IDR Working Group Rajiv Asati
Internet Draft Cisco Systems
Intended status: Informational
Expires: March 2009
October 27, 2008
BGP Bestpath Selection Criteria
draft-asati-idr-bgp-bestpath-selection-criteria-00.txt
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Abstract
BGP specification [RFC4271] prescribes 'BGP next-hop reachability' as
one of the key 'Route Resolvability Condition' that must be satisfied
before the BGP bestpath candidate selection. This condition, however,
may not be sufficient (as explained in the Appendix section) and
desire further granularity.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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].
Table of Contents
1. Introduction...................................................2
2. Route Resolvability Condition - Modification...................3
3. Conclusions....................................................4
4. Security Considerations........................................4
5. IANA Considerations............................................4
6. Acknowledgments................................................4
7. Appendix.......................................................5
8. References.....................................................7
Author's Addresses................................................8
Intellectual Property Statement...................................8
Disclaimer of Validity............................................9
1. Introduction
As per BGP specification [RFC4271], when a router receives a BGP
path, BGP must qualify it as the valid candidate prior to the BGP
bestpath selection using the 'Route Resolvability Condition'
(section#9.1.2.1 of RFC4271]. After the path gets qualified as the
bestpath candidate, it becomes eligible to be the bestpath, and may
get advertised out to the neigbhor(s), if it became the bestpath.
However, in BGP networks that utilize data plane protocol other than
IP, such as MPLS etc. to forward the received traffic towards the
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next-hop, the above qualification condition may not be sufficient. In
fact, this may expose the BGP networks to experience traffic
blackholing i.e. traffic loss, due to malfunctioning of the chosen
data plane protocol to the next-hop. This is explained further in the
Appendix section.
This document defines further granularity to the "Route Resolvability
Condition" by (a) resolving the BGP next-hop reachability in the
forwarding database of a particular data plane protocol, and (b)
optionally including the BGP next-hop "path availability" check.
2. Route Resolvability Condition - Modification
This document proposes two amendments to 'Route Resolvability
Condition', which is defined in RFC4271, in consideration for a
particular data plane protocol.
1) The next-hop reachability SHOULD be resolved in a particular data
plane protocol.
For example, if a BGP IPv4/v6 or VPNv4/v6 path wants to use MPLS data
plane to the next-hop, as determined by the policy, then the BGP
'next-hop reachability' should be resolved using the MPLS data plane.
In another example, if BGP path wants to use the IP data plane to the
next-hop, as determined by the policy, then BGP 'next-hop
reachability' should be resolved using the IP data plane. The latter
example covers MPLS-in-IP encapsulation techniques such as [RFC4817],
[RFC4023] etc.
The selection of particular data plane is a matter of a policy, and
is outside the scope of this document. A dynamic signaling such as
draft-ietf-softwire-encaps-safi [ENCAP] may be used to convey the
data plane protocol chosen by the policy.
This check may be limited to confirming the availability of the valid
forwarding entry for the next-hop in the forwarding database of the
chosen data plane protocol.
2) The 'path availability' check for the BGP next-hop MAY be
performed. This criterion checks for the functioning path to the
next-hop in a particular data plane protocol.
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The path availability check may be performed by any of the OAM data-
plane liveness mechanisms associated with the data plane that is used
to reach the Next Hop. The data plane protocol for this criterion
must be the same as the one selected by the previous criterion (#1).
The mechanism(s) to perform the "path availability" check and the
selection of particular data plane are a matter of a policy and
outside the scope of this document.
For example, if a BGP VPNv4 path wants to use the MPLS as the data
plane protocol to the next-hop, then MPLS path availability to the
next-hop should be evaluated i.e. liveness of MPLS LSP to the next-
hop should be validated.
3. Conclusions
Both amendments discussed in section 2 provide further clarity and
granularity to help the BGP speaker to either continue to advertise a
BGP path's reachability or withdraw the BGP path's reachability,
based on the consideration for the path's next-hop reachability
and/or availability in a particular data plane.
4. Security Considerations
This draft doesn't impose any additional security constraints.
5. IANA Considerations
None.
6. Acknowledgments
Yakov Rekhter provided critical suggestions and feedback to improve
this document. Thanks to John Scudder and Chandrashekhar Appanna for
contributing to the discussions that formed the basis of this
document. Thanks to Ilya Varlashkin, who made the case to revive this
document and provided useful feedback.
This document was prepared using 2-Word-v2.0.template.dot.
