How to Construct a Correct and Scalable iBGP Configuration

1
How to Construct a Correct and Scalable iBGP
Configuration

• Mythili Vutukuru
• Joint work with
• Paul Valiant, Swastik Kopparty and Hari
  Balakrishnan

2
BGP
eBGP and iBGP
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eBGP
iBGP
Border router/ Egress
Autonomous System (AS)
Route
3
Our contribution

• Status quo in configuring iBGP
• Full-mesh (not scalable)
• Route reflection (no correctness guarantees)
• New approach (BGPSep) to configure iBGP that is
  both correct and scalable
• Uses the results on graph separators from graph
  theory

4
Outline of the talk

• More background
• What is the status quo?
• What are the problems with it?
• Problem statement
• Our solution

5
Background - iBGP
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• iBGP sessions run on TCP
• Overlay over the intra-domain routing protocol or
  Interior Gateway Protocol (IGP) like OSPF
• Routing messages and data packets forwarded via
  IGP within AS
• iBGP Rule to avoid routing loops
• Routes from iBGP session not propagated to
  another iBGP session

iBGP
IGP
Route
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Approach1 Full-mesh iBGP
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B
A
D

• Every router has an iBGP session to every border
  router
• Large number of iBGP sessions not scalable

C
iBGP session
Route
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Scalable alternative Route reflection
Route reflector
B
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A

• Reflects routes to and from client iBGP
  sessions
• Avoids full-mesh
• Hierarchy of reflectors

D
C
Client iBGP session
Route
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Problems with route reflection 1

• Problem 1 Routers may not choose best route
• Why? Route reflector reflects only its best route

B
A
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Client session
D
Route
Data packets
C

• B chooses the sub-optimal route through C
• In full-mesh B would have chosen route through A
• Sub-optimal path assignments

Lower cost to egress
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Problem2 Forwarding loops
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To 18.0.0.1
IGP
To 18.0.0.1
Client iBGP session
Route
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IGP link
Data packets
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Background - Summary
iBGP configuration Correctness Scalability
Full-mesh v
Route reflection v
We need v v
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Outline of the talk

• More background
• What is the status quo?
• What are the problems with it?
• Problem statement
• Our solution

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Problem Statement

• Input IGP (IP-level connectivity) graph
• Output iBGP configuration
• Route reflectors and clients
• iBGP sessions
• Constraints
• Emulate full-mesh
• More scalable than full-mesh
• Previous work GW02 how to check for
  correctness, not how to construct correct
  configurations
• GW02 T. Griffin and G. Wilfong, On the
  Correctness of iBGP Configuration, In Proc. ACM
  SIGCOMM 2002, Pittsburg, PA, August 2002.

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Outline of the talk

• More background
• What is the status quo?
• What are the problems with it?
• Problem statement
• Our solution

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Key insight for emulating full-mesh

• For every BGP router P, every egress E
• P and E have iBGP session, OR
• P should be the client of a route reflector on
  the shortest path between P and E

A
R
C
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B
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Our solution BGPSep
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S

• S is graph separator
• Nodes in graph separator S are route reflectors
• u in G1 or G2, v in S u is a client of v
• Full-mesh in G1, G2 !!!
• Recurse on G1, G2
• Recursion terminates when component small enough

D
A
?
B
G2
G1
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Evaluation

• 2.5 to 5X fewer iBGP sessions on ISP topologies
  Source Rocketfuel

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Features of BGPSep

• Efficient implementation
• 100 lines of Matlab
• Runs in a few seconds
• Convenient to use
• Need not be re-run when link costs change or
  nodes/links fail
• Small number of top-level route reflectors
• Not incremental need to be re-run when
  nodes/links added

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Conclusion

• First algorithm to construct correct iBGP
  configurations with route reflection.
• Reduces iBGP sessions compared to full-mesh
• Efficient implementation
• Convenient and practical to use
• Questions?

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Questions?
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Best route selection

• BGP best route selection rules
• Local Pref
• AS path length
• MED
• IGP cost to egress
• Best route selected by route reflector might not
  be the best route for the client

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Route reflection

• 2 types of iBGP sessions
• Client iBGP session
• Normal (peer) iBGP session
• Route from client ? all clients and peers
• Route from peer ? all clients
• Multiple route reflectors
• Hierarchy of route reflectors

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Problems with route reflection

• Lack of complete visibility every router is not
  guaranteed to see its best available route.
• Forwarding loops
• Some router along the forwarding path chooses a
  different egress
• Packets do not make progress towards egress and
  loop forever
• Not robust to IGP changes
• IGP link failures trigger forwarding loops
• Full-mesh iBGP has none of these problems

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Key insight for emulating full-mesh

• For every router P, every egress E
• P and E have iBGP session, OR
• P should be the client of a route reflector on
  the shortest path between P and E

To R
R goto D
R
R
To R
To R
Client iBGP session
Route
R goto A
IGP link
Data packets