Understanding Route
Redistribution
ICNP 2007
October 17th, 2007
Franck Le, Geoffrey G. Xie, Hui Zhang
1
Internetwork and Routing
• Common view:
– Intra-domain routing using OSPF, RIP
– Inter-domain routing using BGP
• In reality, internetworking is much more complex
– ISP networks:
• OSPF routes to be redistributed into BGP (and vice versa)
– Enterprise networks:
• When BGP is not used, needs mechanism to distribute
routes among OSPF, RIP, EIGRP domains
• Also, needs to distribute routes among multiple OSPF
domains
2
What is Route Re-Distribution (RR)?
By default, OSPF routers have no visibility of RIP routers
RIP
B
A
OSPF
D
C
E
Office branch 1
Office branch 2
RIP OSPF Local
FIB
router ospf 27
redistribute rip metric
200 subnets route-map
rip2ospf
distance ospf external
200
!
route-map rip2ospf permit
100
match ip address 100
set tag 22
set metric-type-1
3
How Does RR Compare to BGP?
• In many scenarios, RR, not BGP, is used to interconnect
network domains,
• Even when BGP is used, RR is required to connect BGP
and IGP
• RR can implement policy, like BGP
• Unlike BGP, RR is NOT a protocol
– RR is just a configuration mechanism, used
separately at each router
RR is more commonly used than BGP, but much
less understood, and much more error-prone
4
Problem Statements
• Given an internetwork with RR
configurations, what are the loop-free and
convergence properties?
• What are the guidelines of using RR if one
wants to have loop-free and convergent
internetwork?
5
Synthesis of the Paper
• Model that reasons about the loop-free and
convergence properties
• Sufficient condition to guarantee loop-free
and convergence properties
6
Outline
1.
2.
3.
4.
Introduction to Route Redistribution (RR)
Illustration of routing anomalies
A Model for RR
Sufficient condition for loop-free and
convergent RR
7
Route Selection Process
Office branch 1
Office branch 2
RIP
OSPF
P
D
B
P
E
A
RIP
C
OSPF
FIB
Local
P
Signaling
Data path
8
Route Selection Process
Office branch 1
Office branch 2
RIP
OSPF
D
B
P
A
P
P
E
C
P
Selected routing process
RIP
OSPF
Local
120
110
0/1
FIB
P
Signaling
Data path
9
Route Redistribution Process
Office branch 1
Office branch 2
RIP
OSPF
B
D
A
P
C
E
RIP Update
RIP
OSPF
Local
120
110
0/1
FIB
P
Signaling
Data path
10
Outline
1.
2.
3.
4.
Introduction to Route Redistribution (RR)
Illustration of routing anomalies
A Model for RR
Sufficient condition for loop-free and
convergent RR
11
Instabilities
• Wide range of possible routing instabilities
• No general guideline to configure RR
12
Formation of Routing Loops
RIP(120)
Next-hop: D P
B
D
OSPF Local
FIB
P
Next-hop: C
E
P
C
A
Next-hop: B
P
RIP
Next-hop: EOSPF(110)
RIP
OSPF Local
Signaling
Data path
FIB
13
Outline
1.
2.
3.
4.
