Internet DRAFT - draft-ietf-idr-best-external
draft-ietf-idr-best-external
Network Working Group P. Marques
Internet-Draft
Expires: July 6, 2012 R. Fernando
E. Chen
P. Mohapatra
Cisco Systems
H. Gredler
Juniper Networks
January 3, 2012
Advertisement of the best external route in BGP
draft-ietf-idr-best-external-05
Abstract
The current BGP-4 protocol specification [RFC4271] states that the
selection process chooses the best path for a given route which is
added to the Loc-Rib and advertised to all peers.
Previous versions [RFC1771] of the specification defined a different
rule for Internal BGP Updates. Given that Internal paths are not re-
advertised to Internal peers, it was specified that the best of the
external paths, as determined by the path selection tie breaking
algorithm, would be advertised to Internal peers.
This document extends that procedure to operate in environments where
Route Reflection [RFC4456] or Confederations [RFC5065] are used and
explains why advertising the additional routing information can
improve convergence time without causing routing loops.
Additional benefits include reduction of inter-domain churn and
avoidance of permanent route oscillation.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on July 6, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Generalization . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Algorithm for selection of the Adj-RIB-OUT path . . . . . . . 7
5. Advertisement Rules . . . . . . . . . . . . . . . . . . . . . 9
6. Consistency between routing and forwarding . . . . . . . . . . 10
7. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Fast Connectivity Restoration . . . . . . . . . . . . . . . . 13
9. Inter-Domain Churn Reduction . . . . . . . . . . . . . . . . . 14
10. Reducing Persistent IBGP oscillation . . . . . . . . . . . . . 15
11. Deployment Considerations . . . . . . . . . . . . . . . . . . 16
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
14. Security Considerations . . . . . . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
Earlier versions of the BGP-4 protocol specification [RFC1771]
prescribed different route advertisement rules for Internal and
External peers. While the overall best path would be advertised to
External peers, Internal peers are advertised the best of the
externally received paths.
This Internal advertisement rule was never implemented as specified
and was latter dropped from the protocol. There is a trade-off in
advertising the "best-external" route versus the behavior that became
common standard of not advertising the route when the selected best
path is received from an Internal peer. By not advertising
information in this case it is possible to reduce state both in the
local BGP speaker as well as in the network overall. Early BGP
implementations where very concerned with reducing state as they
where limited to relatively low memory footprints (e.g. 16 MB).
There is also the possible concern regarding advertising a path
different than the path that has been selected for forwarding.
However, advertising the best external route, when different from the
best route, presents additional information into an IBGP mesh which
may be of value for several purposes including:
o Faster restoration of connectivity. By providing additional
paths, that may be used to fail over in case the primary path
becomes invalid or is withdrawn.
o Reducing inter-domain churn and traffic black-holing due to the
readily available alternate path.
o Reducing the potential for situations of permanent IBGP route
oscillation [RFC3345].
o Improving selection of lower MED routes from the same neighboring
AS.
This document defines procedures to select the best external route
for each peer. It also describes how above benefits are realized
with best external route announcement with the help of certain
scenarios.
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2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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3. Generalization
The BGP-4 protocol [RFC1771] has extended with two alternative
mechanisms that provide ways to reduce the operational complexity of
route distribution within an AS: Route Reflection [RFC4456] and
Confederations [RFC5065]. It is important to be able to express
route advertisement rules in the context of both of these mechanisms.
When Route Reflection is used, Internal peers are further classified
depending of the reflection cluster they belong to. Non-client
internal peers form one BGP peering mesh. Each set of RR clients
with the same "cluster-id" configuration forms a separate mesh.
When selecting the path to add to the Adj-RIB-OUT, this document
specifies that the path that originate from the same mesh MAY be
excluded from consideration. This results in an Adj-RIB-OUT
selection per mesh (the set of non-client peers or a specific
cluster).
Similarly, when BGP Confederations are used, each confederated AS is
a BGP mesh. As with the Route Reflection scenario, when selecting
the path to add to the Adj-RIB-OUT, routes from the same mesh MAY be
excluded.
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4. Algorithm for selection of the Adj-RIB-OUT path
The objective of this protocol extension is to improve the quality of
the routing information known to a particular BGP mesh with minimum
additional cost in terms of processing and state.
