Border Gateway Protocol BGP

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Border Gateway Protocol BGP
Hady S. Abdel Salam
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In a typical computer network with up to a few
hundreds of hosts, Routers have 2 Goals to
achieve Routing (Finding Optimal Route)
Packet Forwarding
Many Routing Algorithms are currently available
and technically they are suitable for such
deployments. e.g. Distant Vector, RIP,
OSPF, etc
Routing for a very large network like the
Internet is something different.
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To be able to see the differences, we need to
have a closer look at the current structure of
the Internet.
The Internet can be viewed as a large collection
of interconnected autonomous systems.

• Autonomous System (AS)is a group of networks
  under the technical administration of the same
  entity.
• e.g. Universities Networks (ODU Network).
• Big Corporations Networks (IBM).
• Commercial ISP (Verizon Network).

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Basically, ASs are free choosing their internal
routing protocols. However an external routing
protocol must be agreed upon to allow routing
between different ASs.

• This way we have 2 types of routing Protocols
• Interior Gateway Protocols to route inside the
  ASs.
• Exterior Gateway Protocols to route between ASs

Can we use the same routing protocol internally
inside ASs and externally to route between them?
MOST PROBABLY NO ...
Why ?
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Interior Gateway Protocol (IGP) runs inside
autonomous systems. Main Goal Technically find
Optimal path. (e.g. minimum
number of hops, minimum Delay)
AS 200
AS 100
OSPF
RIP
BGP
Exterior Gateway Protocol (EGP) runs between
autonomous Systems. Main Goal Scalable, enables
routing policies. ( e.g. Political Policies
Dont route through bad guys ASs,
Business policies Dont use routers of this ISP
)
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BGP Overview

• BGP has gone through several improvements since
  its original version, BGP-1, in 1989 till the
  deployment of BGP-4 in 1993.
• BGP-4 support aggregation and CIDR.
• BGP is a path vector protocol used to exchange
  routing information between ASs. Routing
  information includes sequence of ASs visited to
  reach some network prefix destination.
• The path information associated with the routes
  is used to
• Enable loop prevention Enforce routing
  policies Avoid Count to infinity problem.

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How does BGP Work ?

• Routers in ASs that are connected to external Ass
  are called Border Routers (Speakers).
• Each router knows exactly the different routes
  available inside its own AS.
• BGP Speakers start communication sessions to
  exchange routing information.
• To do that, BGP uses 4 different types of
  messages OPEN, UPDATE, NOTIFICATION, KEEPALIVE.

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How does BGP Work ? Cont.
Initially, when a BGP session is established
between a set of BGP speakers, all candidate BGP
routes are exchanged.
After the session has been established and the
initial route exchange has occurred, only
incremental updates are sent. These updates may
include route withdrawal.
The incremental update approach has improved CPU,
bandwidth overhead compared with complete
periodic updates used in old protocols.
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How does BGP Support Incremental Updates ?

• To be able to make incremental updates, BGP must
  have a table to keep track of its routing table
  version.
• If the table changes, BGP increments the table
  version number.
• KEEPALIVE messages are very light messages that
  are sent periodically between BGP neighbors to
  ensure that the connection is kept alive.

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BGP Message Header Format

• 16-byte Marker field
• Used for authentication so no one should tamper
  with the routing tables.
• 2-byte Length field
• Message length including header.
• 1-byte Type field
• There are 4 different types of messages
• OPEN
• UPDATE
• NOTIFICATION
• KEEPALIVE

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Open Message Format

• 1-Byte Version BGP-1, BGP-2, BGP-3 or BGP-4.
• 2-Byte AS number of sending router.
• 2-Byte Hold Timer indicates the maximum amount
  of time in seconds that may elapse between the
  receipt of successive messages.
• 4-Byte BGP Identifier BGP ID. Router ID (RID),
  which is calculated as the highest IP address on
  the router.

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Update Message Format

• Update Message can withdraw 0 or more routes but
  it cant advertise more than one route.
• 2-Byte Unfeasible Routes length. Followed by
  withdrawn routes.
• 2-Byte Total path attributes length Followed by
  Path attributes.
• ltLength (1-Byte), Prefix (Variable)gt pairs, that
  represents advertised networks.

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Notification Message Format

• 1-Byte The Error code indicates the type of the
  notification.
• 1-Byte The Error sub code provides more specific
  information about the nature of the error.
• The Data field contains data relevant to the
  error, such as an illegal AS number

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KEEPALIVE Message

• The KEEPALIVE message is a 19-byte BGP message
  header with no data following it.
• KEEPALIVE messages are periodic messages
  exchanged between peers to determine whether
  peers are still reachable or not.
• KEEPALIVE messages are sent at a rate that
  ensures that the hold time will not expire (the
  session is considered alive).
• A recommended KEEPALIVE rate is one-third of the
  Hold Timer value.

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BGP Session Creation Finite State Machine
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References

• RFC 1771, Border Gateway Protocol-4.
• Russ White, Danny McPherson, Sangli Srihari,
  Practical BGP-4, Addison Wesley, July, 2004.
  Chapter 1.
• Internetworking Technology Handbook, Cisco
  Systems Website, http//www.cisco.com/univercd/cc/
  td/doc/cisintwk/ito_doc/bgp.htm
• Sam Halabi, Danny McPherson, Internet Routing
  Architectures, Cisco Press, Second Edition,
  Chapter 4, Chapter 5.

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• Thank You