Introduction to Route Redistribution (RR)
Illustration of routing anomalies
A Model for RR
Sufficient condition for loop-free and
convergent RR
14
Challenges
• Too many network elements
– Hundreds or thousands of routers
• Different router processing order
– Routers may process signaling messages in
different order (message delay, router load)
– Different order can result in different outcome
15
Solutions
• Too many network elements
– Abstractions: routing instances
– Logics: route selection, RR, network-wide RR
• Different router processing order
– Activation sequence1
1
L. Gao and J. Rexford, Stable Internet Routing Without Global Coordination,
in Proc. ACM SIGMETRICS, 2000
16
A Model for RR
• Abstracts the dynamic exchange of routing
information for a prefix P
• Allows to predict paths
17
Route Propagation Graph
• Routing instance
2
(110)
• Originating routing instance
1
(120)
• Configured redistribution
• Actual redistribution
• Route vs. no route
1
(120)
80, A, 90
2
(110)
1
(120)
80, A, 90
2
(110)
1
(120)
80, A, 90
2
(110)
• Variables: CL, S
18
Illustration of Model
B
C
D
E
A
F
G
I
H
J
P
K
OSPF1
RIP
0
Local
(0)
N
M
L
A
OSPF2
RIP
F
1
RIP
(120)
F
E
E
L
2
OSPF1
(110)
3
RIP
(120)
L
H
4
OSPF2
(110)
H
19
Illustration of Model
Sequence 1
0
Local
(0)
F
1
RIP
(120)
A
L
2
OSPF1
(110)
F
L
E
Signaling
H
4
OSPF2
(110)
E
Data path
S(t=1) = {A}
CL(t=0) = {A}
S(t=6) = {A, F}
CL(t=6) = { }
S(t=2) = {F}
CL(t=1) = {E, F}
H
S(t=3) = {L}
CL(t=2) = {E, L}
S(t=5) = {E}
CL(t=5) = {A, F}
3
RIP
(120)
CL(t=3) = {E, H}
S(t=4) = {H}
CL(t=4) = {E}
20
Route Redistribution Configuration Cycle Detection (RRC-CD) Problem
• Given a RR configuration, determining
whether there is an activation sequence
such that the redistributions converge to
state including a cycle of active
redistributions is NP-hard
21
Outline
1.
2.
3.
4.
Introduction to Route Redistribution (RR)
Illustration of routing anomalies
A Model for RR
Sufficient condition for loop-free and
convergent RR
22
Sufficient condition for safety
•
Pruning of Route Propagation Graph
– For each redistributing router, only conserve
redistributions from the routing processes
with lowest administrative distances
•
Rationale
– Focus on preferred redistributions
1
(100)
A
2
(70)
A
3
(120)
A
4
(90)
23
Sufficient condition
If resulting graph satisfies
1. Every redistributing router redistributes from a
single routing instance
(predictable outcome)
2. For all vertice, there is a redistribution path from a
originating vertex
(active redistribution)
3. The graph is acyclic
(no cycle)
Then, the redistributions converge to an acyclic
routing state
 No route oscillations
 No forwarding loops
24
Application of Sufficient
Condition
0
Local
(0)
A
F
1
RIP
(120)
F
E
E
L
2
OSPF1
(110)
3
RIP
(120)
L
H
4
OSPF2
(110)
H
25
Application of Sufficient
Condition
0
Local
(0)
A
80, F
1
RIP
(120)
F, 80
80, E
E, 80
L
2
OSPF1
(110)
3
RIP
(120)
L
H
4
OSPF2
(110)
H
Modifications
26
Application of Sufficient
Condition
0
Local
(0)
A
80, F
1
RIP
(120)
F, 80
80, E
E, 80
L
2
OSPF1
(110)
3
RIP
(120)
L
H
4
OSPF2
(110)
H
Pruning
27
Application of Sufficient
Condition
0
Local
(0)
A
1
RIP
(120)
80, F
2
OSPF1
(110)
L
3
RIP
(120)
80, E
4
OSPF2
(110)
H
Pruning
28
Application of Sufficient
Condition
0
Local
(0)
A
1
RIP
(120)
80, F
2
OSPF1
(110)
L
3
RIP
(120)
80, E
4
OSPF2
(110)
1. Every redistributing router is redistributing
from a single routing instance.
2. For all vertice, there is a redistribution path
from a originating vertex.
3. The graph is acyclic.
H
29
Summary
• Internetwork is far more complex with RR
than the conceptual model of BGP/OSPF
• RR serves a fundamental need, but is not
well-understood or even well-designed
• First formal study route-free and
convergence properties of RR
internetwork
– Model
– Sufficient condition
30
Future Work
• If one were to re-design the RR, what
should be the solution that supports all the
RR applications but avoid the pitfalls?
31