OSPF Link Overload

When a node is being prepared for a planned maintenance or upgrade, provides mechanisms to advertise the node being in an overload state by setting all outgoing link costs to MAX-METRIC (0xffff). These procedures are specific to the maintenance activity on a node and cannot be used when a single link attached to the node requires maintenance. In traffic-engineering deployments, LSPs need to be moved away from the link without disrupting the services. It is useful to be able to advertise the impending maintenance activity on the link and to have LSP re-routing policies at the ingress to route the LSPs away from the link. Many OSPFv2 or OSPFv3 deployments run on overlay networks provisioned by means of pseudo-wires or L2-circuits. When the devices in the underlying network go for maintenance, it is useful to divert the traffic away from the node before the maintenance is actually scheduled. Since the nodes in the underlying network are not visible to OSPF, the existing stub router mechanism described in cannot be used. Application specific to this use case is described in This document provides mechanisms to advertise link overload state in the flexible encodings provided by RI LSA( ) for OSPFv2 and OSPFv3. Throughout this document, OSPF is used when the text applies to both OSPFv2 and OSPFv3. OSPFv2 or OSPFv3 is used when the text is specific to one version of the OSPF protocol.

The motivation of this document is to reduce manual intervention during maintenance activities. The following objectives help to accomplish this in a range of deployment scenarios. Advertise impending maintenance activity so that the traffic from both directions can be diverted away from the link. Allow the solution to be backward compatible so that nodes that do not understand the new advertisement do not cause routing loops. Advertise the maintenance activity to other nodes in the network so that LSP ingress routers/controllers can learn the impending maintenance activity and apply specific policies to re-route the LSP for traffic-engineering based deployments. Allow the link to be used as last resort link to prevent traffic disruption when alternate paths are not available.

The Link Overload sub-TLV is defined as below. This sub-TLV is attached to the Link TLV and carried in RI LSA for OSPFv2 and OSPFv3

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type : TBA (suggested value 35) Length: 0

The link overload information can be flood in area scoped RI LSA or link scoped RI LSA or both based on the need of the application. describes applications requiring area scope as well as link scope Link-overload information. The Link TLV and the link-overload sub-tlv MAY appear in any instance of the RI-LSA.

For OSPFv2, Link overload Sub-TLV is carried in the Link TLV as defined in . Link TLV is carried in area scoped RI LSA . When there are more than one parallel links between two nodes, the link carrying link-overload information, need to be uniquely identified among the parallel links. Remote interface IP address sub-tlv as defined by is also carried in the Link TLV which is used by the remote nodes to uniquely identify the overloaded link. For OSPFv3, Link-overload sub-tlv is carried in Link TLV as defined in Link TLV is carried in the OSPFv3 area scoped RI-LSA

The link local scope RI-LSA corresponds to the link on which the LSA arrives and there is no need to explicitly carry the link TLV. The Link overload sub-TLV is carried in the RI-LSA for both OSPFv2 and OSPFv3.

The Link Overload sub-TLV indicates that the link identified in which it is carried is overloaded. The node that has the link to be taken out of service SHOULD originate the Link Overload sub-TLV in the Link TLV in the RI LSA as defined in . The link-overload information is carried as a property of the link and is flooded across the area. This information can be used by ingress routers or controllers to take special actions. Application specific to this use case is described in . The precise action taken by the remote node at the other end of the link identified as overloaded depends on the link type.

The node that has the link to be taken out of service SHOULD set metric of the link to MAX-METRIC (0xffff) and re- originate the Router-LSA. The TE metric SHOULD be set to MAX-TE-METRIC-1 (0xfffffffe) and the node SHOULD re-originate the TE Link Opaque LSAs. When a Link Overload sub-TLV is received for a point-to-point link either by link local or area scoped RI-LSA, the remote node SHOULD identify the local link which corresponds to the overloaded link and set the metric to MAX-METRIC (0xffff). The remote node MUST re-originate the router-LSA with the changed metric and flood into the OSPF area. The TE metric SHOULD be set to MAX-TE-METRIC-1 (0xfffffffe) and the TE opaque LSA for the link MUST be re-originated with new value. In multi-topology deployments , the Link overload Sub-TLV carried in an RI LSA corresponds to all the topologies the link belongs to. The receiver node SHOULD change the metric in the reverse direction corresponding to all the topologies to which the reverse link belongs. When the originator of the Link Overload sub-TLV purges the RI-LSA or re-originates it without the Link Overload sub-TLV, the remote node must re-originate the appropriate LSAs with the metric and TE metric values set to their original values.

