OSPF Routing with
Cross-Address Family Traffic Engineering Tunnels
Cisco Systems, Inc.
De Kleetlaan 6a
Diegem
1831
Belgium
as@cisco.com
Futurewei Technologies, Inc.
2330 Central Expressway
Santa Clara
CA
95050
United States of America
alvaro.retana@futurewei.com
michael_barnes@usa.net
Routing
LSR
OSPF
IPv4
IPv6
TE
MPLS
When using Traffic Engineering (TE) in a dual-stack IPv4/IPv6
network, the Multiprotocol Label Switching (MPLS) TE Label Switched
Path (LSP) infrastructure may be duplicated, even if the destination
IPv4 and IPv6 addresses belong to the same remote router.
In order to
achieve an integrated MPLS TE LSP infrastructure, OSPF routes must be
computed over MPLS TE tunnels created using information propagated in
another OSPF instance. This issue is solved by advertising cross-address
family (X-AF) OSPF TE information.
This document describes an update to RFC 5786 that allows for the easy
identification of a router's local X-AF IP addresses.
Introduction
TE extensions to OSPFv2
and OSPFv3 have been
described to support intra-area
TE in IPv4 and IPv6 networks, respectively. In both cases, the TE
database provides a tight coupling between the routed protocol and
advertised TE signaling information. In other words, any use of the TE
database is limited to IPv4 for OSPFv2
and IPv6 for OSPFv3 .
In a dual-stack network, it may be desirable to set up common MPLS TE
LSPs to carry traffic destined to addresses from different address
families on a router. The use of common LSPs eases potential scalability
and management concerns by halving the number of LSPs in the
network. Besides, it allows operators to group traffic based on
business characteristics, class of service, and/or applications;
the operators are not constrained by the network protocol used.
For example, an LSP created based on MPLS TE information propagated
by an OSPFv2 instance can be used to transport both IPv4 and IPv6
traffic, as opposed to using both OSPFv2 and OSPFv3 to provision a
separate LSP for each address family. Even if, in some cases, the address-family-specific traffic is to be separated, calculation from a common TE
database may prove to be operationally beneficial.
During the SPF calculation on the TE tunnel
head-end router, OSPF
computes shortcut routes using TE tunnels. A commonly used algorithm for
computing shortcuts is defined in . For that or
any similar algorithm to work with a common MPLS TE infrastructure in a
dual-stack network, a requirement is to reliably map the X-AF addresses
to the corresponding tail-end router. This mapping is a challenge
because the Link State Advertisements (LSAs) containing the routing
information are carried in one
OSPF instance, while the TE calculations may be done using a TE database
from a different OSPF instance.
A simple solution to this problem is to rely on the Router ID to
identify a node in the corresponding OSPFv2 and OSPFv3 Link State
Databases (LSDBs). This solution would mandate both instances on the
same router to be configured with the same Router ID. However, relying
on the correctness of configuration puts additional burden and cost on
the operation of the network. The network becomes even more difficult to
manage if OSPFv2 and OSPFv3 topologies do not match exactly, for example,
if area borders are chosen differently in the two protocols. Also, if
the routing processes do fall out of sync (e.g., having different Router
IDs for local administrative reasons), there is no defined way for other
routers to discover such misalignment and to take corrective measures
(such as to avoid routing traffic through affected TE tunnels or
alerting the network administrators). The use of misaligned Router IDs
may result in delivering the traffic to the wrong tail-end router, which
could lead to suboptimal routing or even traffic loops.
This document describes an update to that
allows for the easy identification of a router's local X-AF IP
addresses. defined the Node IPv4 Local Address
and Node IPv6 Local Address sub-TLVs of the Node Attribute TLV for a
router to advertise additional local IPv4 and IPv6 addresses. However,
did not describe the advertisement and usage of
these sub-TLVs when the address family of the advertised local address
differed from the address family of the OSPF traffic engineering
protocol.
This document updates so that a router can
also announce one or more local X-AF addresses using the corresponding
Local Address sub-TLV. Routers using the Node
Attribute TLV can include non-TE-enabled interface addresses in
their OSPF TE advertisements and also use the same sub-TLVs to carry
X-AF information, facilitating the mapping described above.
The method specified in this document can also be used to compute the
X-AF mapping of the egress Label Switching Router (LSR) for sub-LSPs of
a Point-to-Multipoint LSP . Considerations of
using Point-to-Multipoint MPLS TE for X-AF traffic forwarding is outside
the scope of this document.
Requirements Language
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are
to be interpreted as described in BCP 14
when, and only when, they appear in all capitals,
as shown here.
Operation
To implement the X-AF routing technique described in this document,
OSPFv2 will advertise the Node IPv6 Local Address sub-TLV and OSPFv3
will advertise the Node IPv4 Local Address sub-TLV, possibly in addition
to advertising other IP addresses as documented by .
Multiple instances of OSPFv3 are needed if it is used for both IPv4
and IPv6 . The operation in this section is
described with OSPFv2 as the protocol used for IPv4; that is the most
common case. The case of OSPFv3 being used for IPv4 follows the same
procedure as what is indicated for OSPFv2 below.
