Network Working Group D. McPherson

Network Working Group D. McPherson
Request for Comments: 3345 TCB
Category: Informational V. Gill
AOL Time Warner, Inc.
D. Walton
A. Retana
Cisco Systems, Inc.
August 2002

Border Gateway Protocol (BGP) Persistent Route Oscillation Condition

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

In particular configurations, the BGP scaling mechanisms defined in
"BGP Route Reflection - An Alternative to Full Mesh IBGP" and
"Autonomous System Confederations for BGP" will introduce persistent
BGP route oscillation. This document discusses the two types of
persistent route oscillation that have been identified, describes
when these conditions will occur, and provides some network design
guidelines to avoid introducing such occurrences.

1. Introduction

The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol. The primary function of a BGP speaking system is
to exchange network reachability information with other BGP systems.

In particular configurations, the BGP [1] scaling mechanisms defined
in "BGP Route Reflection - An Alternative to Full Mesh IBGP" [2] and
"Autonomous System Confederations for BGP" [3] will introduce
persistent BGP route oscillation.

The problem is inherent in the way BGP works: locally defined routing
policies may conflict globally, and certain types of conflicts can
cause persistent oscillation of the protocol. Given current
practices, we happen to see the problem manifest itself in the
context of MED + route reflectors or confederations.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

The current specification of BGP-4 [4] states that the
MULTI_EXIT_DISC is only comparable between routes learned from the
same neighboring AS. This limitation is consistent with the
description of the attribute: "The MULTI_EXIT_DISC attribute may be
used on external (inter-AS) links to discriminate among multiple exit
or entry points to the same neighboring AS." [1,4]

In a full mesh iBGP network, all the internal routers have complete
visibility of the available exit points into a neighboring AS. The
comparison of the MULTI_EXIT_DISC for only some paths is not a
problem.

Because of the scalability implications of a full mesh iBGP network,
two alternatives have been standardized: route reflectors [2] and AS
confederations [3]. Both alternatives describe methods by which
route distribution may be achieved without a full iBGP mesh in an AS.

The route reflector alternative defines the ability to re-advertise
(reflect) iBGP-learned routes to other iBGP peers once the best path
is selected [2]. AS Confederations specify the operation of a
collection of autonomous systems under a common administration as a
single entity (i.e. from the outside, the internal topology and the
existence of separate autonomous systems are not visible). In both
cases, the reduction of the iBGP full mesh results in the fact that
not all the BGP speakers in the AS have complete visibility of the
available exit points into a neighboring AS. In fact, the visibility
may be partial and inconsistent depending on the location (and
function) of the router in the AS.

In certain topologies involving either route reflectors or
confederations (detailed description later in this document), the
partial visibility of the available exit points into a neighboring AS
may result in an inconsistent best path selection decision as the
routers don't have all the relevant information. If the
inconsistencies span more than one peering router, they may result in
a persistent route oscillation. The best path selection rules
applied in this document are consistent with the current
specification [4].

The persistent route oscillation behavior is deterministic and can be
avoided by employing some rudimentary BGP network design principles
until protocol enhancements resolve the problem.

In the following sections a taxonomy of the types of oscillations is
presented and a description of the set of conditions that will
trigger route oscillations is given. We continue by providing
several network design alternatives that remove the potential of this
occurrence.

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It is the intent of the authors that this document serve to increase
operator awareness of the problem, as well as to trigger discussion
and subsequent proposals for potential protocol enhancements that
remove the possibility of this to occur.

The oscillations are classified into Type I and Type II depending
upon the criteria documented below.

2. Discussion of Type I Churn

In the following two subsections we provide configurations under
which Type I Churn will occur. We begin with a discussion of the
problem when using Route Reflection, and then discuss the problem as
it relates to AS Confederations.

In general, Type I Churn occurs only when BOTH of the following
conditions are met:

1) a single-level Route Reflection or AS Confederations design is
used in the network AND

2) the network accepts the BGP MULTI_EXIT_DISC (MED) attribute
from two or more ASs for a single prefix and the MED values are
unique.

