Network Working Group M. Jork
Request for Comments: 5443 GENBAND
Category: Informational A. Atlas
Cisco Systems, Inc.
March 2009 LDP IGP Synchronization
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In certain networks, there is dependency on the edge-to-edge Label
Switched Paths (LSPs) setup by the Label Distribution Protocol (LDP),
e.g., networks that are used for Multiprotocol Label Switching (MPLS)
Virtual Private Network (VPN) applications. For such applications,
it is not possible to rely on Internet Protocol (IP) forwarding if
the MPLS LSP is not operating appropriately. Blackholing of labeled
traffic can occur in situations where the Interior Gateway Protocol
(IGP) is operational on a link on which LDP is not. While the link
could still be used for IP forwarding, it is not useful for MPLS
forwarding, for example, MPLS VPN applications or Border Gateway
Protocol (BGP) route-free cores. This document describes a mechanism
to avoid traffic loss due to this condition without introducing any
Table of Contents
1. Introduction ....................................................21.1. Conventions Used in This Document ..........................32. Proposed Solution ...............................................33. Applicability ...................................................44. Interaction with TE Tunnels .....................................55. Security Considerations .........................................56. References ......................................................66.1. Normative References .......................................66.2. Informative References .....................................67. Acknowledgments .................................................61. Introduction
LDP [RFC5036] establishes MPLS LSPs along the shortest path to a
destination as determined by IP forwarding. In a common network
design, LDP is used to provide Label Switched Paths throughout the
complete network domain covered by an IGP such as Open Shortest Path
First (OSPF) [RFC2328] or Intermediate System to Intermediate System
(IS-IS) [ISO.10589.1992]; i.e., all links in the domain have IGP as
well as LDP adjacencies.
A variety of services a network provider may want to deploy over an
LDP-enabled network depend on the availability of edge-to-edge label
switched paths. In a layer 2 (L2) or layer 3 (L3) VPN scenario, for
example, a given Provider-Edge (PE) router relies on the availability
of a complete MPLS forwarding path to the other PE routers for the
VPNs it serves. This means that all the links along the IP shortest
path from one PE router to the other need to have operational LDP
sessions, and the necessary label binding must have been exchanged
over those sessions. If only one link along the IP shortest path is
not covered by an LDP session, a blackhole exists and services
depending on MPLS forwarding will fail. This might be a transient or
a persistent error condition. Some of the reasons for this could be:
- A configuration error.
- An implementation bug.
- The link has just come up and has an IGP adjacency but LDP has
either not yet established an adjacency or session, or has not yet
distributed all the label bindings.
The LDP protocol has currently no way to correct the issue. LDP is
not a routing protocol; it cannot re-direct traffic to an alternate
1.1. Conventions Used in This Document
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].
2. Proposed Solution
The problem described above exists because LDP is tied to
IP-forwarding decisions but no coupling between the IGP and LDP
operational state on a given link exists. If IGP is operational on a
link but LDP is not, a potential network problem exists. So the
solution described by this document is to discourage a link from
being used for IP forwarding as long as LDP is not fully operational.
This has some similarity to the mechanism specified in [RFC3137],
which allows an OSPF router to advertise that it should not be used
as a transit router. One difference is that [RFC3137] raises the
link costs on all (stub) router links, while the mechanism described
here applies on a per-link basis.
In detail: when LDP is not "fully operational" (see below) on a given
link, the IGP will advertise the link with maximum cost to avoid any
transit traffic over it. In the case of OSPF, this cost is
LSInfinity (16-bit value 0xFFFF), as proposed in [RFC3137]. In the
case of ISIS, the maximum metric value is 2^24-2 (0xFFFFFE). Indeed,
if a link is configured with 2^24-1 (the maximum link metric per
[RFC5305]), then this link is not advertised in the topology. It is
important to keep the link in the topology to allow IP traffic to use
the link as a last resort in case of massive failure.
LDP is considered fully operational on a link when an LDP hello
adjacency exists on it, a suitable associated LDP session (matching
the LDP Identifier of the hello adjacency) is established to the peer
at the other end of the link, and all label bindings have been
exchanged over the session. At the present time, the latter
condition cannot generally be verified by a router and some
estimation may have to be used. A simple implementation strategy is
to use a configurable hold-down timer to allow LDP session
establishment before declaring LDP fully operational. The default
timer is not defined in this document due to concerns of the large
variations of the Label Information Base (LIB) table size and
equipment capabilities. In addition, there is a current work in
progress on LDP End-of-LIB as specified in [End-of-LIB], which
enables the LDP speaker to signal the completion of its initial
advertisement following session establishment. When LDP End-of-LIB
is implemented, the configurable hold-down timer is no longer needed.
