evolution of routing techniques
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Delivery, Forwarding, RoutingIP and MaskRouting TableUnicast Routing Protocols
AGENDA
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What is routing? Routing is the process of selecting a path in a
network along which the packets shall be sentto a destination
Routing consists of A Router
A set of routing protocols
A routing information base (RIB) One or more routing algorithms
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Why is routing required?
For practical limitation ofphysicalconnections
For efficiently managing the network traffic
For efficient usage of network resources
For catering to different types of services
For congestion control
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At which layer is routing done? Generally routing is done at network layer Multi layer layer routing and Cross layer rout ing is also
prevalent nowadays
Firewalls are often integrated with routers
APPLICATION LAYER
PRESENTATION LAYER
SESSION LAYER
TRANSPORT LAYER
NETWORK LAYER
DATA LAYER
PHYSICAL LAYER
APPLICATION LAYER
SOCKET LAYER
ROUTING LAYER
LINK LAYER
DEVICE DRIVERs
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Router, Switch and Hub The basic difference is varying intelligence
Least expensive and complicated. Nointelligence
J ust directs incoming packets from one portto other
HUB
More expensive and intelligent Knows which port is carrying the traffic from
which host/interfaceSWITCH
Most expensive and intelligent, Mostcomplicated
Learns about its neighboring conditions,manipulates data traffic
ROUTER
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How does a router work?
One
Packet arrives at the router fordelivery
Router reads destination IPaddress
Two
Router searches RoutingTable
Determines next hop of thepacket
Three
Router forwards the packet tonext hop
Packet is said to be routed
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Direct and Indirect Delivery
To rest of Network
Source SourceDestination
DestinationDirect
Indirect
Direct
Direct
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Logical addressing: IP and MASK
Classful addressing1st 2nd 3rd 4th
0
10
110
1110
1111
1st 2nd 3rd 4th
0-127
128-191
192-223
224-239
240-255
Classes
Class A
Class B
Class C
Class D
Class E
Class No. of Blocks Block Size Application
A 128 16,777,216 Unicast
B 16,384 65,536 Unicast
C 2,097,152 256 Unicast
D 1 268,435,456 Multicast
E 1 268,435,456 Reserved
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Logical addressing: IP and MASK Mask: A 32 bit number made ofn contiguous 1s
followed by (32-n) contiguous 0s (n
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Logical addressing: IP and MASK Classless addressing: No more classes but a
block of addresses are assigned, provided thefollowing restrictions are strictly followed The addresses in the block must be contiguous
The number of addresses must be a power of 2 The first address must be evenly divisible by the total
number of addresses allocated
Mask is a better way to define a block
An example: Given an IP address205.16.37.39/28
What are the first, last and the total number ofaddresses assigned?
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Logical addressing: IP and MASK
Binary equivalent of mask /28: 11111111 11111111 11111111 11110000 (255.255.255.240)
Binary equivalent of the address: 11001101 00010000 00100101 00100111 (205.16.37.39)
First address: Set the right most 4 bits to 0: 11001101 00010000 00100101 0010000 (205.16.37.32)
Last address: Set the right most 4 bits to 1: 11001101 00010000 00100101 00101111 (205.16.37.47)
Number of addresses: 232-n = 24 =16
So, in general a address in classless addressing ismentioned as: x.y.z.t/n
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Network Address When a organization is allocated a block of addresses,
normally (not always) the first address is treated as thenetwork address
It is not assigned to any device, it defines the organization
itself to the rest of the world
REST of the WORLD
Network Address:205.16.37.32
All packets with receiveraddress 205.16.37.32 to205.16.37.47 are routed tox.y.z.t/n
205.16.37.32/28
205.16.39.33/28 205.16.39.47/28
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Routing Table
A host or a router maintains a routing table withan entry for each specific destination
The table can be STATIC or DYNAMIC
Static Routing Table: Contains information entered manually by the
administratorat the time ofcreation
Cannot be modified automatically when there is anychange in the Internet
Dynamic Routing Table: Capable ofupdating the table with the help of routing
protocols and algorithms automatically
Only option for managing any large network of today
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A quick look at a system routing table
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Network Configuration of a Systemform the Routing Table
A UNIX server gives the following result with netstatand ifconfig command $ netstat nr
Kernel IP rout ing table
$ ifconfig eth0 Eth0 Link encap:Ethernet Hwaddr 00:B0:D0:DF:09:5D
Inet addr: 153.18.17.11 Bcast: 153.18.31.255 Mask:255.255.240.0
What is the network configuration of the server?
