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A Whirlwind Tour of Interdomain Routing. Aaron Wagner awagner@eecs.berkeley.edu EE 122 Class February 9, 2001. Overview. What I hope you will learn in the next half-hour: What interdomain routing is and how it came to be What BGP is and how it works BGP ’s strengths and weaknesses.

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A Whirlwind Tour of Interdomain Routing

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A whirlwind tour of interdomain routing

A Whirlwind Tour of Interdomain Routing

Aaron Wagner


EE 122 Class

February 9, 2001



  • What I hope you will learn in the next half-hour:

    • What interdomain routing is and how it came to be

    • What BGP is and how it works

    • BGP’s strengths and weaknesses

The need for routing domains

The Need for Routing Domains

  • Prior to 1982, the Internet used a single routing protocol (GGP).

  • This architecture had disadvantages:

    • Wasteful to store complete routing table in every router

    • Routing traffic became excessive as the Internet grew

  • In 1982, the Internet split into “Autonomous Systems” (AS’s) or “Routing Domains”: collections of routers and hosts under common administration.

  • An AS uses an intradomain (RIP, OSPF) protocol of its choice for routing within the AS.

  • A common interdomain (EGP, then BGP) protocol is used for routing between AS’s.

Challenges of interdomain routing

Challenges of Interdomain Routing

  • “Link costs” cannot be compared between different AS’s.

    • How to avoid routing loops?

  • The scale of the problem is larger than the intradomain one:

    • Larger routing tables

    • Routers separated by greater distances: higher delays and lower reliability for messages passed

  • Different AS’s may have conflicting routing objectives…

    … due to prior business agreements.

    … due to different approaches to the speed vs. reliability tradeoff.

Border gateway protocol bgp overview

Border Gateway Protocol (BGP): Overview

  • BGP is the currently-deployed interdomain routing protocol.

    • First introduced in 1989. Many modifications since.

  • BGP is a path-vector protocol: route announcements include the complete list of AS’s to reach the destination (AS-path).

    • Makes loop suppression very simple

      – Avoids comparisons of metrics between different AS’s

Bgp operation a high level example




AS 11537

AS 556

AS 25


BGP Operation: A High-Level Example

128.32/16 25

128.32/16 25


128.32/16 X 556 11537 25

128.32/16 11537 25

128.32/16 556 11537 25

128.32/16 556 11537 25

128.32/16 11537 25

Bgp operation messaging

BGP Operation: Messaging

  • Each AS has one or more BGP speakers.

    • Speakers learn of internal routes through the intradomain routing protocol.

    • Speakers inform BGP speakers in neighboring domains of local destinations.

    • Other routers in the domain can send traffic destined outside the domain to a speaker: they do not need a complete routing table.

  • BGP speakers exchange entire routing table only once. Thereafter, updates (announcements and withdrawals)

  • BGP speakers communicate using TCP (port 179)

    (+) Keeps retransmission and reordering of updates out of protocol

    (–) Routing updates “back off” during congestion

  • Speakers send “keep-alive” messages at regular intervals.

Bgp operation a low level example

BGP Speakers

d R2 B

Other Routers

d R2 B

Data Link

BGP Session

d R2 B

d R2 B


BGP Operation: A Low-Level Example


e R2 B C

e R2 B C



d R4

d R4


e R3 C

e R3 C





Bgp operation choosing routes

BGP Operation: Choosing Routes

  • Each AS defines a mapping from routes to nonnegative numbers: speakers use it to choose routes.

  • The number to which a route is assigned is called its “local-preference.”

  • When a BGP speaker receives two routes to a destination:

    • It chooses the one with higher local-preference, if possible.

    • Otherwise, it chooses the one with shorter AS-path, if possible.

    • Ties are broken using the IP address of the next-hop router.

  • A speaker may advertise at most one route per destination to other speakers, so the last step always breaks a tie.

Bgp operation choosing routes1

BGP Operation: Choosing Routes

  • Sample mapping from routes to local-preferences:

    If 556 is in AS-Path, return 200.

    Else if destination is 128.32/16 and AS-Path does not contain 556, return 150.

    Otherwise, return 100.

  • BGP speakers can be programmed to exclude certain routes from consideration.

    • Examples:

      • CALREN excludes routes from UCB with destinations other than 128.32/16, 169.229/16, and 136.152/16.

Non convergence of bgp

(Non)convergence of BGP

  • We say a routing protocol converges if routers settle on a set of routes and no new routing updates are sent.

  • One can prove that distance-vector and link-state routing protocols will always converge.

  • It was recently discovered (1996) that BGP does not necessarily converge.

Key points

Key Points

  • The Internet is too large for a flat routing architecture.

  • So, the Internet is split into Autonomous Systems (AS’s)

    • Routing problem separated into inter- and intradomain routing.

