is
the construction of VPN
CN_3.41
Switching and Forwarding ATM (Asynchronous Transfer Mode)
Connection-oriented packet-switched network
Packets are called cells5 byte header + 48 byte payload
Fixed length packets are easier to switch in hardwareSimpler to designEnables parallelism
CN_3.42
Switching and Forwarding ATM
User-Network Interface (UNI) Host-to-switch format GFC: Generic Flow Control VCI: Virtual Circuit Identifier Type: management, congestion control CLP: Cell Loss Priority HEC: Header Error Check (CRC-8)
Network-Network Interface (NNI) Switch-to-switch format GFC becomes part of VPI field
CN_3.43
Source Routing All the information about network
topology that is required to switch a packet across the network is provided by the source host
Switching and Forwarding
CN_3.44
Bridges and Spanning Tree Algorithm
(IEEE 802.1D)
CN_3.45
Functions of a Bridge
MAC layer device which relays frames among physically separated LANs and makes the physical LANs appear as one logical LAN to the end stations
Preamble SFD DA SA LEN LLC PAD FCS
7 1 6 6 2 4 Bytes
CN_3.46
Functions of a Bridge Basic Functions:
Frame Forwarding Learning and Filtering Resolving Possible Loops in the
Topology Additional Functions:
Congestion Control (Enough Buffer) Static Filtering (Security) Translation (Multi-Bridge) Routing (Multi-Bridge) Segmentation
CN_3.47
A Simple Bridge Example
Bridge
LAN A
LAN B
1 2 3
5 6 7
4
Stations
CN_3.48
Design Considerations No modifications to the content or
format of the frames Contain enough buffer space to meet
peak demands Contain addressing and routing
intelligence A bridge may connect more than two
networks Why Bridged LANs (BLAN) ?
Reliability Performance Security Geography
CN_3.49
Bridge Routing
The Bridges must be equipped with a routing capability
The routing decision may not always be a simple one (loop)
Topology changes have to be considered
A bridge knows all the station addresses (Filtering Database)
CN_3.50
BLAN Example (Without loop)
ID=10
ID=40
Bridge 1
LAN 1
LAN 2
LAN 3
LAN 4
Bridge 4
1 2
LAN 5 LAN 6
Bridge 2ID=20
C D EF
A B
2 3
1
2
1 Bridge 3ID=30 2
1
CN_3.51
Bridged LAN (BLAN) Example with Loop
4
LAN 1
Bridge 1 Bridge 2
Bridge 4 Bridge 5 Bridge 6 Bridge 7
LAN 2 LAN 3
LAN 4LAN 5
5 6
1
2 3
LAN 6
Bridge 3
Station
CN_3.52
Bridge Protocol Architecture
User Data
LLC-H
MAC-H
t1, t8t2, t7
t3, t4, t5, t6 LLC-H User Data MAC-T
User Data
PHY
Bridge
Station A
LAN 1
MACPHYB CPHY
USER
LLCMAC
t1t2t3 t
4t5 t6
t7t8
Station D
LAN 2
MACMACPHY
USER
LLC
CN_3.53
Spanning Tree Routing Frame Forwarding and Filtering
Use the destination MAC address (DMAC) field in each MAC frame
A bridge maintains a filtering database with entries:
[Address, Port, Time]
Address Learning Use the source MAC address (SMAC) field in each
MAC frame If the element is already in the database, the
entry is updated and the timer is reset If the element is not in the database, a new entry
is created with its own timerPreamble SFD DMAC SMAC LEN LLC PAD FCS
7 1 6 6 2 4 Bytes
CN_3.54
Filtering Database Examples
LAN 2
LAN 3
Bridge 2
LAN 4
LAN 1
Bridge1 1
2
C D
E
A B
F
A 1 19 B 1 17 C 2 24 D 3 3 E 1 6 F 1 13
Filtering Database ( Bridge 2 ) 1
2 3
MAC AddrPort Time (S)A 2 20 B 2 18 C 2 25 D 2 4 E 1 5 F 1 12
Filtering Database ( Bridge 1 )
MAC Addr Port Time(S)
CN_3.55
Forwarding and
Address Learning Algorithm
Frame fromPort x
Add SMAC, port (x)and Timer (0) into FDB
Forward to belonging
Port
Filter Forward to
all ports ( except port
x )
Change to port
X, reset timer
End
Y
N
Y
Y
N
N
AddressLearning
FrameForwarding
DMAC in FDB?
Belong to Port x ?
SMAC in FDB ?
