BORDER GATEWAY PROTOCOL

BASICS UNDERSTANDING

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 TABLE OF CONTENTS

BGP …………………………………………………………………………………………………………….3
WHY DO WE NEED BGP? ………………………………………………………………………….…3

ANTONOMOUS NUMBER ………………………………………………………………………………3

AS RANGES – PRIVATE – PUBLIC …………………………………………………………………3

BASIC UNDERSTANDING OVER BGP ……………………………………………………………4

CONFIGURATION ………………………………………………………………………………………….4

• Router 1
• Router 2

BGP PARAMETERS …………………………………………………………………………………………5

BGP NEXT HOP …………………………………………………………………………………………….5

BGP ADCHANGE …………………………………………………………………………………………...5

BGP SUMMARY ………………………………………………………………………………………………6

CONCLUSION ……………………………………………………………………………………………......6

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BGP - The Border Gateway Protocol
BGP is one of a family of IP Routing protocols, and is an Exterior Gateway Protocol (EGP)
designed to distribute routing information between ASs. It is the routing protocol that glues
the Internet together.
WHY DO WE NEED BGP?

Nowadays almost everything is connected to the Internet. In the picture above we have a
customer network connected to an ISP (Internet Service Provider). Our ISP is making sure we
have Internet access. Our ISP has given us a single public IP address we can use to access the
Internet. To make sure everyone on our LAN at the customer side can access the Internet we
are using NAT/PAT (Network / Port address translation) to translate our internal private IP
addresses to this single public IP address.
ANTONOMOUS NUMBER

An AS is a collection of networks under a single administrative domain. The Internet is nothing

For routing between the different autonomous systems, we use an EGP (external gateway
protocol). The only EGP we use nowadays is BGP.

AS RANGES – PRIVATE – PUBLIC

Autonomous system numbers are 16-bit which means we have 65535 numbers to choose from.
Just like private and public IP addresses, we have a range of public and private AS numbers.

Range 1 – 64511 are globally unique AS numbers and range 64512 – 65535 are private
autonomous system numbers.

BGP has two types:

a) External BGP: used between autonomous systems

We can use BGP to advertise our address space to the ISPs but what are the ISPS going to
advertise to our customer through BGP? There are a number of options:

a) They advertise only a default route.

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BASIC UNDERSTANDING OVER BGP – SIMPLE LAB - OUTPUT LEARNING

BGP is considered an application layer protocol, and uses TCP to transport its updates. If the
BGP message is too large for the underlying network infrastructure, it will still segment the
message into multiple TCP segments, which will then be placed in multiple IP packets and so
on to be sent.
CONFIGURATION – we have just defined neighbors along with Network for the purpose of
creating neighborship and output results to understand reachability between 2 different AS.
ROUTER 1

ROUTER 2

By default, every 60 seconds, BGP will scan its next-hop addresses within the BGP table. This
scan checks to see that there is a route in the local routing table to reach each one of those
next-hop addresses. If a route is not found for a particular next-hop address, the entry in the
BGP table is removed.
Here is the neighborship created between both AS – as you can noticed that adjancey logs
initaited – TCP connection initiated at port 179 – BGP routing performed in such a way that IPs
/ neighbors must be pingable and reachable – if all the required steps completed than the

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next step is of Open sent and Open request – If an else any error occurred so it move back to
Active state. Open sent basically one time Hello message – Hold timer is 180 seconds – BGP
aggrements must match for its formation – Below are aggrements.
a) AS – Number
b) Anthentication
c) Hold timer
d) Version
e) Optional parameters

If all above parameters match than it will smoothly proceed at Established state and Neighbor
adjancy formed – Now we can exchange our database with our neighbor – One thing to notice
that – It keep sending Keep alive message at default set 60 Seconds – If 3 consecutive keep
alive message expired than the whole process again repeat to create adjancy.
BGP SUMMARY AT BOTH ROUTERS – PARAMETERS MATCH – AGGREMENTS SUCCESS

ADJCHANGE CREATED

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NEXT HOP DEFINED AND VALID ROUTE
*> -- Means valid route – Best possible path

CONCLUSION - The Border Gateway Protocol (BGP) is the main routing system for the internet
backbone and is used by every major service provider. BGP provides a map for routers to
funnel packets from their original source to their final destination. A BGP session is
established with a peer interface through the use of a finite state machine and standard
message types. The peer can be in the same autonomous system or part of a different
vendor’s network.
BGP sends information about the reachable networks it knows to all of its peers. Based on the
routes it receives in return, it constructs a local routing information base consisting of the
best route to each network. In turn, BGP sends its selected routes to the main system routing
base, where they can potentially be selected as the best route. BGP is a scalable and efficient
standard, but suffers from high memory requirements and security issues, and is prone to
serious misconfiguration problems

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