Behavior
Prof. Routing Gao
Persistent Oscillation
ECE697A Fall 2003 Advanced Computer Networks
Outline
So far,
Persistent Oscillation due to EBGP
Persistent Oscillation due to IBGP
Routing oscillation in IBGP with route reflection Correctness of IBGP configuration Guidelines to ensure stability and correctness
Route Oscillation
Route oscillation can be caused by EBGP routing policy Can also be caused by IBGP routing policy
Focus on route reflection cases
With route reflection With confederation
Route Reflection
For scalability of IBGP
AS100
AS200
Case Study
A and B are route reflectors in AS0
1, 50, 40, 120 are IGP metrics of links Bold numbers are MED value
IBGP Oscillation
A uses r2, B uses r3 B tells A r3 A uses r1, B uses r3 A tells B r1 B uses r1 B tells A r1, implicitly withdraw r3 A uses r2 A tells B r2 B uses r3
Cause of Oscillation
Nondeterministic ranking among routes
MED comparisons only take place among routes which learned from the same neighboring AS The order of best path comparisons
Partial view of routes among Route Reflectors
r1 better than r3 (low IGP metric) r3 better than r2 (low MED value) r2 better than r1 (low IGP metric)
Solution
In practice
In theory
Set metric among inter RR clusters higher than intra RR cluster Do not accept MED or compare MED value among all routes Modify the IBGP bestpath selection and export route procedures IBGP router advertises a set of good exit paths to all its IBGP peers instead of a single best exit path
How to Define Good Exit Paths ?
Good exit paths are
So at each IBGP router
Paths with the highest LOCAL_PREF Paths with the lowest AS PATH length Paths with the minimum MED value from each AS Not just the single best path Has the same set of paths to choose from Chooses the same best path to destination
Another Example: Without MED
R0, R1 and R2 are route reflectors B0, B1 and B2 are clients in the cluster of dashed oval lines Dashed are IBGP sessions Solid lines are physical connections labeled with IGP weights
Another Example: Without MED
Oscillating without stable state
R0 prefers P1 over P0 R1 prefers P2 over P1 R2 prefers P0 over P2
Anomalies Due to Improper Configuration
Signaling anomalies
Even IBGP routers converge on a set of routes
IBGP routers never converge on a set of routes
Still have problem on data forwarding path
Forwarding Anomalies
P1 and P2 announce the same prefix Existing a forwarding loop between C1 and C2
C1 uses C1C2R1 route C2 uses C2C1R2 route
Correctness of IBGP Configuration
Signaling anomalies
Define the signaling digraph Gs = (V, As) Three sets of arc(u, v)
An arc in Gs represents an IBGP session
Guideline for the correctness of IBGP configuration
OVER: Basic IBGP session between u and v DOWN: from router reflector u to one of its clients v UP: from client u to a router reflector v
Sufficient Condition For Signaling Correctness
Similar to guidelines in EBGP stability
OVER edge vs. PeertoPeer edge DOWN edge vs. ProvidertoCustomer edge UP edge vs. CustomertoProvider edge
Sufficient Condition
All nodes u rank exit ro...