CN_3.56
Addresses Learning Example
1. A -> E2. B -> D3. C -> B4. D -> A5. E -> C
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
CN_3.57
Addresses Learning Example (AE)
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
A 2
E A
E A
E A
E A
E A
A 1 A 1
CN_3.58
Addresses Learning Example (BD)
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
A 2A 1 A 1
BD
BD BD
BD BD
B 2 B 1B 2
CN_3.59
Addresses Learning Example (CB)
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
A 2A 1 A 1B 2 B 1B 2
B C
B C
C 1C 2
CN_3.60
Addresses Learning Example (DA)
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
A 2A 1 A 1B 2 B 1B 2C 1C 2D 3
D 1D 2
A DA D
A D
CN_3.61
Addresses Learning Example (EC)
2
A
1
FDB FDB
Bridge X Bridge Y Bridge Z
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
MAC Port MAC Port MAC Port
B
C
D
E
12 3 1
2
A 2A 1 A 1B 2 B 1B 2C 1C 2D 3
D 1D 2
C E
E 3E 2
C EC E
CN_3.62
Bridge X
A
B
LAN 1
Bridge Yt0
t12
21
1
LAN 2
Loop Problems and Resolution
Loops provides reliability Loops make frames duplication Loops make wrong address
learningt2
B AB A
B A B A
CN_3.63
1
Bridge 1
LAN 1
LAN 2
LAN 3
LAN 4 LAN 5
Bridge 2 Bridge 3
Bridge 4
Bridge 5
2
2
1
2
2
1 1
1 2
3
Spanning Tree Example 1
CN_3.64
1
1 2 3
2
43
5
4 5
1
1 2 3
2
43
5
4 5
LAN
Bridge
Spanning Tree
Graph Representation of a BLAN
CN_3.65
ID=10
ID=50
Bridge 1
LAN 1
LAN 2
LAN 3
LAN 4
Bridge 2
Bridge 3Bridge 4
Bridge 5
1 2Root Bridge
1
2 1
2ID=40ID=30
ID=20
1
2
1
2 3
LAN 5
Spanning Tree Example 1 (Continued)
CN_3.66
Spanning Tree Algorithm (requirements)
Bridges Each bridge is assigned a unique identifier
(8 octets): Priority part (two octets): programmable address part (six octets)
A special group MAC address for all bridges :
01-80-C2-00-00-00 (Multicast address)
10000000-00000001-01000011- Each port of a bridge has a unique port
identifier.
CN_3.67
Spanning Tree Algorithm (definitions)
Root Bridge: The bridge with the lowest value of bridge identifier.
Path Cost: For each port, the cost of transmitting a frame onto a LAN.
Root Port: For each bridge, the port on the minimum-cost path to the root bridge.
Root Path Cost: For each bridge, the cost of the path to the root bridge with minimum cost.
Designated Bridge: For each LAN, the bridge that provides the minimum cost path to the root bridge. The only bridge allowed to forward frames to and from the LAN.
Designated Port: The port of the designated bridge that attaches the bridge to the LAN. All internet traffic to and from the LAN pass through the designated port.
CN_3.68
Spanning Tree Example 2
Bridge 1
LAN 1
LAN 2
LAN 3
LAN 4 LAN 5Bridge 5
TC=10 1
ID=10
TC=5 2
ID=50TC: Transmission Cost
TC=10 2
Bridge 2TC=10 1
ID=20
TC=10 2
Bridge 3TC=5 1
ID=30
TC=5 2
Bridge 4TC=5 1
ID=40
TC=5 2
TC=10 1
TC=5 3
CN_3.69
Spanning Tree Example 2 (continued)
D D
R
R D
R
D D
R
RPC: Root Path CostTC: Transmission CostD: Designated PortR: Root Port
LAN 1
LAN 2
LAN 3
LAN 4 LAN 5
Root Bridge
Bridge 5
TC=5 2
ID=50, RPC=10
TC=10 1
TC=5 3 Bridge 2
TC=10 1
ID=20,RPC=10
TC=10 2
Bridge 4TC=5 2
ID=40,RPC=5
TC=5 1
Bridge 3TC=5 1
ID=30,RPC=5
TC=5 2
Bridge 1
TC=10 1
ID=10, RPC=0
TC=10 2
CN_3.70
Spanning Tree Algorithm Three Steps:
1. Determine the root bridge.2. Determine the root port on all
other bridges.3. Determine the designated port on
each LAN.The port with the minimum root path
cost.In the case of two or more bridges
with the same root path cost, the highest-priority bridge is selected.
If the designated bridge has two or more ports attached to this LAN, then the port with the lowest value of identifier is selected.