Towards that goal, it is useful to define a total order between the
Adj-RIB-In routes which provides both the same overall best path as
the algorithm defined in the current BGP-4 specification [RFC4271] as
well as an ordering of alternate routes. Using this total order it
is then computationally efficient to select the path for a specific
Adj-RIB-OUT by excluding the routes that have been received from the
BGP mesh corresponding to the peer (or set of peers).
In order to achieve this, it is helpful to introduce the concept of
path group. A group is the set of paths that compare as equal
through all the steps prior to the MED comparison step (as defined in
section 9.1.2.2 of RFC 4271 [RFC4271] and have been received from the
same neighbor AS.
Paths are ordered within a group via MED or subsequent route
selection rules.
In pseudo-code:
function compare(path_1, path_2) {
cmp_result cmp = selection_steps_before_med(path_1, path_2);
if (cmp != cmp_result.equal) {
return cmp;
}
if (neighbor_as(path_1) == neighbor_as(path_2)) {
return selection_steps_after_med(path_1, path_2);
}
if (is_group_best(path_1)) {
if (!is_group_best(path_2)) {
return cmp_result.greater_than;
}
return selection_steps_after_med(path_1, path_2);
} else {
if (is_group_best(path_2)) {
return cmp_result.less_than;
}
/* Compare the best paths of respective groups */
return compare(group_best(path_1), group_best(path_2));
}
}
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As an example, the following set of received routes:
+------+----+-----+--------+
| Path | AS | MED | rtr_id |
+------+----+-----+--------+
| a | 1 | 10 | 10 |
| | | | |
| b | 2 | 5 | 1 |
| | | | |
| c | 1 | 5 | 5 |
| | | | |
| d | 2 | 20 | 20 |
| | | | |
| e | 2 | 30 | 30 |
| | | | |
| f | 3 | 10 | 20 |
+------+----+-----+--------+
Path Attribute Table
Would yield the following order (from the most to the least
preferred):
b < d < e < c < a < f
In this example, comparison of the best path within each group
provides the sequence (b < c < f). The remaining paths are ordered
in relation to their respective group best.
The first path in the ordering above is the best overall path for a
given NLRI. When selecting a path for a particular Adj-RIB-Out (or
set of RIB-Outs) an implementation MAY choose to select the first
path in the global order which was not received from the same BGP
mesh (as defined above) as the target peer (or peers).
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5. Advertisement Rules
1. When advertising a route to a non-client Internal peer, a BGP
speaker MAY choose to select the first path in order that did not
originate from the same BGP mesh (i.e. the set of non-client
Internal peers) whenever the best overall path has been received
from this mesh and would be suppressed by the Internal BGP non-
readvertisement rule.
2. When advertising a route to a Route Reflection client peer, in
case the overall best path has been received from the same
cluster, a BGP speaker MUST be able to advertise the best overall
path to all the members of the cluster other than the originator,
unless "client-to-client" reflection is disabled. The
implementation MAY choose to advertise an alternate path to the
specific peer that originates the best overall path by excluding
from consideration all paths with the same originator-id.
3. When "client-to-client" reflection is disabled and the cluster is
operating as a mesh, a Route Reflector MAY opt to advertise to
the cluster the preferred path from the set of paths not received
from the cluster. While this deployment mode is currently
uncommon, it can be a practical way to guarantee path diversity
inside the cluster.
4. A confederation border route MAY choose to advertise an alternate
path towards its Internal BGP mesh or towards a con-fed member AS
following the same procedure as defined above.
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6. Consistency between routing and forwarding
The internal update advertisement rules contained in the original
BGP-4 specification [RFC1771] can lead to situations where traffic is
forwarded through a route other than the route advertised by BGP.
Inconsistencies between forwarding and routing are highly
undesirable. Service providers use BGP with the dual objective of
learning reach-ability information and expressing policy over network
resources. The latter assumes that forwarding follows routing
information.
Consider the Autonomous system presented in figure 1, where r1 ... r4
are members of a single IBGP mesh and routes a, b, and c are received
from external peers.
AS 1 (c)
|
+----+ +----+
| r1 |...........| r2 |
+----+ +----+
.
.
.
.
.
.