It is also possible for the non-deterministic ordering of paths to
cause the route oscillation problem. [1] does not specify that paths
should be ordered based on MEDs but it has been proven that non-
deterministic ordering can lead to loops and inconsistent routing
decisions. Most vendors have either implemented deterministic
ordering as default behavior, or provide a knob that permits the
operator to configure the router to order paths in a deterministic
manner based on MEDs.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

2.1. Route Reflection and Type I Churn

We now discuss Type I oscillation as it relates to Route Reflection.
To begin, consider the topology depicted in Figure 1:

---------------------------------------------------------------
/ -------------------- -------------------- \
| / \ / \ |
| | Cluster 1 | | Cluster 2 | |
| | | | | |
| | | *1 | | |
| | Ra(RR) . . . . . . . . . . . . . . Rd(RR) | |
| | . . | | . | |
| | .*5 .*4 | | .*12 | |
| | . . | | . | |
| | Rb(C) Rc(C) | | Re(C) | |
| | . . | | . | |
| \ . . / \ . / |
| ---.------------.--- ---------.---------- |
\ .(10) .(1) AS1 .(0) /
-------.------------.---------------------------.--------------
. . .
------ . ------------ .
/ \ . / \ .
| AS10 | | AS6 |
\ / \ /
------ ------------
. .
. .
. --------------
. / \
| AS100 |- 10.0.0.0/8
\ /
--------------

Figure 1: Example Route Reflection Topology

In Figure 1 AS1 contains two Route Reflector Clusters, Clusters 1 and
2. Each Cluster contains one Route Reflector (RR) (i.e., Ra and Rd,
respectively). An associated 'RR' in parentheses represents each RR.
Cluster 1 contains two RR Clients (Rb and Rc), and Cluster 2 contains
one RR Client (Re). An associated 'C' in parentheses indicates RR
Client status. The dotted lines are used to represent BGP peering
sessions.

The number contained in parentheses on the AS1 EBGP peering sessions
represents the MED value advertised by the peer to be associated with
the 10.0.0.0/8 network reachability advertisement.

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The number following each '*' on the IBGP peering sessions represents
the additive IGP metrics that are to be associated with the BGP
NEXT_HOP attribute for the concerned route. For example, the Ra IGP
metric value associated with a NEXT_HOP learned via Rb would be 5;
while the metric value associated with the NEXT_HOP learned via Re
would be 13.

Table 1 depicts the 10.0.0.0/8 route attributes as seen by routers
Rb, Rc and Re, respectively. Note that the IGP metrics in Figure 1
are only of concern when advertising the route to an IBGP peer.

Router MED AS_PATH
--------------------
Rb 10 10 100
Rc 1 6 100
Re 0 6 100

Table 1: Route Attribute Table

For the following steps 1 through 5, the best path will be marked
with a '*'.

1) Ra has the following installed in its BGP table, with the path
learned via AS2 marked best:

NEXT_HOP
AS_PATH MED IGP Cost
-----------------------
6 100 1 4
* 10 100 10 5

The '10 100' route should not be marked as best, though this is
not the cause of the persistent route oscillation. Ra realizes
it has the wrong route marked as best since the '6 100' path
has a lower IGP metric. As such, Ra makes this change and
advertises an UPDATE message to its neighbors to let them know
that it now considers the '6 100, 1, 4' route as best.

2) Rd receives the UPDATE from Ra, which leaves Rd with the
following installed in its BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-----------------------
* 6 100 0 12
6 100 1 5

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Rd then marks the '6 100, 0, 12' route as best because it has a
lower MED. Rd sends an UPDATE message to its neighbors to let
them know that this is the best route.

3) Ra receives the UPDATE message from Rd and now has the
following in its BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-----------------------
6 100 0 13
6 100 1 4
* 10 100 10 5

The first route (6 100, 0, 13) beats the second route (6 100,
1, 4) because of a lower MED. Then the third route (10 100,
10, 5) beats the first route because of lower IGP metric to
NEXT_HOP. Ra sends an UPDATE message to its peers informing
them of the new best route.

4) Rd receives the UPDATE message from Ra, which leaves Rd with
the following BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-----------------------
6 100 0 12
* 10 100 10 6

Rd selects the '10 100, 10, 6' path as best because of the IGP
metric. Rd sends an UPDATE/withdraw to its peers letting them
know this is the best route.

5) Ra receives the UPDATE message from Rd, which leaves Ra with
the following BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-----------------------
6 100 1 4
* 10 100 10 5

Ra received an UPDATE/withdraw for '6 100, 0, 13', which
changes what is considered the best route for Ra. This is why
Ra has the '10 100, 10, 5' route selected as best in Step 1,
even though '6 100, 1, 4' is actually better.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

At this point, we've made a full loop and are back at Step 1. The
router realizes it is using the incorrect best path, and repeats
the cycle. This is an example of Type I Churn when using Route
Reflection.