CN_3.71
Bridge Port State Diagram
Blocking
After a forward delay time
Listening Learning Forwarding
After a forward delay time
Cancel
Selected asa D or R port
Cancel Cancel
CN_3.72
Bridge Protocol Data Unit (BPDU)
Protocol ID
BPDU TypeFlag
111
8
8
4
22
222
2 Version ID
Root Bridge ID
Bridge ID
RPC
Root Port ID
Message Age Time Limit
Hello Time Forward delay
1
1
2
Bytes
Bytes
(b)Topology Change BPDU
(a)Network Configuration BPDU
Protocol ID
BPDU Type
Version ID
CN_3.73
Spanning Tree Algorithm Example
LAN W
RPC = 35 7
D(W): Designated Port of LAN W
Bridge XTC=15 i
TC=10 j
RPC = 38 1
RPC = 40, R = m 12
RPC = 48, R = n, D(W) = m2
RPC = 48 3
RPC = 20 5
RPC = 35, R = i,D(W) = j
6
Bridge ZTC=10 m
TC=10 n
Bridge YTC=5 l
TC=5 k
RPC = 53, R = kRPC = 58, R = j4 4
RPC = 45, R = m 8
RPC = 40, R = k 8
RPC = 30, R = l,D(W) = k
10
RPC = 30
RPC = 35, R = i11
11
RPC = 25 9
CN_3.74
LAN W
R R
R
D
D: Designated PortR: Root Port
Spanning Tree Algorithm Example (Continued)
Bridge XTC=15 i
TC=10 j
Bridge ZTC=15 m
TC=10 n
Bridge YTC=5 l
TC=5 k
CN_3.75
Spanning Tree Features The spanning tree constructed by the IEEE
802.1D algorithm has the features that for each bridge, the shortest path (minimum root path cost, RPC) to the root bridge is included.
For each LAN, the shortest path (minimum root path cost, RPC) to the root bridge via the designated bridge is included.
So the spanning tree usually is not a minimum cost spanning tree.
The spanning tree of a BLAN (or switches connected network) is predictable or deterministic. Thus, given a BLAN topology (with any loops) and configuration parameters, the spanning tree of the BLAN can be calculated manually.
CN_3.76
LAN 1, DPC = 20
LAN 6, DPC = 0
LAN 3,DPC = 0
LAN 5, DPC = 5
Bridge 8
Bridge 1ID=10,RPC=0
TC=5 1
TC=5 2
ID=80,RPC=5
LAN 7, DPC = 5
Bridge 3 Bridge 4
TC=15 2
ID=40,RPC=15ID=30,RPC=15
TC=15 1
LAN 2, DPC = 10
Bridge 5
ID=50,RPC=5
TC=5
1
LAN 4,DPC = 5
D
D
DD
D
R
R R
R R
R
DRD
Root Bridge
Spanning Tree Example 3
TC=10 3
TC=5 2
TC=15 2
TC=15 1
TC=10 2
TC=10 1
Bridge 2ID=20,RPC=20
TC=5 1
TC=10 2
Bridge 7ID=70,RPC=5
TC=5 1
TC=5 2
Bridge 6ID=60,RPC=10
TC=5 1
TC=5 2
CN_3.77
Spanning Tree Maintenance The transmission of the configuration is
triggered by root. The root will periodically (once every Hello
time) issue a configuration BPDU on all LANs to which it is attached.
A bridge that receives a configuration BPDU on what it decides is its root port passes that information to all LANs for which it believes itself to be the designated bridge.
A cascade of configuration BPDUs throughout the spanning tree.
A bridge may change the spanning tree topology
A TCN BPDU is reliable relayed up the new spanning tree to the root bridge (bridge by bridge).
The root will set the Topology Change flag in all configuration messages transmitted for some time.
CN_3.78
LAN 1, DPC = 20
LAN 6, DPC = 0
LAN 3,DPC = 0
LAN 5, DPC = 5
Bridge 8
Bridge 1ID=10,RPC=0
TC=5 1
TC=5 2
ID=80,RPC=5
LAN 7, DPC = 5
Bridge 3 Bridge 4
TC=15 2
ID=40,RPC=15ID=30,RPC=15
TC=15 1
LAN 2, DPC = 10
Bridge 5
ID=50,RPC=5
TC=5
1
LAN 4,DPC = 5
D
D
DD
D
R
R R
R R
R
DRD
Root Bridge
Spanning Tree Maintenance Example 1
TC=10 3
TC=5 2
TC=15 2
TC=15 1
TC=10 2
TC=10 1
Bridge 2ID=20,RPC=20
TC=5 1
TC=10 2
Bridge 7ID=70,RPC=5
TC=5 1
TC=5 2
Bridge 6ID=60,RPC=10
TC=5 1
TC=5 2
D
15
25
CN_3.79
Spanning Tree Maintenance Example 1
Assume Bridge 60 faults. Then all the Hello BPDUs sent from root
bridge to Bridge 60 will not be forwarded to LAN 2 any more.
The Bridges 30 and 40 in LAN 2 will trigger the timeout event individually which means the Designated bridge 60 for LAN 2 was gone.
Then they will try to serve as the Designated bridge of LAN 2 by forwarding a configuration BPDU.
Assume bridge 40 sends the BPDU first with a RPC = 15.
Then bridge 30 will return another BPDU with RPC=15 since it’s priority is higher than bridge 40 (same RPC, smaller ID).
After two forwarding delays, bridge 30 will become the new Designated bridge of LAN2 and the DPC becomes 15.
CN_3.80
Spanning Tree Maintenance Example 1
Also the DPC of LAN 1 is changed from 15 to 25.