On a node that implements X-AF routing, each OSPF instance
advertises, using the Node Local Address sub-TLV, all X-AF IPv6 (for
OSPFv2 instance) or IPv4 (for OSPFv3) addresses local to the router that
can be used by the Constrained Shortest Path First (CSPF) to calculate MPLS TE LSPs:
- The OSPF instance MUST advertise the IP address listed in the Router
Address TLV of
the X-AF instance maintaining the TE database.
- The OSPF instance SHOULD include additional local addresses
advertised by the X-AF OSPF instance in its Node Local Address
sub-TLVs.
- An implementation MAY advertise other local X-AF addresses.
When TE information is advertised in an OSPF instance, both natively
(i.e., as per RFC or )
and as X-AF Node Attribute TLV, it is left to local configuration to
determine which TE database is used to compute routes for the OSPF
instance.
On Area Border Routers (ABRs), each advertised X-AF IP address MUST be
advertised into, at most, one area. If OSPFv2 and OSPFv3 ABRs coincide
(i.e., the areas for all OSPFv2 and OSPFv3 interfaces
are the same), then the X-AF addresses MUST be advertised into the same
area in both instances. This allows other ABRs connected to the same set
of areas to know with which area to associate computed MPLS TE
tunnels.
During the X-AF routing calculation, X-AF IP addresses are used to
map locally created LSPs to tail-end routers in the
LSDB. The mapping algorithm can be described as:
- Walk the list of all MPLS TE tunnels for which the computing
router is a head end. For each MPLS TE tunnel T:
-
- If T's destination address is from the same address family as the
OSPF instance associated with the LSDB, then the extensions defined
in this document do not apply.
- Otherwise, it is a X-AF MPLS TE tunnel. Note the tunnel's destination
IP address.
- Walk the X-AF IP addresses in the LSDBs of all connected areas.
If a matching IP address is found, advertised by router R in area A,
then mark the tunnel T as belonging to area A and terminating on
tail-end router R. Assign the intra-area SPF cost to reach router R
within area A as the IGP cost of tunnel T.
After completing this calculation, each TE tunnel is associated with
an area and tail-end router in terms of the routing LSDB of the
computing OSPF instance and has a cost.
The algorithm described above is to be used only if the Node Local
Address sub-TLV includes X-AF information.
Note that, for clarity of description, the mapping algorithm is
specified as a single calculation. Implementations may choose to support equivalent mapping
functionality without implementing the algorithm as described.
As an example, consider a router in a dual-stack network
using OSPFv2 and OSPFv3 for IPv4 and IPv6 routing, respectively. Suppose the OSPFv2
instance is used to propagate MPLS TE information and the router is
configured to accept TE LSPs terminating at local addresses 198.51.100.1
and 198.51.100.2. The router advertises in OSPFv2 the IPv4 address
198.51.100.1 in the Router Address TLV, the additional local IPv4
address 198.51.100.2 in the Node IPv4 Local Address sub-TLV, and other
TE TLVs as required by . If the
OSPFv3 instance in the network is enabled for X-AF TE routing (that is,
to use MPLS TE LSPs computed by OSPFv2 for IPv6 routing), then the
OSPFv3 instance of the router will advertise the Node IPv4 Local Address
sub-TLV listing the local IPv4 addresses 198.51.100.1 and 198.51.100.2.
Other routers in the OSPFv3 network will use this information to
reliably identify this router as the egress LSR for MPLS TE LSPs
terminating at either 198.51.100.1 or 198.51.100.2.
Backward Compatibility
Only routers that serve as endpoints for one or more TE tunnels MUST
be upgraded to support the procedures described herein:
- Tunnel tail-end routers advertise the Node IPv4 Local Address
sub-TLV and/or the Node IPv6 Local Address sub-TLV.
- Tunnel head-end routers perform the X-AF routing calculation.
Both the endpoints MUST be upgraded before the tail end starts
advertising the X-AF information. Other routers in the network do not
need to support X-AF procedures.
Automatically Switched Optical Networks
updates
by
defining extensions to be used in an Automatically Switched Optical
Network (ASON). The Local TE Router ID sub-TLV is required for
determining ASON reachability. The implication is that if the Local TE
Router ID sub-TLV is present in the Node Attribute TLV, then the
procedures in apply, regardless of whether
any X-AF information is advertised.
Security Considerations
This document describes the use of the Local Address sub-TLVs to
provide X-AF information. The advertisement of these sub-TLVs, in any
OSPF instance, is not precluded by . As such, no
new security threats are introduced beyond the considerations in OSPFv2, OSPFv3,
and .
The X-AF information is not used for SPF computation or normal
routing, so the mechanism specified here has no effect on IP routing.
However, generating incorrect information or tampering with the
sub-TLVs may have an effect on traffic engineering computations.
Specifically, TE traffic may be delivered to the wrong tail-end router,
which could lead to suboptimal routing, traffic loops, or exposing
the traffic to attacker inspection or modification. These threats are
already present in other TE-related specifications, and their
considerations apply here as well, including
and .
IANA Considerations
This document has no IANA actions.
References
Normative References
Informative References
Acknowledgements
The authors would like to thank Peter Psenak and Eric Osborne for
early discussions and Acee Lindem for discussing compatibility with ASON
extensions. Also, Eric Vyncke, Ben Kaduk, and Roman Danyliw provided
useful comments.
We would also like to thank the authors of RFC 5786 for laying down
the foundation for this work.