2.2. AS Confederations and Type I Churn

Now we provide an example of Type I Churn occurring with AS
Confederations. To begin, consider the topology depicted in Figure
2:

---------------------------------------------------------------
/ -------------------- -------------------- \
| / \ / \ |
| | Sub-AS 65000 | | Sub-AS 65001 | |
| | | | | |
| | | *1 | | |
| | Ra . . . . . . . . . . . . . . . . . Rd | |
| | . . | | . | |
| | .*3 .*2 | | .*6 | |
| | . . | | . | |
| | Rb . . . . . Rc | | Re | |
| | . *5 . | | . | |
| \ . . / \ . / |
| ---.------------.--- ---------.---------- |
\ .(10) .(1) AS1 .(0) /
-------.------------.---------------------------.--------------
. . .
------ . ------------ .
/ \ . / \ .
| AS10 | | AS6 |
\ / \ /
------ ------------
. .
. .
. --------------
. / \
| AS100 |- 10.0.0.0/8
\ /
--------------

Figure 2: Example AS Confederations Topology

The number contained in parentheses on each AS1 EBGP peering session
represents the MED value advertised by the peer to be associated with
the 10.0.0.0/8 network reachability advertisement.

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The number following each '*' on the IBGP peering sessions represents
the additive IGP metrics that are to be associated with the BGP
NEXT_HOP attribute for the concerned route.

For example, the Ra IGP metric value associated with a NEXT_HOP
learned via Rb would be 3; while the metric value associated with the
NEXT_HOP learned via Re would be 6.

Table 2 depicts the 10.0.0.0/8 route attributes as seen by routers
Rb, Rc and Re, respectively. Note that the IGP metrics in Figure 2
are only of concern when advertising the route to an IBGP peer.

Router MED AS_PATH
--------------------
Rb 10 10 100
Rc 1 6 100
Re 0 6 100

Table 2: Route Attribute Table

For the following steps 1 through 6 the best route will be marked
with an '*'.

1) Ra has the following BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
(65001) 6 100 0 7
6 100 1 2

The '10 100' route is selected as best and is advertised to Rd,
though this is not the cause of the persistent route
oscillation.

2) Rd has the following in its BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
6 100 0 6
* (65000) 10 100 10 4

The '(65000) 10 100' route is selected as best because it has
the lowest IGP metric. As a result, Rd sends an
UPDATE/withdraw to Ra for the '6 100' route that it had
previously advertised.

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3) Ra receives the withdraw from Rd. Ra now has the following in
its BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
6 100 1 2

Ra received a withdraw for '(65001) 6 100', which changes what
is considered the best route for Ra. Ra does not compute the
best path for a prefix unless its best route was withdrawn.
This is why Ra has the '10 100, 10, 3' route selected as best,
even though the '6 100, 1, 2' route is better.

4) Ra's periodic BGP scanner runs and realizes that the '6 100'
route is better because of the lower IGP metric. Ra sends an
UPDATE/withdraw to Rd for the '10 100' route since Ra is now
using the '6 100' path as its best route.

Ra's BGP table looks like this:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
10 100 10 3
* 6 100 1 2

5) Rd receives the UPDATE from Ra and now has the following in its
BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
(65000) 6 100 1 3
* 6 100 0 6

Rd selects the '6 100, 0, 6' route as best because of the lower
MED value. Rd sends an UPDATE message to Ra, reporting that '6
100, 0, 6' is now the best route.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

6) Ra receives the UPDATE from Rd. Ra now has the following in
its BGP table:

NEXT_HOP
AS_PATH MED IGP Cost
-------------------------------
* 10 100 10 3
(65001) 6 100 0 7
6 100 1 2

At this point we have made a full cycle and are back to step 1.
This is an example of Type I Churn with AS Confederations.

2.3. Potential Workarounds for Type I Churn

There are a number of alternatives that can be employed to avoid this
problem:

1) When using Route Reflection make sure that the inter-Cluster
links have a higher IGP metric than the intra-Cluster links.
This is the preferred choice when using Route Reflection. Had
the inter-Cluster IGP metrics been much larger than the intra-
Cluster IGP metrics, the above would not have occurred.

2) When using AS Confederations ensure that the inter-Sub-AS links
have a higher IGP metric than the intra-Sub-AS links. This is
the preferred option when using AS Confederations. Had the
inter-Sub-AS IGP metrics been much larger than the intra-Sub-AS
IGP metrics, the above would not have occurred.

3) Do not accept MEDs from peers (this may not be a feasible
alternative).

4) Utilize other BGP attributes higher in the decision process so
that the BGP decision algorithm never reaches the MED step. As
using this completely overrides MEDs, Option 3 may make more
sense.