Bridge 30 then sends a Topology Change Notification (TCN) BPDU to root bridge.
The root will set the Topology Change flag in all configuration messages transmitted for some time.
CN_3.81
Final configuration of example 1
LAN 1, DPC = 25
LAN 6, DPC = 0
LAN 3,DPC = 0
LAN 5, DPC = 5
Bridge 8
Bridge 1ID=10,RPC=0
TC=5 1
TC=5 2
ID=80,RPC=5
LAN 7, DPC = 5
Bridge 3 Bridge 4
TC=10 2
ID=40,RPC=10ID=30,RPC=10
TC=15 1
LAN 2, DPC = 15
Bridge 5
ID=50,RPC=5
TC=5
1
LAN 4,DPC = 5
D
D
DD
D
R
R R
R R
R
DD
Root Bridge
TC=10 3
TC=5 2
TC=10 2
TC=15 1
TC=10 2
TC=10 1
Bridge 2ID=20,RPC=20
TC=5 1
TC=10 2
Bridge 7ID=70,RPC=5
TC=5 1
TC=5 2
Bridge 6ID=60
CN_3.82
LAN 1, DPC = 20
LAN 6, DPC = 0
LAN 3,DPC = 0
LAN 5, DPC = 5
Bridge 8
Bridge 1ID=10,RPC=0
TC=5 1
TC=5 2
ID=80,RPC=5
LAN 7, DPC = 5
Bridge 3 Bridge 4
TC=15 2
ID=40,RPC=15ID=30,RPC=15
TC=15 1
LAN 2, DPC = 10
Bridge 5
ID=50,RPC=5
TC=5
1
LAN 4,DPC = 5
D
D
DD
D
R
R R
R R
R
DRD
Root Bridge
Spanning Tree Maintenance Example 2
TC=10 3
TC=5 2
TC=15 2
TC=15 1
TC=10 2
TC=10 1
Bridge 2ID=20,RPC=20
TC=5 1
TC=10 2
Bridge 7ID=70,RPC=5
TC=5 1
TC=5 2
Bridge 6ID=60,RPC=10
TC=5 1
TC=5 2
RR
R
00
Root Bridge
2525
CN_3.83
Spanning Tree Maintenance Example 2
Assume LAN 3 faults. Then all the Hello BPDUs sent from root
bridge to LAN 3 will be lost. All the ports connected to LAN 3, including
port 2 of bridge 30, port 2 0f bridge 40, port 1 of bridge 50, and port 1 of bridge 80, will become “blocked” state from “forwarding” state.
All these bridges are now don’t have “R” port (root port) and then try to be a root bridge.
Bridges 30 and 40 still can receive the Hello BPDU from port 1, so they will change their root port to port 1.
CN_3.84
Spanning Tree Maintenance Example 2
Bridges 50 and 80 will exchange BPDU to compete as a new root follow the STP protocol.
Assume bridge 80 sends the BPDU first with a RPC = 0.
Then bridge 50 will return another BPDU with RPC=0 since it’s priority is higher than bridge 80 (smaller ID).
After two forwarding delays, bridge 50 will become the new root bridge and the port 1 of bridge 80 will become a root port.
Finally, we have two separated (disconnected) spanning trees.
CN_3.85
Final configuration of example 2LAN 1, DPC = 20
LAN 6, DPC = 0
LAN 3
LAN 5, DPC = 5
Bridge 8
Bridge 1ID=10,RPC=0
TC=5 1
TC=5 2
ID=80,RPC=5
LAN 7, DPC = 5
Bridge 3 Bridge 4
TC=10 2
ID=40,RPC=25ID=30,RPC=25
TC=15 1
LAN 2, DPC = 10
Bridge 5
ID=50,RPC=0
TC=5
1
LAN 4,DPC = 0
D
DD
D
R R
R R
R
DRD
Root Bridge
TC=10 3
TC=5 2
TC=10 2
TC=15 1
TC=10 2
TC=10 1
Bridge 2ID=20,RPC=20
TC=5 1
TC=10 2
Bridge 7ID=70,RPC=5
TC=5 1
TC=5 2
Bridge 6ID=60,RPC=10
TC=5 1
TC=5 2
CN_3.86
3.2 Internetworking What is internetwork ?
An arbitrary collection of networks interconnected to provide some sort of host-host to packet delivery service
A simple internetwork where H represents hosts and R represents routers
CN_3.87
Internetworking What is IP ?
IP stands for Internet Protocol Key tool used today to build scalable,
heterogeneous internetworks It runs on all the nodes in a collection of networks
and defines the infrastructure that allows these nodes and networks to function as a single logical internetwork
A simple internetwork showing the protocol layers
CN_3.88
IP Service Model Packet Delivery Model
Connectionless model for data delivery Best-effort delivery (unreliable service)
packets are lost packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time
Global Addressing Scheme Provides a way to identify all hosts in the
network
CN_3.89
Layer 3 router uses store and forward scheme to forward incoming IP packets (datagrams). IP Address Lookup (Forwarding Table
constructed by routing protocols, such as RIP, OSPF, BGP, etc)
IP/MAC mapping table The IP address lookup is longest prefix
matching lookup. Forward IP packet into next hop if the
destination IP is found in the Forwarding Table. Otherwise, forward to default port.