Destination Gateway Mask Flags Iface
153.18.16.0 0.0.0.0 255.255.240.0 U eth0
127.0.0.0 0.0.0.0 255.0.0.0 U lo
0.0.0.0 153.18.31.254 0.0.0.0 UG eth0
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Network Configuration from theRouting Table
Rest of the Internet
153.18.16.0/20
eth0
00:B0:D0:DF:09:5D
DefaultRouter
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Forwarding
It means placing the packet in its route to itsdestination
Requires a host or a router to have a routing table
When the host has a packet to send or the routerhas received a packet, it looks up this routingtable to determine route to the final destination
Routing techniques caters to optimizing this table
as maintain a full-fledged look-up table isimpossible to maintain
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Forwarding Techniques Next-hop method Vs Route Method
N1N2 N3
R1 R2Host A
Host B
Routing tables based on routing
Destination Route
Host B R1, R2, host B
Destination Route
Host B R2, host B
Destination Route
Host B Host B
Routing tables based on Next-hop
Destination Route
Host B R1
Destination Route
Host B R2
Destination Route
Host B Host B
ForA
ForR1
ForR2
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Forwarding Techniques Network-Specific method Vs Host Specific method
System
DCBA
N2N1
Routing table for host S based onhost-specific method
Destination Next Hop
A R1
B R1
C R1
D R1
Routing table for host S based onnetwork-specific method
Destination Next Hop
N2 R1
R1
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Forwarding Techniques
Default Method: Using a default router
N1 N2
R1
R2Host A
Rest of the Internet
DefaultRouter
Destinat ion Next Hop
N2 R2
Any other R1
Routingtable forhost A
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Forwarding Process
In classless addressing, at least 4 columns are required
The routing table is searched based on the network addressand mask
Mask NetworkAddress
Next-hop
Address
Interface
ExtractDestinationAddress
SearchTable
Forwarding Module
To ARP
Next hop addressand interface no.
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Managing Routing Table in ClasslessAddressing
Address aggregation: Blocks of addresses of differentinterface and mask are aggregated into one single block inrouting table
Several levels of aggregation are possible
140.24.7.0/26
140.24.7.64/26
140.24.7.128/26
140.24.7.192/26
Org 1
Org 2
Org 3
Org 4
m0
m1
m2
m3
m4 m0 m1
R1 R2
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Managing Routing Table in ClasslessAddressing
Address aggregation: Routing tables for router R1and router R2 For R2, any packet with destination addresses 140.24.7.0 to
140.24.7.255 are sent to interface m0 regardless of any ofthe organizations
Mask NA NHA Iface
/26 140.24.7.0 m0
/26 140.24.7.64 m1
/26 140.24.7.128 m2
/26 140.24.7.192 m3
/0 0.0.0.0 Default m4
Mask NA NHA Iface
/24 140.24.7.0 m0
/0 0.0.0.0 Default m1
Routing table for R1 Routing table for R2
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Longest Mask Matching
What happens if Org. 4 is not geographically close to the other
3 Orgs?
Can we still use Address Aggregation and assign the block140.24.7.192/26 to Org. 4?
