----------------
                  /                \
                 /                  \
                /                    \
   +------+   +----+    Access     +----+
   |eNodeB|---|CSG1|    Ring 1     |ASG1|-------------
   +------+   +----+               +----+             \
                \                    /                 \
                 \                  /                   +----+    +---+
                  \             +----+                  |RSG1|----|RNC|
                   -------------|    |    Aggregate     +----+    +---+
                                |ASG2|      Ring          |
                   -------------|    |                  +----+    +---+
                  /             +----+                  |RSG2|----|RNC|
                 /                  \                   +----+    +---+
                /                    \                 /
   +------+   +----+     Access     +----+            /
   |eNodeB|---|CSG2|     Ring 2     |ASG3|------------
   +------+   +----+                +----+
               \                     /
                \                   /
                 \                 /
                  -----------------

         

Figure 2: Mobile Backhaul Network

This section lists several examples of how implementations might use the Node administrative tags. These examples are given only to demonstrate generic usefulness of the router tagging mechanism. Implementation supporting this specification is not required to implement any of the use cases. It is also worth noting that in some described use cases routers configured to advertise tags help other routers in their calculations but do not themselves implement the same functionality.

1. Service auto-discovery
   
   Router tagging may be used to automatically discover group of routers sharing a particular service.
   
   For example, service provider might desire to establish full mesh of MPLS TE tunnels between all PE routers in the area of MPLS VPN network. Marking all PE routers with a tag and configuring devices with a policy to create MPLS TE tunnels to all other devices advertising this tag will automate maintenance of the full mesh. When new PE router is added to the area, all other PE devices will open TE tunnels to it without the need of reconfiguring them.
2. Fast-Rerouting policy
   
   Increased deployment of Loop Free Alternates (LFA) as defined in [RFC5286] poses operation and management challenges. [I-D.litkowski-rtgwg-lfa-manageability] proposes policies which, when implemented, will ease LFA operation concerns.
   
   One of the proposed refinements is to be able to group the nodes in IGP domain with administrative tags and engineer the LFA based on configured policies.

   (a)

   Administrative limitation of LFA scope
   
   Service provider access infrastructure is frequently designed in layered approach with each layer of devices serving different purposes and thus having different hardware capabilities and configured software features. When LFA repair paths are being computed, it may be desirable to exclude devices from being considered as LFA candidates based on their layer.
   
   For example, if the access infrastructure is divided into the Access, Distribution and Core layers it may be desirable for a Distribution device to compute LFA only via Distribution or Core devices but not via Access devices. This may be due to features enabled on Access routers; due to capacity limitations or due to the security requirements. Managing such a policy via configuration of the router computing LFA is cumbersome and error prone.
   
   With the Node administrative tags it is possible to assign a tag to each layer and implement LFA policy of computing LFA repair paths only via neighbors which advertise the Core or Distribution tag. This requires minimal per-node configuration and network automatically adapts when new links or routers are added.

   (b)

   LFA calculation optimization
   
   Calculation of LFA paths may require significant resources of the router. One execution of Dijkstra algorithm is required for each neighbor eligible to become next hop of repair paths. Thus a router with a few hundreds of neighbors may need to execute the algorithm hundreds of times before the best (or even valid) repair path is found. Manually excluding from the calculation neighbors which are known to provide no valid LFA (such as single-connected routers) may significantly reduce number of Dijkstra algorithm runs.
   
   LFA calculation policy may be configured so that routers advertising certain tag value are excluded from LFA calculation even if they are otherwise suitable.

3. Controlling Remote LFA tunnel termination
   
   [I-D.ietf-rtgwg-remote-lfa] proposed method of tunneling traffic after connected link failure to extend the basic LFA coverage and algorithm to find tunnel tail-end routers fitting LFA requirement. In most cases proposed algorithm finds more than one candidate tail-end router. In real life network it may be desirable to exclude some nodes from the list of candidates based on the local policy. This may be either due to known limitations of the node (the router does accept targeted LDP sessions required to implement Remote LFA tunneling) or due to administrative requirements (for example, it may be desirable to choose tail-end router among co-located devices).
   
   The Node administrative tag delivers simple and scalable solution. Remote LFA can be configured with a policy to accept during the tail-end router calculation as candidates only routers advertising certain tag. Tagging routers allows to both exclude nodes not capable of serving as Remote LFA tunnel tail-ends and to define a region from which tail-end router must be selected.
4. Mobile backhaul network service deployment
   
   The topology of mobile backhaul network usually adopts ring topology to save fiber resource and it is divided into the aggregate network and the access network. Cell Site Gateways(CSGs) connects the eNodeBs and RNC(Radio Network Controller) Site Gateways(RSGs)connects the RNCs. The mobile traffic is transported from CSGs to RSGs. The network takes a typical aggregate traffic model that more than one access rings will attach to one pair of aggregate site gateways(ASGs) and more than one aggregate rings will attach to one pair of RSGs.
   
   
   
   A typical mobile backhaul network with access rings and aggregate links is shown in figure above. The mobile backhaul networks deploy traffic engineering due to the strict Service Level Agreements(SLA). The TE paths may have additional constraints to avoid passing via different access rings or to get completely disjoint backup TE paths. The mobile backhaul networks towards the access side change frequently due to the growing mobile traffic and addition of new eNodeBs. It's complex to satisfy the requirements using cost, link color or explicit path configurations. The node administrative tag defined in this document can be effectively used to solve the problem for mobile backhaul networks. The nodes in different rings can be assigned with specific tags. TE path computation can be enhanced to consider additional constraints based on node administrative tags.

6. Security Considerations

This document does not introduce any further security issues other than those discussed in [RFC2328] and [RFC5340].

7. IANA Considerations

IANA maintains the registry for the TLVs. OSPF Administrative Tags will require one new type code for the TLV defined in this document.

8. Acknowledgments

Thanks to Bharath R and Pushpasis Sarakar for useful inputs. Thanks to Chris Bowers for providing useful inputs to remove ambiguity related to tag-ordering.

9. References

9.1. 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 1998.
[RFC3630]	Katz, D., Kompella, K. and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, September 2003.
[RFC4970]	Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 4970, July 2007.
[RFC5340]	Coltun, R., Ferguson, D., Moy, J. and A. Lindem, "OSPF for IPv6", RFC 5340, July 2008.

9.2. Informative References

[I-D.ietf-rtgwg-remote-lfa]	Bryant, S., Filsfils, C., Previdi, S., Shand, M. and S. Ning, "Remote LFA FRR", Internet-Draft draft-ietf-rtgwg-remote-lfa-02, May 2013.
[I-D.litkowski-rtgwg-lfa-manageability]	Litkowski, S., Decraene, B., Filsfils, C. and K. Raza, "Operational management of Loop Free Alternates", Internet-Draft draft-litkowski-rtgwg-lfa-manageability-01, February 2013.
[RFC5286]	Atlas, A. and A. Zinin, "Basic Specification for IP Fast Reroute: Loop-Free Alternates", RFC 5286, September 2008.

Authors' Addresses