5) Always compare BGP MEDs, regardless of whether or not they were
obtained from a single AS. This is probably a bad idea since
MEDs may be derived in a number of ways, and are typically done
so as a matter of operator-specific policy. As such, comparing
MED values for a single prefix learned from multiple ASs is
ill-advised. Of course, this mostly defeats the purpose of
MEDs, and as such, Option 3 may be a more viable alternative.

6) Use a full IBGP mesh. This is not a feasible solution for ASs
with a large number of BGP speakers.

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3. Discussion of Type II Churn

In the following subsection we provide configurations under which
Type II Churn will occur when using AS Confederations. For the sake
of brevity, we avoid similar discussion of the occurrence when using
Route Reflection.

In general, Type II churn occurs only when BOTH of the following
conditions are met:

1) More than one tier of Route Reflection or Sub-ASs is used in
the network AND

2) the network accepts the BGP MULTI_EXIT_DISC (MED) attribute
from two or more ASs for a single prefix and the MED values are
unique.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

3.1. AS Confederations and Type II Churn

Let's now examine the occurrence of Type II Churn as it relates to AS
Confederations. Figure 3 provides our sample topology:

---------------------------------------------------------------
/ ------------------- \
| AS 1 / Sub-AS 65500 \ |
| | | |
| | Rc . . . . Rd | |
| | . *2 . | |
| \ . . / |
| .-----------------. |
| .*40 .*40 |
| --------------.----- --.----------------- |
| / . \ / . \ |
| | Sub-AS . | | . Sub-AS | |
| | 65501 . | | . 65502 | |
| | Rb | | Re | |
| | . | | . . | |
| | .*10 | | *2. .*3 | |
| | . | | . . | |
| | Ra | | . Rg . . . Rf | |
| \ . / . . / |
| ----------.---------- . -------------.------- |
\ .(0) .(1) .() /
----------------.---------------.-------------------.----------

. . .
--------- . ---------
|AS 200 | |AS 300 |
--------- ---------
. .
. .
-------------------
| AS 400 | - 10.0.0.0/8
-------------------

Figure 3: Example AS Confederations Topology

In Figure 3 AS 1 contains three Sub-ASs, 65500, 65501 and 65502. No
RR is used within the Sub-AS, and as such, all routers within each
Sub-AS are fully meshed. Ra and Rb are members of Sub-AS 65501. Rc
and Rd are members of Sub-AS 65500. Ra and Rg are EBGP peering with
AS 200, router Rf has an EBGP peering with AS 300. AS 200 and AS 300
provide transit for AS 400, and in particular, the 10/8 network. The
dotted lines are used to represent BGP peering sessions.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

The number following each '*' on the BGP peering sessions represents
the additive IGP metrics that are to be associated with the BGP
NEXT_HOP. The number contained in parentheses on each AS 1 EBGP
peering session represents the MED value advertised by the peer to be
associated with the network reachability advertisement (10.0.0.0/8).

Rc, Rd and Re are the primary routers involved in the churn, and as
such, will be the only BGP tables that we will monitor step by step.

For the following steps 1 through 8 each router's best route will be
marked with a '*'.

1) Re receives the AS 400 10.0.0.0/8 route advertisement via AS
200 from Rg and AS 300 from Rf. Re selects the path via Rg and
AS 200 because of IGP metric (Re didn't consider MED because
the advertisements were received from different ASs).

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Re * 200 400 1 2
300 400 3

Re sends an UPDATE message to Rd advertising its new best path
'200 400, 1'.

2) The '200 400, 0' path was advertised from Ra to Rb, and then
from Rb to Rc. Rd learns the '200 400, 1' path from Re.

NEXT_HOP
Router AS_PATH MED IGP Cost
-------------------------------
Rc * 200 400 0 50
Rd * 200 400 1 42
Re 300 400 3
* 200 400 1 2

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3) Rc and Rd advertise their best paths to each other; Rd selects
'200 400, 0' because of the MED.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc * 200 400 0 50
200 400 1 44
Rd * 200 400 0 52
200 400 1 42
Re 300 400 3
* 200 400 1 2

Rd has a new best path so it sends an UPDATE to to Re,
announcing the new path and an UPDATE/withdraw for '200 400, 1'
to Rc.

4) Re now selects '300 400' (with no MED) because '200 400, 0'
beats '200 400, 1' based on MED and '300 400' beats '200 400,
0' because of IGP metric.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc * 200 400 0 50
Rd * 200 400 0 52
200 400 1 42
Re * 300 400 3
200 400 0 92

Re has a new best path and sends an UPDATE to Rd for '300 400'.

