Chapter 11
Unicast
Routing
Protocols
TCP/IP Protocol Suite
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Chapter
Outline
11.1 Introduction
11.2 Intra- and Inter-Domain
Routing
11.3 Distance Vector Routing
11.4 RIP
11.5 Link State Routing
11.6 OSPF
11.7 Path Vector Routing
11.8 BGP
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11-1 INTRODUCTION
An internet is a combination of networks connected
by routers. When a datagram goes from a source to
a destination, it will probably pass through many
routers until it reaches the router attached to the
destination network.
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11-2 INTER- AND INTRA-DOMAIN
ROUTING
Today, an internet can be so large that one routing
protocol cannot handle the task of updating the
routing tables of all routers. For this reason, an
internet is divided into autonomous systems. An
autonomous system (AS) is a group of networks and
routers under the authority of a single administration.
Routing inside an autonomous system is called intradomain routing. Routing between autonomous
systems is called inter-domain routing
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Figure 11.1 Autonomous systems
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How does it work ?
Tx
• A certain “cost” or
“metric” is assigned
each network
Rx
• If #hops was the metric, we would want to
traverse the least number of networks in going
from Tx to Rx
• In figuring out the best
route from Tx to Rx,
the set of networks
with the smallest sum
is chosen
• More generically, the
set of networks best
meeting the “metric’s”
objective is chosen
• If max throughput was the metric, a fiber optic
network would have a better metric than a
coaxial network.
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Figure 11.2 Popular routing protocols
• RIP – Routing Information Protocol – treats each network the same
(assigns the same cost for each network)
• OSPF – Open Shortest Path First protocol – assigns a cost for passing
through a network based on the type of service required – routes
through the network can have different cost – each router would have
several tables
• BGP – Border Gateway Protocol – is an exterior routing protocol that
uses a policy that defines what paths should be chosen
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Routing Protocols vs. Algorithms
 Routing protocols allow routers to share info with
one another dynamically - as the Internet makes
changes, the routing protocols allow routers to
inform other routers
 Routers communicate to their neighboring
routers - gossip
 Routing protocols implement the procedures
for combining info received from other routers
 Routing Algorithms – decision making analysis –
the “brains” – using the info provided
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11-3 DISTANCE VECTOR ROUTING
 Bellman-Ford Algorithm
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Figure 11.4 The fact behind Bellman-Ford algorithm
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RIP Algorithm
Distance Vector Algorithm built from Bellman-Ford Algorithm
Recall: each router sends message to it’s neighbor
For the router receiving a RIP response
1st – add one to hop count for each destination advertised
2nd – repeat the following steps for each advertised destination
1. If destination is not in table
add destination to table
2. Else if destination is in table
1. If next-hop field is the same
replace entry in table with advertised one
2. Else next-hop different
replace entry if advertised hop count is less
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Example 11.1
Figure 11.5 shows the initial routing table for an AS. Note that the figure
does not mean that all routing tables have been created at the same time;
each router creates its own routing table when it is booted.
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Figure 11.6 Example 11.2
4
2
3
Net5 , 1Net4 , 1Net2 , 1
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Figure 11.7 Example 11.3
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RIP Algorithm
•
Router receives RIP message for some router C
•
The RIP message list destination networks, corresponding hop count
•
1st step: increment hop count
• Net1: no news, don’t change
• Net2: same next hop, so replace
2 with 5
• Net 3: new router, so add
• Net 6: different next hop, new
hop count less, so replace
• Net 8: different next hop, new
hop count the same, don’t
change
• Net 9: different next hop, new
hop count larger, do not
change
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11-4 RIP
The Routing Information Protocol (RIP) is an intradomain (interior) routing protocol used inside an
autonomous system. It is a very simple protocol
based on distance vector routing. RIP implements
distance vector routing directly with some
considerations.
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Figure 11.10 Example of a domain using RIP
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Figure 11.11 RIP message format
• Command – 8-bit field specifying the type of message: response (2) or request (1)
• Version – 8-bit field specifying RIP version
• Family – 16-bit specifying protocol family (TCP/IP=2)
• Network Address – address of the destination network
• Distance – 32-bit field defining the hop count from advertising router to destination
network
NOTE: Request can be issued by a newly added router or by a router seeking certain info
NOTE: 2 response types: Solicited – response to request, Unsolicited – periodic updates
Gray fields repeated for each destination network
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Example
What is the periodic response sent by router R1 in the figure
below. Assume R1 knows about the whole autonomous system.
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Solution
R1 can advertise three networks 144.2.7.0, 144.2.9.0, and
144.2.12.0. The periodic response (update packet) is shown below
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Figure 11.13 Solution to Example 11.4
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Figure 11.14 RIP timers
• Periodic Timer – each router has timer set to 25-35 secs and when the
timer counts down, an update message is sent
• Expiration Timer – governs the validity of the next-hop – when router
receives next-hop update, timer is set to 180 sec. If there is a problem
and the router doesn’t receive it’s 30 sec update, the route info expires
(invalid) after the 180 sec count down – then the hop count is set to 16
(infinity)
• Garbage Collection Timer – once the route expires, this timer is set to
120 sec and counts downs – allows neighbors time to become aware of
invalidity – after count down, info is purged
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Example 11.5
A routing table has 20 entries. It does not receive information
about five routes for 200 s. How many timers are running at this
time?
Solution
The 21 timers are listed below:
Periodic timer: 1
Expiration timer: 20 − 5 = 15
Garbage collection timer: 5
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RIP Problem – Count to Infinity
Any decrease in cost propagates quickly, but any increase in
cost propagates slowly.
If a link is broken, this takes some time --- count to infinity
 Slow Convergence – the time it takes a change in the Internet
to propagate through the rest of the Internet – recall the
periodic updates with neighbors. DEPENDING ON THE
DATA RATE, millions or billions of bits could be sent in that
time – therefore possibly lost
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Figure 11.8 Two-node instability
• RA and RB has hop counts 1 and
2, respectively for X
• X goes down – RA can update
fast due to direct connection –
sets hop count to ∞
• RA has to wait 30 sec to update
RB (this is the problem)
• In meanwhile, RB sends update
to RA with hop count 2 for X
(incremented to 3)
• Now when RA finally send the
update to RB, it sends a hop
count of 3 (incremented to 4) –
RA thinks it’s another route to X
• This INSTABILITY (back-andforth) continues until both set
hop count to ∞
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Note
RIP uses the services of UDP on wellknown port 520.
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Figure 11.2 Popular routing protocols
• RIP – Routing Information Protocol – treats each network the same
(assigns the same cost for each network)
• OSPF – Open Shortest Path First protocol – assigns a cost for passing
through a network based on the type of service required – routes
through the network can have different cost – each router would have
several tables
• BGP – Border Gateway Protocol – is an exterior routing protocol that
uses a policy that defines what paths should be chosen
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11-5 LINK STATE ROUTING
Link state routing has a different philosophy from
that of distance vector routing. In link state routing, if
each node in the domain has the entire topology of
the domain—the list of nodes and links, how they
are connected including the type, cost (metric), and
the condition of the links (up or down)—the node can
use the Dijkstra algorithm to build a routing table.
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Figure 11.17 Concept of Link state routing
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Figure 11.18 Link state knowledge
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Figure 11.19 Forming shortest path three for router A in a graph
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Figure 11.19 Continued
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Figure 11.19 Continued
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Example 11.6
To show that the shortest path tree for each node is different,
we found the shortest path tree as seen by node C (Figure
11.20). We leave the detail as an exercise.
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Figure 11.20 Example 11.6
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11-6 OSPF
The Open Shortest Path First (OSPF) protocol is an
intra-domain routing protocol based on link state
routing. Its domain is also an autonomous system.
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Figure 11.21 Areas in an autonomous system
• Divide autonomous system (AS) into areas
• Routers with in an area floods the area with routing info – router sends to all it’s
neighbors and each neighbor sends to all it’s neighbors and etc..
• At the border of an area, special routers called area border routers are used to (1)
summarize info about an area and (2) send info amongst areas
• A special area called the backbone is used to tie together all of the areas – backbone is
primary area and all other areas are secondary areas – backbone area uses backbone
routers. Each area has an ID (backbone’s Id is 0)
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OSPF: Open Shortest Path First