New router Architecture with L3 switching Fabric ASICs and IP address lookup ASICs (hardware lookup)
Wire-speed forwarding design Gbps, 10Gbps, 100Gbps, …
Not Plug-and-Play
How Layer 3 Routers Work ?
IP Next140.114.77.0 Directly140.114.78.0 Directly140.114.79.0 Router Z
IP MACIP(A) MAC(A)IP(B) MAC(B)IP(Y) MAC(Y)IP(X) MAC(X)
CN_3.90
IP Datagram Header Format
version IHL Type of Service Total length
Identification Flags Fragment Offset
Time to Live Protocol Header Checksum
Source IP AddressDestination IP Address
Options + PaddingData
0 3 8 15 19 31
CN_3.91
Type of Service (ToS) of IP
Precedence D T R O O
Precedence111 Network Control110 Internetwork Control101 CRITIC/ECP100 Flash Override011 Flash010 Immediate001 Priority000 Routine
Delay0 Normal1 Low
Throughput, Reliability0 Normal1 High
0 DF MF
DF 0 May Fragment 1 Don't Fragment
MF 0 Last Fragment 1 More Fragment
0 1 2 3 4 5 6 7
Flags0 1 2
CN_3.92
How datagrams are delivered in an Internet ?
R H
WAN
LAN
R
R R
R
RB
H
H
H
A
H
H
H
H
Datagram
LAN
LAN
LAN
CN_3.93
Routers
MACPHY
HOST X
LAN 1 LAN 2
Higher LayerProtocols
MACPHY
HOST Y
A BMACPHY
NetworkROUTER
Network
LAN n
IP MAC
140.114.78.69
140.114.78.68
IP = 140.114.78.0Mask= 255.255.255.0
IP = 140.114.77.0Mask= 255.255.255.0
140.114.78.66140.114.77.65
140.114.77.60
140.114.77.62 IP(A) MAC(A)IP(B) MAC(B)IP(Y) MAC(Y)
IP Next140.114.77.0 Directly140.114.78.0 Directly140.114.79.0 Router Z
A B
ROUTER Z
LAN mIP = 140.114.79.0Mask= 255.255.255.0
IP(X) MAC(X)Higher LayerProtocols
MACPHY
NetworkA B
Y B
MAC(Y) MAC(B) IP(Y) IP(B) IP DatagramMAC(R) MAC(B) IP(A) IP(B) IP DatagramMAC(A) MAC(R) IP(A) IP(B) IP Datagram
CN_3.94
Intra-LAN and Inter-LAN Communications
B -> Y (Intra LAN)
B -> A (Inter-LAN)
MAC(Y) MAC(B) IP(Y) IP(B) IP Datagram
MAC(R) MAC(B) IP(A) IP(B) IP Datagram
MAC(A) MAC(R) IP(A) IP(B) IP Datagram
CN_3.95
An Internet Routing Example
Network10.0.0.0 F Network
20.0.0.0 G Network30.0.0.0 H Network
40.0.0.0
10.0.0.5
20.0.0.5
20.0.0.6
30.0.0.6
30.0.0.7
40.0.0.7
20.0.0.0 Deliver Direct30.0.0.0 Deliver Direct10.0.0.0 20.0.0.540.0.0.0 30.0.0.7
To reach hosts Route toon network this address
• Routing Table
CN_3.96
Router Characteristics Network Layer Routing
Network layer protocol dependent Filter MAC broadcast and multicast packets Easy to support mixed media Packet fragmentation and reassembly Filtering on network addresses and
information Accounting
Direct Communication Between Endpoints and Routers Highly configurable and hard to get right Handle speed mismatch Congestion control and avoidance
CN_3.97
Router Characteristics (Continued)
Routing Protocols Interconnect layer 3 networks and exploit
arbitrary topologies Determine which route to take Static routing Dynamic routing protocol support
RIP: Routing Information Protocol OSPF: Open Shortest Path First
Provides reliability with alternate routes Router Management
Troubleshooting capabilities Name-Address mapping services
CN_3.98
Differences Between Bridges and Routers
Bridges RoutersOperation at Layer 2 Operation at
Layer 3Protocol Independent Protocol DependentAutomatic Address Learning/Filtering Administration
Required for Address,Interface and Routes
Pass MAC Multicast/Broadcast MAC M/B can be Filtered
Lower Cost Higher CostNo Flow/Congestion Control Flow/Congestion
ControlLimited Security Complex SecurityTransparent to End Systems Non-
TransparencyWell Suited for Simple/Small Networks For WAN, Larger
NetworksNo Frames Segmentation/Reassembly Frames
Segmentation/ReassemblySpanning Tree Based Routing Optimal Routing
and Load Sharing Plug and Play Requires Central
Administrator
CN_3.99
IP Fragmentation and Reassembly
Each network has some MTU (Maximum Transmission Unit) Ethernet (1500 bytes), FDDI (4500 bytes)
Strategy Fragmentation occurs in a router when it
receives a datagram that it wants to forward over a network which has MTU < datagram
Reassembly is done at the receiving host All the fragments carry the same
identifier in the Ident field Fragments are self-contained datagrams IP does not recover from missing
fragments
CN_3.100
IP Fragmentation and Reassembly
IP datagrams traversing the sequence of physical networks
CN_3.101
IP Fragmentation and Reassembly
Header fields used in IP fragmentation. (a) Unfragmented packet;
(b) fragmented packets.