5) Rd selects the '300 400' path because of IGP metric.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc * 200 400 0 50
Rd 200 400 0 52
* 300 400 43
Re * 300 400 3
200 400 0 92
200 400 1 2

Rd has a new best path so it sends an UPDATE to Rc and a
UPDATE/withdraw to Re for '200 400, 0'.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

6) Rc selects '300 400' because of the IGP metric. Re selects
'200 400, 1' because of the IGP metric.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc 200 400 0 50
* 300 400 45
Rd 200 400 0 52
* 300 400 43
Re 300 400 3
* 200 400 1 2

Rc sends an UPDATE/withdraw for '200 400, 0' to Rd. Re sends
an UPDATE for '200 400, 1' to Rd.

7) Rd selects '200 400, 1' as its new best path based on the IGP
metric.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc 200 400 0 50
* 300 400 45
Rd * 200 400 1 42
Re 300 400 3
* 200 400 1 2

Rd sends an UPDATE to Rc, announcing '200 400, 1' and
implicitly withdraws '300 400'.

8) Rc selects '200 400, 0'.

NEXT_HOP
Router AS_PATH MED IGP Cost
------------------------------
Rc * 200 400 0 50
200 400 1 44
Rd * 200 400 1 42
Re 300 400 3
* 200 400 1 2

At this point we are back to Step 2 and are in a loop.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

3.2. Potential Workarounds for Type II Churn

1) Do not accept MEDs from peers (this may not be a feasible
alternative).

2) Utilize other BGP attributes higher in the decision process so
that the BGP decision algorithm selects a single AS before it
reaches the MED step. For example, if local-pref were set based
on the advertising AS, then you first eliminate all routes except
those in a single AS. In the example, router Re would pick either
X or Y based on your local-pref and never change selections.

This leaves two simple workarounds for the two types of problems.

Type I: Make inter-cluster or inter-sub-AS link metrics higher
than intra-cluster or intra-sub-AS metrics.

Type II: Make route selections based on local-pref assigned to the
advertising AS first and then use IGP cost and MED to make
selection among routes from the same AS.

Note that this requires per-prefix policies, as well as near
intimate knowledge of other networks by the network operator. The
authors are not aware of ANY [large] provider today that performs
per-prefix policies on routes learned from peers. Implicitly
removing this dynamic portion of route selection does not appear
to be a viable option in today's networks. The main point is that
an available workaround using local-pref so that no two AS's
advertise a given prefix at the same local-pref solves type II
churn.

3) Always compare BGP MEDs, regardless of whether or not they were
obtained from a single AS. This is probably a bad idea since MEDs
may be derived in a number of ways, and are typically done so as a
matter of operator-specific policy and largely a function of
available metric space provided by the employed IGP. As such,
comparing MED values for a single prefix learned from multiple ASs
is ill-advised. This mostly defeats the purpose of MEDs; Option 1
may be a more viable alternative.

4) Do not use more than one tier of Route Reflection or Sub-ASs in
the network. The risk of route oscillation should be considered
when designing networks that might use a multi-tiered routing
isolation architecture.

5) In a RR topology, mesh the clients. For confederations, mesh the
border routers at each level in the hierarchy. In Figure 3, for
example, if Rb and Re are peers, then there's no churn.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

4. Future Work

It should be stated that protocol enhancements regarding this problem
must be pursued. Imposing network design requirements, such as those
outlined above, are clearly an unreasonable long-term solution.
Problems such as this should not occur under 'default' protocol
configurations.

5. Security Considerations

This discussion introduces no new security concerns to BGP or other
specifications referenced in this document.

6. Acknowledgments

The authors would like to thank Curtis Villamizar, Tim Griffin, John
Scudder, Ron Da Silva, Jeffrey Haas and Bill Fenner.

7. References

[1] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
1771, March 1995.

[2] Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection - An
Alternative to Full Mesh IBGP", RFC 2796, April 2000.

[3] Traina, P., McPherson, D. and J. Scudder, J., "Autonomous System
Confederations for BGP", RFC 3065, February 2001.

[4] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
Work in Progress.

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

8. Authors' Addresses

Danny McPherson
TCB
EMail: [email protected]

Vijay Gill
AOL Time Warner, Inc.
12100 Sunrise Valley Drive
Reston, VA 20191
EMail: [email protected]

Daniel Walton
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
EMail: [email protected]

Alvaro Retana
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
EMail: [email protected]

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RFC 3345 BGP Persistent Route Oscillation Condition August 2002

9. Full Copyright Statement

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Acknowledgement

Funding for the RFC Editor function is currently provided by the
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