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A Whirlwind Tour of Interdomain Routing. Aaron Wagner [email protected] 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

A Whirlwind Tour of Interdomain Routing

Aaron Wagner

[email protected]

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.

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