CN_3.102
Global Addresses Properties
globally unique hierarchical: network + host 4 Billion IP address, half are A type, ¼ is B type,
and 1/8 is C type Format
Dot notation 10.3.2.4 128.96.33.81 192.12.69.77
CN_3.103
IP Datagram Forwarding Strategy
every datagram contains destination's address if directly connected to destination network, then
forward to host if not directly connected to destination network,
then forward to some router forwarding table maps network number into next
hop each host has a default router each router maintains a forwarding table
Example (router R2)
CN_3.104
IP Datagram Forwarding Algorithmif (NetworkNum of destination = NetworkNum of one of my
interfaces) thendeliver packet to destination over that interface
elseif (NetworkNum of destination is in my forwarding table) thendeliver packet to NextHop routerelsedeliver packet to default router
For a host with only one interface and only a default router in its forwarding table, this simplifies to
if (NetworkNum of destination = my NetworkNum)thendeliver packet to destination directly
elsedeliver packet to default router
CN_3.105
Subnetting Add another level to address/routing
hierarchy: subnet Subnet masks define variable partition of
host part of class A and B addresses Subnets visible only within site
CN_3.106
Subnetting
Forwarding Table at Router R1
CN_3.107
SubnettingForwarding Algorithm
D = destination IP addressfor each entry < SubnetNum, SubnetMask, NextHop>
D1 = SubnetMask & Dif D1 = SubnetNum
if NextHop is an interface deliver datagram directly to destinationelse deliver datagram to NextHop (a router)
CN_3.108
SubnettingNotes Would use a default router if nothing
matches Not necessary for all ones in subnet mask to
be contiguous Can put multiple subnets on one physical
network Subnets not visible from the rest of the
Internet
CN_3.109
Classless Addressing Classless Inter-Domain Routing
A technique that addresses two scaling concerns in the Internet The growth of backbone routing table as more
and more network numbers need to be stored in them
Potential exhaustion of the 32-bit address space
Address assignment efficiency Arises because of the IP address structure with
class A, B, and C addresses Forces us to hand out network address space in
fixed-size chunks of three very different sizes– A network with two hosts needs a class C address
» Address assignment efficiency = 2/255 = 0.78– A network with 256 hosts needs a class B address
» Address assignment efficiency = 256/65535 = 0.39
CN_3.110
Classless Addressing Exhaustion of IP address space centers on
exhaustion of the class B network numbers Solution
Say “NO” to any Autonomous System (AS) that requests a class B address unless they can show a need for something close to 64K addresses
Instead give them an appropriate number of class C addresses
For any AS with at least 256 hosts, we can guarantee an address space utilization of at least 50%
What is the problem with this solution?
CN_3.111
Classless Addressing Problem with this solution
Excessive storage requirement at the routers.
If a single AS has, say 16 class C network numbers assigned to it, Every Internet backbone router needs 16
entries in its routing tables for that AS This is true, even if the path to every one
of these networks is the same If we had assigned a class B address
to the AS The same routing information can be
stored in one entry Efficiency = 16 × 255 / 65, 536 = 6.2%
CN_3.112
Classless Addressing CIDR tries to balance the desire to
minimize the number of routes that a router needs to know against the need to hand out addresses efficiently.