  • Interdomain routing presents unique challenges over intradomain.

  • The current interdomain routing protocol is the Border Gateway Protocol (BGP), a path-vector protocol.

  • Unlike other routing protocols, BGP does not necessarily converge.

    Buzzwords: Interdomain Routing, BGP, BGP Speakers, AS, AS Path, Local Preference, Path Vector Routing Protocol.

A whirlwind tour of interdomain routing

Extra Slides

Interdomain routing by the numbers

Interdomain Routing by the numbers

Statistics on the Internet at Large

Statistics on one BGP Speaker

From www.telstra.net/ops/bgptable.html, 9/23/00

From rs.arin.net and www.netsizer.com, 9/23/00

When does a network become an as

When does a network become an AS?

  • To receive an AS number, a network must

    • Be multi-homed (i.e. have more than one connection to the rest of the network)

    • Be capable of running BGP

    • Pay 500 USD

      There is no minimum network size to become an AS.

  • Example: a small company with a single provider is considered part of the provider’s AS.

Nonconvergence of bgp an example




Nonconvergence of BGP: An Example

  • Consider an internet with 4 AS’s,

  • Focus on a destination d in AS 0.

  • AS 1 will accept routes 10 and 120 to d, but prefers 120.

  • AS 2 will accept routes 20 and 230 to d, but prefers 230.

  • AS 3 will accept routes 30 and 310 to d, but prefers 310.


Nonconvergence of bgp an example1

























Nonconvergence of BGP: An Example

d 2 3 0

d 2 0

d 3 0

d 2 0

d 1 0

d 3 1 0

Classless interdomain routing 1991

Classless InterDomain Routing (1991)

  • A temporary solution to two immediate dangers:

    • Class-B address space exhaustion

      • Class-A allows 16,777,216 hosts: “too large”

      • Class-C allows 255 hosts: “too small”

      • Class-B allows 65,536 hosts: “just right”

    • Only 16384 class-B network numbers available

    • Last Class-B address would have been assigned in March 1994.

      2. Routing table explosion

  • Idea: assign multiple consecutive Class-C addresses in place of Class-B and aggregate.

  • Cidr an example

    CIDR: An Example

    Assign IP addresses to maximize aggregation.



    197.8.0/24 W

    AS W

    197.8.0/22 Y W X

    197.8.1/24 X


    AS Z


    AS Y

    AS X

    Cidr cont

    CIDR, cont.

    • Imperfect technique: old addresses, multihoming, changing providers, etc.

    From www.telstra.net/ops/bgptable.html, 9/23/00






    Labovitz, C., Malan, G., Jahanian, F. “Origins of Internet routing instability.” Proceedings of INFOCOM'99: Conference on Computer Communications, Piscataway, NJ, USA: IEEE, 1999. p.218-26 vol.1.

    Labovitz, C., Malan, G., Jahanian, F. “Internet routing instability.” IEEE/ACM Transactions on Networking, vol.6, (no.5), IEEE; ACM, Oct. 1998. p.515-28.

    Labovitz, C., Ahuja, A., Jahanian, F. “Experimental study of Internet stability and backbone failures.” Proceedings of the 29th Annual International Symposium on Fault-Tolerant Computing, June 1999, Los Alamitos, CA: IEEE Comput. Soc, 1999. p.278-85.

    Varadhan, K. Govindan, R. Estrin, D. “Persistent route oscillations in inter-domain routing.” Computer Networks, vol.32, (no.1), Elsevier, Jan. 2000.

    Griffin, T., Wilfong, G. “An analysis of BGP convergence properties.” Computer Communication Review, vol.29, (no.4), Oct. 1999. p.277-88.

    Griffin, T., Shepherd, F.B.; Wilfong, G. “Policy disputes in path-vector protocols.” Proceedings Seventh International Conference on Network Protocols (ICNP'99), 1999. p.21-30.

    Griffin, T., Wilfong, G. “A safe path vector protocol.” Proceedings of IEEE INFOCOM 2000. Piscataway, NJ, USA: IEEE, 2000. p.490-9 vol.2.

    Gao, L., Rexford, J. “Stable Internet routing without global coordination.” Performance Evaluation Review, vol.28, (no.1), June 2000. p.307-17.

    Stewart III, J. BGP-4: Inter-Domain Routing in the Internet. Reading, MA. Addison-Wesley, 1999.

    Black, U. IP Routing Protocols. Upper Saddle River, NJ. Prentice-Hall, 2000.

    Huitema, C. Routing in the Internet.2nd ed. Upper Saddle River, NJ. Prentice-Hall, 2000.

    Bgp traffic status

    BGP Traffic: Status

    Number of


    Received by a

    Speaker at


    3/03/00 to


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