Broadcast or NBMA networks in OSPF are represented by a star topology where the Designated Router (DR) is the central point to which all other routers on the broadcast or NBMA network connect logically. As a result, routers on the broadcast or NBMA network advertise only their adjacency to the DR. Routers that do not act as DR do not form or advertise adjacencies with each other. For the Broadcast links, the MAX-METRIC on the remote link cannot be changed since all the neighbours are on same link. Setting the link cost to MAX-METRIC would impact paths going via all neighbours. The node that has the link to be taken out of service SHOULD set metric of the link to MAX-METRIC (0xffff) and re-originate the Router-LSA. The TE metric SHOULD be set to MAX-TE-METRIC-1(0xfffffffe) and the node SHOULD re-originate the TE Link Opaque LSAs. For a broadcast link, the two part metric as described in is used. The node originating the Link Overload sub-TLV MUST set the metric in the Network-to-Router Metric sub-TLV to MAX-METRIC 0xffff for OSPFv2 and OSPFv3 and re-originate the LSAs the TLV is carried-in. The nodes that receive the two part metric should follow the procedures described in . The backward compatibility procedures described in should be followed to ensure loop free routing.

Operation for the point-to-multipoint links is similar to the point-to-point links. When a Link Overload sub-TLV is received for a point-to-multipoint link the remote node SHOULD identify the link which corresponds to the overloaded link and set the metric to MAX-METRIC (0xffff). The remote node MUST re-originate the Router-LSA with the changed metric and flood into the OSPF area.

Unnumbered interface do not have a unique IP addresses and borrow address from other interfaces. The Link TLV carries the local and remote interface ids to uniquely identify the link when there are more than one parallel links between the nodes. Procedures to obtain interface-id of the remote side is defined in and are applicable to the Link TLV added in the RI LSA for the purpose of carrying the Link overload sub-tlv.

The mechanism described in the document is fully backward compatible.It is required that the originator of the Link Overload sub-TLV as well as the node at the remote end of the link identified as overloaded understand the extensions defined in this document. In the case of broadcast links, the backward compatibility procedures as described in are applicable. .

---------PE3----------------PE4---------- | | | | CE1---------PE1----------------PE2---------CE2 | | | | ----------------------------------------- Private VLAN Many service providers offer pseudo-wire services to customers using L2 circuits. The IGP protocol that runs in the customer network would also run over the pseudo-wire to create seamless private network for the customer. Service providers want to offer overload kind of functionality when the PE device is taken-out for maintenance. The provider should guarantee that the PE is taken out for maintenance only after the service is successfully diverted on an alternate path. There can be large number of customers attached to a PE node and the remote end-points for these pseudo-wires are spread across the service provider's network. It is a tedious and error-prone process to change the metric for all pseudo-wires in both directions.The link overload feature simplifies the process by increasing the metric on the link in the reverse direction as well so that traffic in both directions is diverted away from the PE undergoing maintenance. The link-overload feature allows the link to be used as a last resort link so that traffic is not disrupted when alternative paths are not available.

_____________ | | -------------| Controller |-------------- | |____________ | | | | |--------- Primary Path ------------------| PE1---------P1----------------P2---------PE2 | | | | |________P3________| Alternate Path In controller-based deployments where the controller participates in the IGP protocol, the controller can also receive the link-overload information as a warning that link maintenance is imminent. Using this information, the controller can find alternate paths for traffic which use the affected link. The controller can apply various policies and re-route the LSPs away from the link undergoing maintenance. If there are no alternate paths satisfying the traffic engineering constraints, the controller might temporarily relax those constraints and put the service on a different path. In the above example, PE1->PE2 LSP is set-up which satisfies a constraint of 10 GB bandwidth on each link.The links P1->P3 and P3->P2 have only 1 GB capacity. and there is no alternate path satisfying the bandwidth constraint of 10GB. When P1->P2 link is being prepared for maintenance, the controller receives the link-overload information, as there is no alternate path available which satisfies the constraints, controller chooses a path that is less optimal and sets up an alternate path via P1->P3->P2 temporarily. Once the traffic is diverted, P1->P2 link can be taken out for maintenance/upgrade.

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

This specification updates one OSPF registry: OSPF Link TLVs Registry i) TBD – Link Overload sub TLV OSPFV3 Link TLV Registry i) TBD – Link Overload sub TLV OSPF Router Information (RI)TLVs Registry i) TBD - Link TLV

Thanks to Chris Bowers for valuable inputs and edits to the document. Thanks to Jeffrey Zhang and Acee Lindem for inputs.