+----+ +----+
| r3 |...........| r4 | --- ebgp --- AS X
+----+ +----+
/ \
/ \
AS 1 (a) AS 2 (b)
Inconsistency in Routing
+------+----+-----+--------+
| Path | AS | MED | rte_id |
+------+----+-----+--------+
| a | 1 | 10 | 1 |
| | | | |
| b | 2 | 5 | 10 |
| | | | |
| c | 1 | 5 | 5 |
+------+----+-----+--------+
Path Attribute Table
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Following the rules as specified in RFC 1771 [RFC1771], router r3
will select path (b) received from AS 2 as its overall best to
install in the Loc-Rib, since path (b) is preferable to path (c), the
lowest MED route from AS 1. However for the purposes of Internal
Update route selection, it will ignore the presence of path (c), and
elect (a) as its advertisement, via the router-id tie-breaking rule.
In this scenario, router r4 will receive (c) from r1 and (a) from r3.
It will pick the lowest MED route (c) and advertise it out via IBGP
to AS X. However at this point routing is inconsistent with
forwarding as traffic received from AS X will be forwarded towards AS
2, while the IBGP advertisement is being made for an AS 1 path.
Routing policies are typically specified in terms of neighboring
AS-es. In the situation above, assuming that AS 1 is network for
which this AS provides transit services while AS 2 and AS X are peer
networks, one can easily see how the inconsistency between routing
and forwarding would lead to transit being inadvertently provided
between AS X and AS 2. This could lead to persistent forwarding
loops.
Inconsistency between routing and forwarding may happen, whenever a
GP speaker chooses to advertise an external route into IBGP that is
different from the overall best route and its overall best is
external.
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7. Applications
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8. Fast Connectivity Restoration
When two exits are available to reach a particular destination and
one is preferred over the other, the availability of an alternate
path provides fast connectivity restoration when the primary path
fails.
Restoration can be quick since the alternate path is already at hand.
The border router could recompute the backup route and perinatal it
in FIB ready to be switched when the primary goes away. Note that
this requires the border router that's the backup to also perinatal
the secondary path and switch to it on failure.
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9. Inter-Domain Churn Reduction
Within an AS, the non availability of backup best leads to a border
router sending a withdraw upstream when the primary fails. This
leads to inter-domain churn and packet loss for the time the network
takes to converge to the alternate path. Having the alternate path
will reduces the churn and eliminates packet loss.
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10. Reducing Persistent IBGP oscillation
Advertising the best external route, according to the algorithm
described in this document will reduce the possibility of route
oscillation by introducing additional information into the IBGP
system.
For a permanent oscillation condition to occur, it is necessary that
a circular dependency between paths occurs such that the selection of
a new best path by a router, in response to a received IBGP
advertisement, causes the withdrawal of information that another
router depends on in order to generate the original event.
In vanilla BGP, when only the best overall route is advertised, as in
most implementations, oscillation can occur whenever there are 2 or
clusters/sub-AS-es such that at least one cluster has more than one
path that can potentially contribute to the dependency.
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11. Deployment Considerations
The mechanism specified in the draft allows a BGP speaker to
advertise a route that is not the best route used for forwarding.
This is a departure from the current behavior. However, consistency
in the path selection process across the AS is still guaranteed since
the ingress routers will not choose the best-external route as the
best route for a destination in steady state (for the same reason
that the BGP speaker announcing the best-external route chose an IBGP
route as best instead of the externally learnt route). Though it is
possible to alter this assurance by defining route policies on IBGP
sessions, use of such policies in IBGP is not recommended, especially
with best-external announcement turned on in the network. It is also
worth noting that such inconsistency in routing and forwarding is
mitigated in a tunneled network.
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12. Acknowledgments
This document greatly benefits from the comments of Yakov Rekhter,
John Scudder, Eric Rosen, Jenny Yuan, Jay Borkenhagen, Salkat Ray and
Jakob Heitz.
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13. IANA Considerations
This document has no actions for IANA.
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14. Security Considerations
There are no additional security risks introduced by this design.
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15. References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana,
"Border Gateway Protocol (BGP) Persistent Route
Oscillation Condition", RFC 3345, August 2002.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, April 2006.
[RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous
System Confederations for BGP", RFC 5065, August 2007.
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Authors' Addresses
Pedro Marques
Email: pedro.r.marques@gmail.com
Rex Fernando
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Email: rex@cisco.com
Enke Chen
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Email: enkechen@cisco.com
Pradosh Mohapatra
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
US
Email: pmohapat@cisco.com
Hannes Gredler
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
US
Email: hannes@juniper.net
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