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This Internet Draft describes a new well-known BGP community, LAST_RESORT.
This community provides a simple and easily deployable solution to BGP "wedgies".
Initial deployment is expected to be achieved by voluntary use in the network operator community-at-large.
Long-term adoption via software enforcement of the well-known aspect of the community, will improve global behavior, and simplify router configurations.
This Internet Draft is intended to result in this draft or a related draft(s) being placed on the Standards Track for idr.
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
[RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
Intended Status: Proposed Standard.
Even when all the best current practises are observed, operational problems may be experienced when running a BGP network.
One particularly thorny problem is BGP "wedgies" (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [RFC4264].
While not often articulated, the common problem is use of local policy setting within AS boundaries, which often overrides the original intent of "backup" BGP announcements.
It is somewhat ironic that such local policies are often achieved by use of BGP Communities, and that the lack of a common choice, i.e. "well known", community, is one source of the problem.
The BGP "wedgie" problem occurs as a result of "intended policy" information not being available. Specifically, this is the lack of an explicit global mechanism for expressing de-preferring announcements via "back-up" providers. In essence, local policy could be described as "not well informed".
To solve the problem, a new BGP well-known Community, LAST_RESORT, is proposed. This is a new value to be assigned by IANA.
A BGP speaker receiving a prefix from an EBGP neighbor with LAST_RESORT MUST assignment the lowest-possible preference value of LOCAL_PREFERENCE. This LOCAL_PREFERENCE MUST NOT be able to be over-ridden without first removing the LAST_RESORT community from the prefix. A BGP speaker receiving a prefix from an IBGP neighbor with LAST_RESORT MUST ignore the LAST_RESORT community.
The distinction between EBGP and IBGP ensures backwards compatibility, particularly when a heterogeneous set of routers in an AS doing IBGP exists.
The restriction preventing overriding LAST_RESORT ensures that local policy mechanisms do not interfere with LAST_RESORT. This also facilitates ease in deployment, as most router configurations would not require modification.
The main reason for establishing the LAST_RESORT Community is to permit the global implementation of actual "backup only" announcements.
It is not to facilitate change of policies, or to circumvent local policies. Rather, it is to make possible the implementation of policies where those have been negotiated by two or more parties.
Currently, there are several documented scenarios in the "Wedgies" RFC (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [RFC4264] where the mutually desired policy is either unable to be implemented, or does not deterministically reach the desired state.
Application of the LAST_RESORT Community on announcements sent to a backup provider, permits these problems to be resolved.
The same prefix is announced to both the primary and backup provider.
When announced to the primary provider, the LAST_RESORT Community is NOT set.
When announced to the backup provider, the LAST_RESORT Community IS set.
The propagation of the LAST_RESORT instance will be limited by the availability of paths, and inhibited by the existence of paths which do not have LAST_RESORT applied to them.
In Figure 1 (of Appendix A (BGP Wedgie Examples)), the LAST_RESORT instance will be seen by the backup provider, and be passed with LAST_RESORT to the backup provider's transit provider.
The latter will prefer any other instance without LAST_RESORT, even if it has policy for applying a LOCAL_PREFERENCE to the received prefix instances.
Should the other instance be withdrawn, the LAST_RESORT will be selected and subsequently propagated.
No additional security considerations beyond those already present in BGP are introduced.
IANA will need to assign a new code points for BGP Well-Known Communities for LAST_RESORT.
The author wishes to acknowledge the helpful guidance of Joe Abley and Tony Li. The author also wishes to acknowledge the assistance and suggestions of Joel M. Halpern in simplifying the original "Backup-only" concept to that of a BGP community, and of Olivier Bonaventure in clarifying the LOCAL_PREFERENCE mechanisms.
| [RFC4264] | Griffin, T. and G. Huston, “BGP Wedgies,” RFC 4264, November 2005 (TXT). |
| [RFC4271] | Rekhter, Y., Li, T., and S. Hares, “A Border Gateway Protocol 4 (BGP-4),” RFC 4271, January 2006 (TXT). |
| [RFC1997] | Chandrasekeran, R., Traina, P., and T. Li, “BGP Communities Attribute,” RFC 1997, August 1996 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
The following examples from RFC 4264 (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [RFC4264] show the effects of the proposed changes, in resolving "wedgie" issues.
+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |
+----+ |
|AS 2| |
+----+ |
|provider |
| |
|customer |customer
+-------+ +----------+
backup| |primary
+----+
|AS 1|
+----+
| Figure 1 |
This state will be reached regardless of sequence of disconnects and reconnects.
+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |customer
+----+ +----+
|AS 2| |AS 5|
+----+ +----+
|provider |provider
| |
|customer |customer
+-------+ +----------+
backup| |primary for 192.9.200.0/25
primary| |backup for 192.9.200.128/25
+----+
|AS 1|
+----+
| Figure 2 |
+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
||provider |providerS
|+-----------+ |
|customer |customer |
+----+ +----+ |
|AS 2|-------|AS 5| |
+----+ peer +----+ |
|provider |provider |
| | |
|customer +-+customer |customer
+-------+ |+----------+
backup| ||primary
+----+
|AS 1|
+----+
| Figure 3 |
Any announcement that AS 3 hears from AS 2 will always be marked LAST_RESORT. (The same will be true of AS 5.) Thus, any combination of break/restore on any links in any order, will always result in the desired state being reached.
| Brian Dickson | |
| Afilias Canada, Inc | |
| 4141 Yonge St, | |
| Suite 204 | |
| North York, ON M2P 2A8 | |
| Canada | |
| Email: | brian.peter.dickson@gmail.com |
| URI: | www.afilias.info |
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