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7. Appendix
7.1. Problem Applicability
In IP networks using BGP, a router would continue to attract traffic
by advertising the BGP prefix reachability to neighbor(s) as long as
the router had a route to the next-hop in its routing table, but
independent of whether the router has a functional forwarding path to
the next-hop. This may cause the forwarded traffic to be dropped
inside the IP network.
In MPLS or MPLS VPN networks [RFC4364], the same problem is observed
if the functional MPLS LSP to the next-hop is not available (due to
the forwarding path error on any node along the path to the next-
hop).
The following MPLS/VPN topology clarifies the problem -
<-eBGP/IGP-> <-------MP-BGP------> <-eBGP/IGP->
CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~
^
======PE1-PE2 LSP==> ^
^
a.b.c.d
Figure 1 MPLS VPN Network
In the network illustrated in Figure 1, the PE1 to PE2 LSP may be
non-functional due to any reason such as corrupted MPLS Forwarding
Table entry, or the missing MPLS Forwarding table entry, or LDP
binding defect, or down LDP session between the P routers (with
independent label distribution control) etc. In such a situation, it
is clear that the CE1->CE2 traffic inserted into the MPLS network by
PE1 will get dropped inside the MPLS network.
It is undesirable to have PE1 continue to convey to the CE1 router
that PE1 (and the MPLS network) is still the next-hop for the remote
VPN reachability, without being sure of the corresponding LSP health.
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7.1.1. Multi-Homed VPN Site
If the remote VPN site is dual-homed to both PE2 and PE3, then PE1
may learn two VPNv4 paths to the prefix a.b.c.d. via PE2 and PE3
routers, as shown below in Figure 2. PE1 may select the bestpath for
the prefix a.b.c.d via PE2 (say, for which the PE1->PE2 LSP is mal-
functioning) and advertise that bestpath to CE1 in the context of
figure 2.
<------MP-BGP------>
CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~
\ / ^
\~~~~~~~~~~PE3~~~~~~~/ ^
^
a.b.c.d
Figure 2 MPLS VPN Network - CE2 Dual-Homing
This causes CE1 to likely send the traffic destined to prefix a.b.c.d
to the PE1 router, which forwards the traffic over the malfunctioning
LSP to PE2. It is clear that this MPLS encapsulated VPN traffic ends
up getting dropped or blackholed somewhere inside the MPLS network.
It is desirable to force PE1 to select an alternate bestpath via that
next-hop (such as PE3), whose LSP is correctly functioning.
7.1.2. Single-Homed VPN Site with Site-to-Site Backup Connectivity
The local VPN site may have a backup/dial-up link available at the CE
router, but the backup link will not even be activated as long as the
CE's routing table continues to point to the PE router as the next-
hop (over the MPLS/VPN network).
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<------MP-BGP------>
CE1~~~~~~~~~PE1~~~MPLS Network~~~PE2~~~~~~~~CE2~~
\ / ^
\~~~~~~~~~~~~~~backup path~~~~~~~~~~~~~~/ ^
^
a.b.c.d
Figure 3 MPLS VPN Network - CE1-CE2 Backup connection
Unless PE2 withdraws the route via the routing protocol used on the
PE-CE link, CE1 will not be able to activate the backup link (barring
any tracking functionality) to the remote VPN site.
In summary, if PE1 could appropriately qualify the BGP VPNv4
bestpath, then the VPN traffic outage could likely be avoided. Even
if the VPN site was not multi-homed, it is desirable to force PE1 to
withdraw the path from CE1 to improve the CE-to-CE convergence. This
document proposes a mechanism to achieve the optimal BGP behavior at
PE.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4364] Rosen E. and Rekhter Y., "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC4364, February 2006.
[RFC4271] Rekhter, Y., Li T., and Hares S.(editors), "A Border
Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006
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8.2. Informative References
[ENCAP] Rosen, E., Mohapatra, P., "BGP Encapsulation SAFI and BGP
Tunnel Encapsulation Attribute", draft-ietf-softwire-
encaps-safi-03.txt, work in progress.
[RFC4023] Rosen, et al., "Encapsulating MPLS in IP or Generic Routing
Encapsulation", RFC4023, March 2005.
[RFC4817] Townsley, et al., "Encapsulation of MPLS over Layer 2
Tunneling Protocol Version 3", RFC4817, Nov 2006.
Author's Addresses
Rajiv Asati
Cisco Systems
7025 Kit Creek Road
RTP, NC 27560 USA
Email: rajiva@cisco.com
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