Transcript Unicast Routing Protocols: RIP, OSPF, and BGP

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.


In distance vector routing, the least cost route between any
two nodes is the route with minimum distance. In this
protocol each node maintains a vector (table) of minimum
distances to every node.
Updates are send to directly connected
neighbors only.

The Routing Information Protocol (RIP) is an
intra-domain (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.
Periodic timer
It controls the advertising of regular update
message (25 ~ 30 sec).
Expiration timer
It governs the validity of a route (180 sec)
The route is considered expired and the hop
count of the route is set to 16.
Garbage collection timer
A invalid route is not purged from the routing
table until this timer expires (120 sec).



Route tag (autonomous system number) able
receiving information from inter domain routing
protocol.
Support classless addressing and CIDR (classless
interdomain routing).
Authentication
prevent
from
unauthorised
advertisement.


RIP messages are encapsulated with UDP
Datagram.
Port assign to rip in UDP is 520.



RIP treat all network as equal.
Periodic timer is of short time.
Not for large network .(infinity at 16).
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.
Updates are send to entire network.ie network have
visibility of entire network of that area.

The Open Shortest Path First (OSPF) protocol is an
intradomain routing protocol based on link state
routing. Its domain is also an autonomous system.

A collection of networks with area ID

Routers inside an area flood the area with routing
information

Area border routers summarize the information
about the area and send it to other areas

Backbone area and backbone routers

All of the area inside an AS must be connected to the
backbone



Administrator assign cost to each route.
Cost based on types of services.(minimum
delay,maximum throughput etc).
1 router can have many routing tables based on
different types of service.







Network mask :defines network mask over which hello
message is send.
Hello interval : seconds between hello messages.
E Flag : if set then stub area.
T Flag : if set then router support multiple metric.
Priority : highest priority router chosen as designated router.
Dead interval : seconds must pass before neighbor is
consider dead.
Neighbor routing address : Current list of all the neighbours
from which the sending router has received the hello
message.

OSPF packets encapsulated in IP Datagram.
RIP




OSPF
Keeps only track of
closest router for each
destination address.
It is simpler ,older,easy
to configure,uses less
CPU .
Only metric used is
hope count.
Can be used in small
business needs.




Uses tree to keep track of
entire paths.
More overhead .Provide
more options,better,faster
reaction time to faliure
(convergence).
Build cost according to
router’s services.
Widely used for business
enterprises & isp’s.




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.
Need for a third routing protocol.

Path vector routing is similar to distance vector
routing. There is at least one node, called the
speaker node, in each AS that creates a routing
table and advertises it to speaker nodes in the
neighboring ASs..


Speaker node : a node working on behalf of
complete AS.A speaker node advertises the path, not
the metric of the nodes, in its AS or other ASs.
Initialization : At beginning each speaker node
can only the reachability of nodes inside its
autonomous system.
• Sharing
– A speaker in an AS shares its table with immediate
neighbors
• Updating
– Adding the nodes that are not in its routing table
and adding its own AS and the AS that sent the
table
– The routing table shows the path completely

Border Gateway Protocol (BGP) is an interdomain
routing protocol using path vector routing. It first
appeared in 1989 and has gone through four
versions.
•
•
•
Stub AS
– Only one connection to another AS (only a source or sink for
data traffic) eg small local ISP.
Multihomed AS
– More than one connection to other AS, but it is still only a
source or sink for data traffic. Eg large corporation connecting
regional AS.
Transit AS
– Multihomed AS that also allows transient traffic eg national and
international isp’s
•
•
•
•



Well Known Mandatory attributes ORIGIN
– The source of the routing information (RIP, OSPF, etc)
AS_PATH
– The list of ASs through which the destination can be reached
NEXT-HOP
– The next router to which the data packet should be sent
Optional attribues
Optional transitive
Optional non transitine




Open : to create neighbor relationship (open
tcp connection).
Update : it is used by a router to withdraw
destination that are previously advertised,
announce a route to new destination or both.
Keep alive : regularly send by routers to
confirm that they are alive or send in response
of open message.
Notification : send when there is error or router
want to close the connection.


Once opened the connection update, keepalive
and notification messages is continued until
cease notification is sent.
BGP uses services of tcp on port 179.