R2
R1R3
140.24.7.0/26
140.24.7.64/26
140.24.7.128/26
140.24.7.192/26
Org 1
Org 2
Org 3
Org 4
m0m1
m2
m3
m0 m2
m1
m0m1
m2
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Longest Mask Matching Answer: YES
Reason: LONGEST MASK MATCHING
The Routing Table is sorted from the longest mask to theshortest mask
Mask NA NHA Iface
/26 140.24.7.0 m0
/26 140.24.7.64 m1
/26 140.24.7.128 m2
/0 0.0.0.0 Default m3
Mask NA NHA Iface
/26 140.24.7.192 m0
/0 0.0.0.0 Default m2
Mask NA NHA Iface/26 140.24.7.192 m1
/24 140.24.7.0 m0
/0 0.0.0.0 Default m2
Routing table for R1Routing table for R2
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Hierarchical Routing
Hierarchical routing can greatly minimize the size ofthe routing tables
For example, a regional ISP is granted a 16,384(214) addresses starting from120.14.64.0/18
It is divided in to 4 sub-blocks each of size 4096 for3 local ISPs. For them the mask is /20
1st local ISP divides its assigns sub-blocks into 8smaller blocks for small ISPs. For them the maskbecomes /23
Each small ISPs divides them into 128 sub-blocksfor households. For them the mask becomes /30,and so on
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Hierarchical Routing The logical representation is displayed here
120.14.64.0/18
Total 16,384
120.14.64.0/20
120.14.64.0/23
120.14.64.0/30
120.14.78.0/30
120.14.78.0/23
120.14.80.0/20
120.14.96.0/22
120.14.112.0/24
120.14.96.0/20
120.14.112.0/20
Total 4096
Total 4096
Total 4096
Total 4096
512
512
ISP 1
ISP 2.1
ISP 3.1
ISP 3.8
ISP 2.2
ISP 2.3
Total 4 Large Orgs.
Total 16 Small Orgs.
128 Each
128 Each
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Geographical Routing
The same concept of hierarchical routing can be
extended in geographical routing To decrease the size of the routing tables further,
segregation is done in geographical level as well
For example, the entire address space is divided
into few large blocks One block is assigned to North America, one to
Asia, one to Africa, one to Europe and so on
So, for all the routers of the ISPs outside Europe,
every router will have one and only entry for all theaddresses assigned to Europe
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Unicast Routing Protocols
Routing protocols are needed to maintain and
update dynamic routing tables A routing protocols is a combination of set ofrules
(algorithms) and procedures
Unicast routing protocols applies where each
incoming packet has to be delivered to one andonly one destination
Router decides the next hope of a packet in aAutonomous Systembased on Optimization
3 most popular and basic Unicast RoutingProtocols are: RIP (Distance Vector routing), OSPF(Path Vector routing) and BGP (Link State routing)
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Autonomous Systems
An Autonomous System or AS is group of
networks and routers under the authority of a singleadministration
Routing inside AS : Intra-domain routing
Routing between AS : Inter-domain routing
AS1
AS4AS3
AS2
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Routing Protocols Scope
RoutingProtocols
Intradomain
Routing
Distance
Vector (RIP)
Link State(OSPF)
Interdomain
Routing
Path Vector
(BGP)
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Distance Vector Routing
In DVR, the least cost route between any two nodes
is the route with minimum distance
Each node maintains a vector (table) of minimumdistance to every node known
There are three steps involved:
Initialization: At the beginning, each node knows thedistance to its immediate neighbors
Sharing: Periodically or in triggered time, the nodesshare their vectors with other nodes
Updating: Based on the shared info, nodes updatestheir vectors about path to indirectly connectednodes
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DVR: Sharing and Updating
Each node will share its routing table on periodic
basis or triggered condition Full routing table needed not be shared. In our
scenario, only column 1 and column 2 will be shared.Next Hop Address (column 3) will be calculated
based on that Receiving a partial table from its neighbor, a node
calculates a temporary updated table
Then each row of the old and new table are
compared based on the next node entry (col. 3) If next node entry is different, the row with smaller cost is
chosen. If there is a tie, old entry is kept
If next node entry is same, the new entry is chosen
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DVR: Updating Table for A
To Cost NextA 0 _
B 5 _
C 2 _
D 3 _
E
To CostA 2
B 4
C 0
D
E 4
To Cost NextA 4 C
B 6 C
C 2 C
D C
E 6 C
To Cost Next
A 0 _
B 5 _
C 2 _
D 3 _
E 6 C
Old Table of ATableReceivedfrom C
ModifiedTable of A
New Table of A
Compare
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DVR: The Finalized TablesTo Cost Next
A 0 _
B 5 _
C 2 _
D 3 _
E 6 C
A
C
B
ED
5
3
2 4
34To Cost Next
A 3 _
B 8 A
C 5 A
D 0 _
E 9 A
To Cost Next
A 5 _
B 0 _
C 4 _
D 8 A
E 3 _
To Cost Next
A 6 C
B 3 _
C 4 _
D 9 C
E 0 _
To Cost Next
A 2 _
B 4 _C 0 _
D 5 A
E 4 _
Table of A Table of B
Table of CTable of D Table of E
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DVR: Two Node Loop Instability
X A B
XX
X
X
A
AA
A
B
B
B
B
2 4
4
4
4
4
.
