Deterministic Route Redistribution
into BGP
Enke Chen ([email protected])
Jenny Yuan ([email protected])
Introduction

Routing protocols download routes to RIB

RIB uses the “administrative distance” to break ties
between routes from different protocols, and then
program the forwarding engine

BGP uses the LOCAL_PREF, AS_PATH, MED, IGP
metric for route selection

Currently the two route selection algorithms are
independent of each other

Non-deterministic routing behavior can occur involving
route redistribution to BGP by RIB

Route redistribution into BGP is common
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The Problem

A is the primary connection, and B the backup.

X is statically routed on R1, R2, and redistributed into
BGP. On R2, X is configured with the least preferred
admin distance.
R3
IBGP
R1
R2
A
B
CUST
X
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The Problem (cont.)

There are potentially two BGP paths for X on R2

One received from R1 via IBGP

One locally redistributed

With the default setting, the ordering of the arrival of the
paths impacts the routing behavior!

When the IBGP path arrives before the locally configured
static route is redistributed into BGP




Then the IBGP path will be downloaded to RIB
RIB will select the IBGP path as the active path due to the
admin distance
The locally configured static route will not be
redistributed
R1, R2 and R3 will converge to the primary path
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The Problem (cont.)

When the locally configured static route is redistributed
into the BGP before the IBGP path arrives


Then the locally redistributed path will be selected as
the BGP best path (due to IGP metric)

It will then be advertised to IBGP peers

R2, and possibly R3 will use the backup path!
Additional route-specific BGP configuration can be
added during route redistribution to eliminate the nondeterministic routing behavior

Expensive operationally.
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The Proposed Solution

Use the “admin distance” in BGP route selection




Add it as the first step in the route selection
Favor the path with a more preferred admin distance
when comparing a locally redistributed path with a
path received from a peer
For a locally redistributed path, its admin distance is
inherited from the RIB
For a path from a peer, its admin distance is the
same value as the admin distance assigned to the
path for route download
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The Benefits

Eliminates the non-deterministic routing
behavior

Avoids the extra BGP configuration

Very low computational overhead

Backward compatible
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Reference & Revision

draft-chen-bgp-redist-00.txt

Future revisions (as suggested by Curtis)


Expand on “admin distance”
Re-title: Use Admin Distance in BGP Route
Selection
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