# Routing Protocols Autonomous System (AS)

Networking and Communications

Oct 29, 2013 (4 years and 6 months ago)

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11/2/2011
1
Routing Protocols
• Two classes of protocols:
1.Interior
• Routing Information Protocol (RIP)
• Open Shortest Path First (OSPF)
2.Exterior
• Border Gateway Protocol (BGP)
Autonomous System (AS)
• What is an AS?
– A set of routers and networks managed by a single
organization.
– The routers within the AS exchange information
using a common routing protocol.
– The AS graph is connected (in the absence of
failure).
– An organization can register their network and get
an AS number.
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Autonomous System (AS)

AS1

AS2

R3

R2

R4

R1

R7
R6
R5
IGP 1

IGP 2

BGP
• Which class of protocols to use?
– Use interior router protocols to exchange
information between routers within an AS.
– Use exterior routing protocol to pass exchange
routing information between routers in different
AS’s.
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Routing Information Protocol (RIP)
￿The Routing Information Protocol (RIP) is one of
the most commonly used Interior Gateway
Protocol on internal networks
￿It is a distance-vector protocol,which employs
Hop Count as the metric.The maximum number
of hops allowed with RIP is 15.
￿RIP uses a distributed version of Bellman-Ford
algorithm.
RIP (contd.)
￿Each node calculates the distances between itself
and all other nodes within the AS and stores this
information as a table.
￿Each node sends its table to all neighboring nodes.
￿When a node receives distance tables from its
neighbors,it calculates the shortest routes to all
other nodes and updates its own table to reflect
any changes.
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Distance Vector Routing Table
Destination
Hop Count
Next Router
Other
Information
115.2.1.00
4
132.35.27.1
126.3.56.6
5
176.21.11.3
165.11.12.3
7
173.23.12.5
188.22.33.2
6
130.22.34.7
195.23.12.8
3
201.23.11.5
Problems
• Slow convergence for larger networks.
• If a network becomes inaccessible, it may
take a long time for all other routing tables to
know this.
– After a number of message transfers.
• Routing loops may take a long time to be
detected.
– Counting to infinity problem.
• Too much bandwidth consumed by routing
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Open Shortest Path First (OSPF)
• Widely used as the interior router protocol
in TCP/IP networks.
• Basic concept:
– Computes a route that incurs the least cost.
• User configurable: delay, data rate, cost, etc.
– Each router maintains a database.
• Topology of the autonomous system to which
the router belongs.
• Vertices and edges.
• Two types of vertices:
– Router
– Network
• Two types of (weighted) edges:
– Two routers connected to each other by direct
– A router is directly connected to a network.
• A router calculates the least-cost path to all
destination networks.
– Using Dijkstra’s algorithm.
– Only the next hop to the destination is used in the
forwarding process.
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– All routers know the same network topology.
– “Hello” packets sent every 10 seconds
(configurable) to neighbors.
from each router.
– Absence of “Hello” packet for 40 seconds indicate
failure of neighbor.
• Causes LSA to be flooded again.
– LSAs re-flooded every 30 minutes anyway.
Authentication
Version Type Message length
Checksum Authentication type
AreaId
0 8 16 31
Data
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• Packet types :
– 1 : Hello (check if neighbor is up)
– 2 : Database Description (synchronize database
at beginning)
– 3 : Link State Request (request specific LSA)
– 4 : Link State Update (LSAs flooded)
– 5 : Link State Acknowledgement (flooded LSAs
are explicitly ack’ed – reliable flooding)
• Authentication type:
– Cleartext
– Encrypted (MD5 Hash, others possible)
• OSPF allows sets of networks to be grouped
together. Such a grouping is called an Area.
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OSPF

1
2
2
1
3
4
Net A

Net B
Net C
Net D
R1

R2

R3

1
Note:
Numbers in
Blue
shows the Cost
to reach that network

5
Network
Cost
Neighbor
R1
A
4
R4
R1
B
1
R2
R2
B
2
R1
R2
C
5
R3
R3
C
1
R2
R3
D
3
R4
R4
A
1
R1
R4
D
2
R3
Network
Cost
Next
Router
A
4
-----
B
1
-----
C
8
R2
D
7
R4
Routing table
of RI
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Border Gateway Protocol (BGP)
What is BGP?
• Most widely used exterior router protocol for
the Internet.
• Allows routers belonging to different
autonomous systems to exchange routing
information.
– Sent as messages over TCP connections.
– The router tables get updated.
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Message Types in BGP
• Four types of messages:
1) Open: used to open a neighbor connection with
another router.
2) Update: used to transmit information about a
single route.
3) Keepalive: used to periodically confirm the
neighbor connection.
condition.
• Types of error conditions reported:
– Message header error – authentication and
syntax.
– Open message error – syntax errors and
unrecognized options.
– Update message error.
– Hold timer expired – used to close a connection if
– Cease – used by a router to close a connection
with another router in the absence of any other
error.
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Functional Procedures in BGP
• Neighbor Acquisition
– Two routers agree to be neighbors by exchanging
messages.
• Neighbor Reachability
– Check if the neighbor is still alive, and is
maintaining the relationship.
• Network Reachability
– Each router maintains a list of the networks that it
can reach, and the preferred routes.
• All modern-day routers support BGP.
– The routers that are managed by ISP’s actually run
BGP.
– Organizational networks in many cases do not run
BGP.
• Rely on the ISP’s routers to route packets to the
outside world.
• Default route will be to the ISP router.
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BGP
￿Runs over TCP:BGP uses TCP for all communication.So
the reliability issues are taken care by TCP.
￿Conserve network bandwidth:BGP doesn’t pass full
information in each update message.Instead full
information is just passed on once and thereafter
successive messages only carries the incremental
changes called deltas.
￿Support for CIDR:BGP supports classless addressing
(CIDR).That it supports a way to send the network