.
.
X 2 _
X _
X 10 B
X 10 B
X _
X 6 A X 14 A
X 6 A
X 6 A
X _
BeforeFailure
AfterFailure
After Areceivesupdatefrom B
After Breceives
updatefrom A
Finally
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Routing Information Protocol
Routing Information Protocol (RIP) is an
implementation of Distance Vector Algorithm withthe following considerations:
1. In an autonomous system, we are dealing withrouters and networks (links). Only routers have
routing tables, networks not2. The destination in a routing table is a network
always
3. The metric used by RIP is the no of hops
needed to reach the destination4. Infinity is defined as 16
5. The next-node column defines the address ofthe router to which packet is to be sent
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Link State Routing
Domain Topology: Here, each node in the domain has
an entire topology of the domain Link State: For each node, the number of other links
and nodes, their connectivity type, cost (metric) andthe condition of the links (Up or Down) constitutes linkstate
Shortest Path Tree: Based on the link states, a nodecan use Dijkstras Algorithmto create a Shortest Path
Treewhich can used as the routing table
There are four sets of operations required
Creation of Link State Packets (LSPs)Flooding of LSPs
Formation of shortest path treeCalculation of routing based on the tree
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Link State Routing:
A
C
B
ED
5
3
2 4
State ofLinksfor A
Initial Condition:
D
A B
E
5
2
3
3
2 4
4
5
4
3
34
3
State of
Linksfor D
State ofLinksfor B
State ofLinksfor E
State of
Linksfor C
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Link State Routing: Dijkstras Algorithm: Formation of Shortest Path Tree
START
STOPTentative l istis empty?
Set root to local node andmove it to tentative list
Among nodes in tentative list, move theones with shortest path to permanent list
Add each unprocessed neighbor of lastmoved node to tentative list if it is not therealready. If neighbor is in tentative list withlarger cumulative cost, replace with new one
YES
NO
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Link State Routing:
A B
0
1. Set root to A and move A to tentative list
Creation of Shortest Path Tree for node A:
A
Permanent List: Empty Tentative List: A(0)
Root
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Link State Routing:
A B
0
3. Move C to permanent List. Add E tentative list
Creation of Shortest Path Tree for node A:
A
Permanent List : A(0), C(2) Tentative List : B(5), D(3), E(6)
Root
5B
2 C
3 D 6E
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Link State Routing:
A B
0
4. Move D to permanent List.
Creation of Shortest Path Tree for node A:
A
Permanent List : A(0), C(2), D(3) Tentative List : B(5), E(6)
Root
5B
2 C
3 D 6E
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Link State Routing:
A B
0
5. Move B to permanent List.
Creation of Shortest Path Tree for node A:
A
Permanent List : A(0), B(5), C(2), D(3) Tentative List : E(6)
Root
5B
2 C
3 D 6E
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i k S i
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Link State Routing:
Calculation of Routing Table from Shortest Path Tree
Node Cost Next
A 0 _
B 5 _
C 2 _
D 3 _
E 6 C
Routing table for node A
We can see that therouting table of A asdeduced by Link State
Routing is the same asDistance Vector Routing
In real scenario, therouting table is
determined by the costassigned to each node bythe administrator
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Open Shortest Path First (OSPF)
OSPF is based on Link State Routing Protocol
Area:A collection of networks, hosts and routers allcontained within an autonomous system
Area Border Routers: Summarizes all the informationabout an area and shares it across
Backbone: A special area among all areas in an ASwhich all other areas must be connected to. Thebackbone always has area code 0
Backbone Routers: Routers in a backbone. A
backbone router can also be area border router Virtual Link: If the connection between an area and
backbone is broken the administrator can create analternate connection between routers
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OSPF I l t ti
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OSPF: Implementation
net net net
net
net
net
net
net netnet
Area 1
Area 2
Area 0 (Backbone)
ABRABR
BR
BR
AS BR
Autonomous System (AS)
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Path Vector Routing
Why DVR and LSR are not suitable for inter-domain
routing? Reason: Scalability
DVR becomes instable and intractable for a largenumber of hops (even more than 16)
LSR needs a huge amount of resource to calculateits shortest paths. It also causes heavy traffic in thenetwork because of flooding of LSP
How path vector routing eliminates them?