CIDR uses aggregate routes Uses a single entry in the forwarding
table to tell the router how to reach a lot of different networks
Breaks the rigid boundaries between address classes
CN_3.113
Classless Addressing Consider an AS with 16 class C network
numbers. Instead of handing out 16 addresses at
random, hand out a block of contiguous class C addresses
Suppose we assign the class C network numbers from 192.4.16 through 192.4.31
Observe that top 20 bits of all the addresses in this range are the same (11000000 00000100 0001) We have created a 20-bit network number (which
is in between class B network number and class C number)
Requires to hand out blocks of class C addresses that share a common prefix
CN_3.114
Classless Addressing Requires to hand out blocks of class C
addresses that share a common prefix The convention is to place a /X after the
prefix where X is the prefix length in bits For example, the 20-bit prefix for all the
networks 192.4.16 through 192.4.31 is represented as 192.4.16/20
By contrast, if we wanted to represent a single class C network number, which is 24 bits long, we would write it 192.4.16/24
CN_3.115
Classless Addressing How do the routing protocols handle
this classless addresses It must understand that the network
number may be of any length Represent network number with a
single pair<length, value>
All routers must understand CIDR addressing
CN_3.116
Classless Addressing
Route aggregation with CIDR
CN_3.117
IP Forwarding Revisited IP forwarding mechanism assumes
that it can find the network number in a packet and then look up that number in the forwarding table
We need to change this assumption in case of CIDR
CIDR means that prefixes may be of any length, from 2 to 32 bits
CN_3.118
IP Forwarding Revisited It is also possible to have prefixes in the
forwarding tables that overlap Some addresses may match more than one prefix
For example, we might find both 171.69 (a 16 bit prefix) and 171.69.10 (a 24 bit prefix) in the forwarding table of a single router
A packet destined to 171.69.10.5 clearly matches both prefixes. The rule is based on the principle of “longest
match” 171.69.10 in this case
A packet destined to 171.69.20.5 would match 171.69 and not 171.69.10
CN_3.119
Address Translation Protocol (ARP)
Map IP addresses into physical addresses destination host next hop router
Techniques encode physical address in host part of IP
address table-based
ARP (Address Resolution Protocol) table of IP to physical address bindings broadcast request if IP address not in
table target machine responds with its physical
address table entries are discarded if not
refreshed
CN_3.120
ARP Packet Format
HardwareType: type of physical network (e.g., Ethernet)
ProtocolType: type of higher layer protocol (e.g., IP)
HLEN & PLEN: length of physical and protocol addresses
Operation: request or response Source/Target Physical/Protocol addresses
CN_3.121
Host Configurations Notes
Ethernet addresses are configured into network by manufacturer and they are unique
IP addresses must be unique on a given internetwork but also must reflect the structure of the internetwork
Most host Operating Systems provide a way to manually configure the IP information for the host
Drawbacks of manual configuration A lot of work to configure all the hosts in a
large network Configuration process is error-prune
Automated Configuration Process is required
CN_3.122
Dynamic Host Configuration Protocol (DHCP)
DHCP server is responsible for providing configuration information to hosts
There is at least one DHCP server for an administrative domain
DHCP server maintains a pool of available addresses
CN_3.123
DHCP Newly booted or
attached host sends DHCPDISCOVER message to a special IP address (255.255.255.255)
DHCP relay agent unicasts the message to DHCP server and waits for the response
CN_3.124
Internet Control Message Protocol (ICMP)
Defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully Destination host unreachable due to link /node
failure Reassembly process failed TTL had reached 0 (so datagrams don't cycle
forever) IP header checksum failed
ICMP-Redirect From router to a source host With a better route information
CN_3.125
Internet Control Message Protocol (ICMP)
Defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully Destination host unreachable due to link /node
failure Reassembly process failed TTL had reached 0 (so datagrams don't cycle
forever) IP header checksum failed
ICMP-Redirect From router to a source host With a better route information
CN_3.126
3.3 RoutingForwarding versus Routing
– Forwarding: – to select an output port based on
destination address and routing table– Routing:
– process by which routing table is built
CN_3.127
Routing• Forwarding table VS Routing table
• Forwarding table • Used when a packet is being
forwarded and so must contain enough information to accomplish the forwarding function
• A row in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such as Ethernet Address of the next hop
• Routing table • Built by the routing algorithm as a
precursor to build the forwarding table
• Generally contains mapping from network numbers to next hops
CN_3.128
Routing
Example rows from (a) routing and (b) forwarding tables
CN_3.129
Routing• Network as a Graph
• The basic problem of routing is to find the lowest-cost path between any two nodes• Where the cost of a path equals the sum of the
costs of all the edges that make up the path
CN_3.130
Routing• For a simple network, we can calculate all
shortest paths and load them into some nonvolatile storage on each node.
