Download ch22 part 3

Chapter 22
Network layer
Delivery, Forwarding and
Routing
(part3)
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Three-Node Instability
Routing Information Protocol (RIP)
RIP is based on Distance vector routing:
RIP implement Distance vector routing with some considerations:

Destination in the routing table is a network (first column defines
network address)
 Metric(distance) is Hop count : is the number of networks that a packet
encounters to reach its final destination
 Infinity is defined as value of 16 .
 Therefore, the Max limited of hops is 15
 suitable for small networks (local area environments)
 Router sends update message to neighbors every 30 sec.
 If router does not receive update message from neighbor X within this
limit, it assumes the link to X has failed and sets the corresponding
minimum cost to 16 (infinity).
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Note
In distance vector routing, each node shares its
routing table with its
immediate neighbors periodically and when there is a
change.
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Routing Information Protocol (RIP)
Routing table
 Has one entry for each destination network of which the router is aware.
 Each entry has destination network address, the shortest distance to reach
the destination in hop count, and next router to which the packet should be
delivered to reach its final destination.
Destination
Hop
Count
Next
Router
163.5.0.0
7
172.6.23.4
197.5.13.0
5
176.3.6.17
189.45.0.0
4
200.5.1.6
115.0.0.0
6
131.4.7.19
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Other information
Example Internetwork
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Initial routing tables in a small autonomous system
• When a router is added to a network, it initializes a
routing table for itself, using its configuration file.
• The table consists only the directly attached networks and
the hop counts, which are initialized to 1.
• The next-hop field, which identifies the next router, is
empty.
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Updating Routing Table for Router A
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Final routing tables
• Each routing table is updated upon receipt of RIP
messages using the RIP updating algorithm.
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Example of updating a routing table
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Example of a domain using RIP
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Link State Routing
•
•
•
•
Each node in the domain has the entire topology of the domain.
Analogous to a city map.
Each node uses the same topology to create a routing table.
but the routing table for each node is unique because the calculations are
based on different interpretations of the topology.
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Building Routing table
1. Creation link state packet (LSP)
2. LSP is a tabular representation of the topology of
3.
4.
5.
6.
the internet
Every router in the same area has the same LSP
After a node has prepared an LSP , it must be
disseminated to all other nodes . The process is
called Flooding
Formation of shortest path tree for each node
(router) using Dijkstra algorithm
Calculation of routing table.
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Building Routing table
Creation link state packet (LSP):
 LSP can carry a large amount of information such as : the node
identity, list of links, sequence number,…etc.
 LSP are generated on two occasions:

When there is a change in topology of the domain. Triggering
of LSP dissemination to inform any node in the domain to
update its topology
 On a periodic basis. The period in this case is much longer
compared to distance vector routing . The timer set for
periodically dissemination is in the range 1 or 2 h.
 It done to ensure that the old LSP is removed
 A longer period ensures that flooding doesn’t create much
traffic on the network
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Building Routing table
Flooding
X
A
C
B
D
X
A
C
B
(a)
X
A
C
B
(c)
The creating node sends a copy
of the LSP out of each interface
 A node receives an LSP
compares it with the copy it has. If
the newly arrived is older than one
it has , it discards
If it is newer the node do the
following :
 It discards the old LSP and
keep the new one
It sends a copy of it out of
each interface except the one
from which the packet
arrived.
D
(b)
D
X
A
C
B
D
(d)
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Dijkstra Algorithm
Calculates the shortest path between two points on a network, using a
graph made up of nodes and edges.
 Algorithm divides the nodes into two sets: tentative and permanent.
It chooses nodes, makes them tentative, examines them, and if they
pass the criteria, makes them permanent.
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Examples of formation of shortest path tree
usingDijkstra algorithm
Example 1:
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Examples of formation of shortest path tree
usingDijkstra algorithm
Example 1:
Routing table for node A
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Open Shortest Path First :OSPF
• OSPF Based on Link state Routing
• OSPF divides an autonomous system into
areas.
Special routers called autonomous system boundary routers are
responsible for dissipating information about other autonomous
systems into the current system
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Areas in an Autonomous System
 Area is a collection of networks, hosts, and routers all
contained within an autonomous system.
 Routers inside an area flood the area with routing
information.
 Area border routers: Summarize the information about the
area and send it to other routers.
 Backbone area [Primary area]: All the areas inside an
autonomous system must be connected to the backbone.
Routers in this area are called as backbone routers. This area
identification number is 0.
 If, due to some problem, the connectivity between a
backbone and an area is broken, a virtual link between routers
must be created by the administration to allow continuity of
the functions of the backbone as the primary area.
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OSPF
 Each router should have the exact topology of the internet at
every moment.
 From this topology, a router can calculate the shortest path between
itself and each network using Dijkstra algorithm
Types of Links
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Types of Links
1. Point-to-point
 Connects two routers without any other router
or host in between.
 Directly connected routers using serial line.
 Only one neighbour.
 No need to assign a network address to this
type of link
 The metrics are the same at the two ends
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Types of Links
2. Transient link
 A network with several routers attached to it.
 Each router has many neighbours.
3. Stub
A network that is connected to only one router. The data
packets enter the network through this single router and
leave the network through this same router.
4. Virtual
When the link between two routers is broken, the administration
may create a virtual link between them, using a longer path that
probably goes through several routers.
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Example of an internet & Graphical representation
 Point to point : between A_D and B-E
 Stub: N5, N4 and N2
 Transient : N1 and N3
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Shortest-path calculation using Dijkstra
Example 2:
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Example 2:
Shortest-path calculation using Dijkstra
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Example 2:
Shortest-path calculation using Dijkstra
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Example 2:
Shortest-path calculation using Dijkstra
Routing Table for Router A
92 permanent
66 permanent
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Shortest-path calculation using Dijkstra
Example 3:
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Shortest-path calculation
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Shortest-path calculation
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Network
Cost
Next
Router
N1
5
----
N2
7
C
N3
10
D
N4
11
B
N5
15
D
OSPF
Reaction to Failure
If a link fails,
 Router sets link distance to infinity & floods the network
with an update packet
 All routers immediately update their link database (LSP)&
recalculate their shortest paths
 Recovery very quick
NOTE:
 OSPF differs from RIP in that each router knows the exact
topology of the network
 OSPF reduces routing bandwidth usage
 OSPF is faster than RIP.
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