Well, it is simply derived from DVR, but does notassign hop count as the metric/cost...
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Path Vector Routing
Speaker node: In path vector routing, a special
node acts on behalf on the entire AS. It summarizesall the information of that AS, creates a routing tableand advertizes it to other ASs
What is advertized?
Not the metrics but the paths in an AS Policy: Every AS will have a well defined policy
Paths are decided upon by the speaker nodes byconsulting the policies in neighboring ASs
Reason: Different ASs will have different policies &priorities associated with them
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P th V t R ti
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Path Vector Routing
A1
D1
C1
B1
A2 A3
A4 A5
C2 C3
B2
B3
B4
D2 D3
D4
Dest. Path
A1 AS1
A2 AS1
A3 AS1
A4 AS1
A5 AS1
Dest. PathC1 AS3
C2 AS3
C3 AS3
Dest. Path
B1 AS2
B2 AS2
B3 AS2
B4 AS2
Dest. Path
D1 AS4D2 AS4
D3 AS4
D4 AS4
AS1
AS2
AS3
AS4
Ta
bleofA1
TableofD1
TableofC1
TableofB1
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Path Vector Routing Stabilized tables of all AS after update:
Dest Path
A1
A5
AS1
AS1
B1
B4
AS1-AS2
AS1-AS2
C1C3
AS1-AS3
AS1-AS3
D1D4
AS1-AS2-AS4
AS1-AS2-AS4
Dest Path
A1A5
AS2-AS1
AS2-AS1
B1
B4
AS2
AS2
C1C3
AS2-AS3
AS2-AS3
D1D4
AS2-AS3-AS4
AS2-AS3-AS4
Dest Path
A1A5
AS3-AS1
AS3-AS1
B1
B4
AS3-AS2
AS3-AS2
C1C3
AS3
AS3
D1D4
AS3-AS4
AS3-AS4
Dest Path
A1A5
AS4-AS3-AS1
AS4-AS3-AS1
B1
B4
AS4-AS3-AS2
AS4-AS3-AS2
C1C3
AS4-AS3
AS4-AS3
D1D4
AS4
AS4
Table of A1 Table of B1 Table of C1 Table of D1
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P th V t R ti
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Path Vector Routing Some important features of updating:
Loop Prevention: The instability of DVR is avoided inPVR; upon receiving a message the router checks tosee if its AS is in the path
Policy Routing: Upon receiving a message a router
checks the path with policy. If an AS in the path isagainst policy it can ignore that
Optimum path: Router find the path that fits theorganization best. A path from AS4 to AS1 can eitherbe AS4->AS3->AS2->AS1 orAS4->AS3->AS1. Herewe will choose the path with less number of ASsinvolvedThis is not a general rule. There are complex criteria which
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B d G t P t l (BGP)
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Border Gateway Protocol (BGP)
BGP was introduced in 1989
Some features of BGP:
Types of AS
Stub AS: An AS which is connected to another AS. Astub is either a source or a sink
Multihomed AS: An AS which is connected to morethan one AS, but it is only a sink or source. Example:A large corporation which is connected to more thanone regional or national ASs
Transit AS: A multihomed AS that allows flow of datatraffic through it. Example: All national andinternational ISPs
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B d G t P t l (BGP)
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Border Gateway Protocol (BGP) Some features of BGP:
BGP Sessions: A BGP session is a connection setupbetween two BGP routers for the sake of exchangingrouter information
A session in BGP is a connection at the TCP level.
External BGP Session (E-BGP):Takes place when two speakernodes exchange routing information
Internal BGP Session (I-BGP):Takes place when a speakernode collects information from other nodes in the its own As
A1
A2 A3
A4 A5
AS1
C1
C2 C3AS3
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