The OSPF is similar to RIP however, it allows the admin the
ability to assign a cost or metric to each route.

The metric can be based on a type of service (ie . Min delay,
max throughput, etc..)

Unlike RIP, sharing or updating is done when there is a
change (not periodically)

For OSPF, the objective is for the routers to contain the full
picture or topology of the Internet – by having this, the
router can figure out the “shortest path” or “least cost” route
between itself and each network

To do this, the Internet is represented by a graph – set of
edges and nodes
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Figure 11.22 Types of links
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Defining edges or connections/links
Point-to-point link
Virtual link
Direct connection between
two routers, no IP address
needed
Bi-directional Edge
When link between two routers are
broken, admin creates a new route
across multiple routers
Transient link
Stub link
Represents
the
network
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Connects to only one router –
packets enter and leave through this
same router
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Figure 11.26 Example of an AS and its graphical representation in OSPF
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Building Routing Table
1. Create the initial state of the links of each node.
2. Disseminate the link state package (LSP) to every other
router, called flooding.
3. Construct a shortest path tree for each node (Dijkstra
Algorithm).
4. Calculate a routing table based on the shortest path tree.
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Routing Table based on Shortest Path Tree
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11-7 DISTANCE VECTOR ROUTING
Distance vector and link state routing are both interior
routing protocols. They can be used inside an
autonomous system. Both of these routing protocols
become intractable when the domain of operation
becomes large. Distance vector routing is subject to
instability if there is more than a few hops in the
domain of operation. Link state routing needs a huge
amount of resources to calculate routing tables. It also
creates heavy traffic because of flooding. There is a
need for a third routing protocol which we call path
vector routing.
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Example 11.10
The difference between the distance vector routing and path
vector routing can be compared to the difference between a
national map and an international map. A national map can tell
us the road to each city and the distance to be traveled if we
choose a particular route; an international map can tell us
which cities exist in each country and which countries should
be passed before reaching that city.
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Figure 11.50 Reachability
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Figure 11.51 Stabilized table for three autonomous system
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Figure 11.52 Routing tables after aggregation
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11-8 BGP
Border Gateway Protocol (BGP) is an interdomain
routing protocol using path vector routing. It first
appeared in 1989 and has gone through four
versions.
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Note
BGP uses the services of TCP
on port 179.
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Summaries:
 To introduce the idea of autonomous systems (ASs) that divide
the Internet into smaller administrative regions.
 To discuss the idea of distance vector routing and the RIP that is
used to implement the idea.
 To discuss the idea of link state routing as the second intra-AS
routing method and OSPF that is used to implement the idea.
 To discuss the idea of path vector routing as the dominant interAS routing method and BGP that is used to implement the idea.
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