The neighbor LDP session is considered fully operational when the
End-of-LIB notification message is received.
This is typically sufficient to deal with the link when it is being
brought up. LDP protocol extensions to indicate the complete
transmission of all currently available label bindings after a
session has come up are conceivable, but not addressed in this
The mechanism described in this document does not entail any protocol
changes and is a local implementation issue.
The problem space and solution specified in this document have also
been discussed in an IEEE Communications Magazine paper
In general, the proposed procedure is applicable in networks where
the availability of LDP-signaled MPLS LSPs and avoidance of
blackholes for MPLS traffic are more important than always choosing
an optimal path for IP-forwarded traffic. Note however that non-
optimal IP forwarding only occurs for a short time after a link comes
up or when there is a genuine problem on a link. In the latter case,
an implementation should issue network management alerts to report
the error condition and enable the operator to address it.
Example network scenarios that benefit from the mechanism described
here are MPLS VPNs and BGP-free core network designs where traffic
can only be forwarded through the core when LDP forwarding state is
The usefulness of this mechanism also depends on the availability of
alternate paths with sufficient bandwidth in the network should one
link be assigned to the maximum cost due to the unavailability of LDP
service over it.
On broadcast links with more than one IGP/LDP peer, the cost-out
procedure can only be applied to the link as a whole and not to an
individual peer. So a policy decision has to be made whether the
unavailability of LDP service to one peer should result in the
traffic being diverted away from all the peers on the link.
4. Interaction with TE Tunnels
In some networks, LDP is used in conjunction with RSVP-TE, which sets
up traffic-engineered tunnels. The path computation for the TE
tunnels is based on the TE link cost that is flooded by the IGP in
addition to the regular IP link cost. The mechanism described in
this document should only be applied to the IP link cost to prevent
unnecessary TE tunnel reroutes.
In order to establish LDP LSPs across a TE tunnel, a targeted LDP
session between the tunnel endpoints needs to exist. This presents a
problem very similar to the case of a regular LDP session over a link
(the case discussed so far): when the TE tunnel is used for IP
forwarding, the targeted LDP session needs to be operational to avoid
LDP connectivity problems. Again, raising the IP cost of the tunnel
while there is no operational LDP session will solve the problem.
When there is no IGP adjacency over the tunnel and the tunnel is not
advertised as a link into the IGP, this becomes a local issue of the
tunnel headend router.
5. Security Considerations
A Denial-of-Service (DoS) attack that brings down LDP service on a
link or prevents it from becoming operational on a link could be one
possible cause of LDP-related traffic blackholing. This document
does not address how to prevent LDP session failure. The mechanism
described here prevents the use of the link where LDP is not
operational while IGP is. Assigning the IGP cost to maximum on such
a link should not introduce new security threats. The operation is
internal to the router to allow LDP and IGP to communicate and react.
Making many LDP links unavailable, however, is a security threat that
can cause dropped traffic due to limited available network capacity.
This may be triggered by operational error or implementation error.
These errors are considered general security issues and implementors
should follow the current best security practice [MPLS-GMPLS-Sec].
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas,
Ed., "LDP Specification", RFC 5036, October 2007.
6.2. Informative References
[End-of-LIB] Asati, R., LDP End-of-LIB, Work in Progress, January
[ISO.10589.1992] International Organization for Standardization,
"Intermediate system to intermediate system intra-
domain-routing routine information exchange protocol
for use in conjunction with the protocol for
providing the connectionless-mode Network Service
(ISO 8473)", ISO Standard 10589, 1992.
[LDP-Fail-Rec] Fang, L., Atlas, A., Chiussi, F., Kompella, K., and
G. Swallow, "LDP Failure Detection and Recovery",
IEEE Communications Magazine, Vol.42, No.10, October
[MPLS-GMPLS-Sec] Fang. L., Ed., "Security Framework for MPLS and
GMPLS Networks", Work in Progress, November 2008.
[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC
3137, June 2001.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
The authors would like to thank Bruno Decraene for his in-depth
discussion and comments, Dave Ward for his helpful review and input,
and Loa Andersson, Ross Callon, Amanda Baber, Francis Dupont, Donald
Eastlake, Russ Housley, Pasi Eronen, Dan Romascanu, Bin Mo, Lan
Zheng, Bob Thomas, and Dave Ojemann for their reviews and comments.
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