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
R
eBGP
iBGP
Border routers/ Egress
BGP routers
Autonomous System (AS)
Internal routers
Route
3
Our contribution

• Status quo in configuring iBGP
• Full-mesh (not scalable)
• Route reflection (no correctness guarantees)
• Problems with both approaches
• New approach to configure iBGP that is both
  correct and scalable
  
• Uses elegant results 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
R

• iBGP sessions run on TCP
• Overlay over the intra-domain routing protocol
  (IGP) like OSPF
  
• Routing messages and data packets forwarded via
  IGP within AS
  
• Routes from iBGP session not propagated to
  another iBGP session

iBGP
IGP
Route
6
Approach1 Full-mesh iBGP
R
B
A
F
E
D

• Every router has an iBGP session to every border
  router
  
• Not scalable

C
iBGP session
Route
7
Approach2 Route reflection
R
Route reflector
B
A
F
E

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

D
C
Client iBGP session
Route
8
Problems with route reflection 1

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

B
A
F
E
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

Lower cost to egress
9
Problem2 Forwarding loops
R goto A
To R
IGP
To R
Client iBGP session
R goto D
Route
IGP link
Data packets
10
Background - Summary
11
Outline of the talk

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

12
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.
  

13
Outline of the talk

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

14
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
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Our solution
R
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

D
A
?
B
G2
G1
R
16
Evaluation

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

17
Conclusion

• First algorithm to construct correct iBGP
  configurations with route reflection.
  
• Efficient implementation
• 2.5 to 10X fewer iBGP sessions compared to
  full-mesh iBGP
  

18
Questions?
19
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