• Such a static approach has several shortcomings• It does not deal with node or link failures• It does not consider the addition of new nodes
or links• It implies that edge costs cannot change
• What is the solution?• Need a distributed and dynamic protocol• Two main classes of protocols
• Distance Vector• Link State
CN_3.131
Distance Vector Each node constructs a one dimensional
array (a vector) containing the “distances” (costs) to all other nodes and distributes that vector to its immediate neighbors
Starting assumption is that each node knows the cost of the link to each of its directly connected neighbors
CN_3.132
Distance Vector
Initial distances stored at each node (global view)
CN_3.133
Distance Vector
Initial routing table at node A
CN_3.134
Distance Vector
Final routing table at node A
CN_3.135
Distance Vector
Final distances stored at each node (global view)
CN_3.136
Distance Vector The distance vector routing algorithm is
sometimes called as Bellman-Ford algorithm
Every T seconds each router sends its table to its neighbor each each router then updates its table based on the new information
Problems include fast response to good new and slow response to bad news. Also too many messages to update
CN_3.137
Distance Vector• When a node detects a link failure
F detects that link to G has failed F sets distance to G to infinity and sends
update to A A sets distance to G to infinity since it uses F to
reach G A receives periodic update from C with 2-hop
path to G A sets distance to G to 3 and sends update to F F decides it can reach G in 4 hops via A
CN_3.138
Distance Vector Slightly different circumstances can prevent the
network from stabilizing Suppose the link from A to E goes down In the next round of updates, A advertises a distance of
infinity to E, but B and C advertise a distance of 2 to E Depending on the exact timing of events, the following
might happen Node B, upon hearing that E can be reached in 2 hops from
C, concludes that it can reach E in 3 hops and advertises this to A
Node A concludes that it can reach E in 4 hops and advertises this to C
Node C concludes that it can reach E in 5 hops; and so on. This cycle stops only when the distances reach some
number that is large enough to be considered infinite– Count-to-infinity problem
CN_3.139
Count-to-infinity Problem Use some relatively small number as an
approximation of infinity For example, the maximum number of hops to get
across a certain network is never going to be more than 16
One technique to improve the time to stabilize routing is called split horizon When a node sends a routing update to its neighbors, it does
not send those routes it learned from each neighbor back to that neighbor
For example, if B has the route (E, 2, A) in its table, then it knows it must have learned this route from A, and so whenever B sends a routing update to A, it does not include the route (E, 2) in that update
CN_3.140
Count-to-infinity Problem In a stronger version of split horizon, called split
horizon with poison reverse B actually sends that back route to A, but it puts
negative information in the route to ensure that A will not eventually use B to get to E
For example, B sends the route (E, ∞) to A
CN_3.141
Routing Information Protocol (RIP)
Example Networkrunning RIP RIPv2 Packet
Format
CN_3.142
Link State RoutingStrategy: Send to all nodes (not just
neighbors) information about directly connected links (not entire routing table).
Link State Packet (LSP) id of the node that created the LSP cost of link to each directly connected neighbor sequence number (SEQNO) time-to-live (TTL) for this packet
Reliable Flooding store most recent LSP from each node forward LSP to all nodes but one that sent it generate new LSP periodically; increment SEQNO start SEQNO at 0 when reboot decrement TTL of each stored LSP; discard when
TTL=0
CN_3.143
Link State Reliable Flooding
Flooding of link-state packets. (a) LSP arrives at node X; (b) X floods LSP to A and C; (c) A and C flood LSP to B (but not X); (d) flooding is complete
CN_3.144
Shortest Path Routing Dijkstra’s Algorithm - Assume non-negative
link weights N: set of nodes in the graph l((i, j): the non-negative cost associated with the
edge between nodes i, j N and l(i, j) = if no edge connects i and j
Let s N be the starting node which executes the algorithm to find shortest paths to all other nodes in N
Two variables used by the algorithm M: set of nodes incorporated so far by the algorithm C(n) : the cost of the path from s to each node n The algorithm
M = {s}For each n in N – {s}
C(n) = l(s, n)while ( N M)M = M {w} such that C(w) is the minimum for all w in (N-M)For each n in (N-M) C(n) = MIN (C(n), C(w) + l(w, n))
CN_3.145
Shortest Path Routing In practice, each switch computes its
routing table directly from the LSP’s it has collected using a realization of Dijkstra’s algorithm called the forward search algorithm
Specifically each switch maintains two lists, known as Tentative and Confirmed
Each of these lists contains a set of entries of the form (Destination, Cost, NextHop)
CN_3.146
Shortest Path Routing The algorithm
Initialize the Confirmed list with an entry for myself; this entry has a cost of 0
For the node just added to the Confirmed list in the previous step, call it node Next, select its LSP
For each neighbor (Neighbor) of Next, calculate the cost (Cost) to reach this Neighbor as the sum of the cost from myself to Next and from Next to Neighbor
If Neighbor is currently on neither the Confirmed nor the Tentative list, then add (Neighbor, Cost, Nexthop) to the Tentative list, where Nexthop is the direction I go to reach Next
If Neighbor is currently on the Tentative list, and the Cost is less than the currently listed cost for the Neighbor, then replace the current entry with (Neighbor, Cost, Nexthop) where Nexthop is the direction I go to reach Next
If the Tentative list is empty, stop. Otherwise, pick the entry from the Tentative list with the lowest cost, move it to the Confirmed list, and return to Step 2.
CN_3.147
Shortest Path Routing
CN_3.148
Open Shortest Path First (OSPF)
OSPF Header Format OSPF Link State Advertisement
CN_3.149
Summary We have looked at some of the issues
involved in building scalable and heterogeneous networks by using switches and routers to interconnect links and networks.
To deal with heterogeneous networks, we have discussed in details the service model of Internetworking Protocol (IP) which forms the basis of today’s routers.
We have discussed in details two major classes of routing algorithms Distance Vector Link State
End of Chapter 3