IS-IS BFD Enabled TLV

The Bidirectional Forwarding Detection protocol is a protocol that allows for detection of a forwarding plane failure between two routers. A router can use to validate that a peer router's forwarding ability is functioning. One specific application of BFD as described in is to verify the forwarding ability of an IS-IS router's adjacencies; however, the method described in does not allow for certain failure scenarios. We will define a TLV that will allow for proper response to the detection of all forwarding failures where the use of BFD is employed with IS-IS.

We observe that to allow for mixed use (i.e., some routers running BFD and some not) does not require a BFD session be established prior to the establishment of an IS-IS adjacency. Thus, if a router A has neighbors B and C, and B does not support BFD, A would still form adjacencies with B and C, and would only establish a BFD session with C. The problem with this solution is that it assumes that the transmission and receipt of IS-IS IIHs shares fate with forwarded data packets. This is not a fair assumption to make given that the primary use of BFD is to protect IPv4 (and IPv6) forwarding and IS-IS does not utilize IPv4 or IPv6 for sending or receiving its hellos. Thus, if we consider our previous example, and if C is currently experiencing an IPv4 forwarding failure that allows for IS-IS IIHs to be sent and received, when A first starts (or restarts) A will assume that C simply does not support BFD, will form an adjacency with C, and may incorrectly forward IPv4 traffic through C.

A simple solution to this problem is for an IS-IS router to advertise that it has BFD enabled on a given interface. It can do this through the inclusion of a TLV in its IIHs, and indeed that is our proposal. When sending an IIH on a BFD enabled interface, a router which supports this extension MUST include the BFD enabled TLV in its IIH. The contents of the TLV MUST indicate what topologies/protocols have been enabled for BFD by including the appropriate MTID/NLPID pairs. When sending an IIH on an interface on which BFD is NOT enabled a router MUST NOT include the BFD enabled TLV.

The following definitions apply to each IS-IS neighbor: For each locally supported MTID/NLPID pair, an ISIS_TOPO_NLPID_BFD_REQUIRED variable is assigned. If BFD is supported by both the local system and the neighbor for the MTID/NLPID this variable is set to TRUE. Otherwise the variable is set to FALSE. For each locally supported MTID, an ISIS_TOPO_BFD_REQUIRED variable is set to the logical OR of all ISIS_TOPO_NLPID_BFD_REQUIRED variables associated with that MTID. An ISIS_BFD_REQUIRED variable is set to the logical AND of all ISIS_TOPO_BFD_REQUIRED variables. For each locally supported MTID/NLPID pair, an ISIS_TOPO_NLPID_STATE variable is assigned. If ISIS_TOPO_NLPID_BFD_REQUIRED is TRUE, this variable follows the BFD session state for that MTID/NLPID (UP == TRUE). Otherwise the variable is set to TRUE. For each locally supported topology (MTID), an ISIS_TOPO_USEABLE variable is set to the logical AND of the set of ISIS_TOPO_NLPID_STATE variables associated with that MTID. An ISIS_NEIGHBOR_USEABLE variable is set to the logical OR of all ISIS_TOPO_USEABLE variables.

Whenever ISIS_BFD_REQUIRED is TRUE the following extensions to the rules for adjacency establishment and maintenance MUST apply: ISIS_NEIGHBOR_USEABLE MUST be TRUE before the adjacency can transition from INIT to UP state When the IS-IS adjacency is UP and ISIS_NEIGHBOR_USEABLE becomes FALSE the IS-IS adjacency MUST transition to DOWN. On a Point-to-Point circuit whenever ISIS_NEIGHBOR_USEABLE is FALSE, the Three-Way adjacency state MUST be set to DOWN in the Point-to-Point Three Way Adjacency TLV in all transmitted IIHs. On a LAN circuit whenever ISIS_NEIGHBOR_USEABLE is FALSE, the IS Neighbors TLV advertising the MAC address of the neighbor MUST be omitted in all transmitted IIHs.

The advertisement of a topology specific IS-neighbor (as well as the use of the neighbor in the topology specific decision process) is determined by the value of ISIS_TOPO_USEABLE for each topology. If ISIS_TOPO_USEABLE is TRUE then the topology specific neighbor is advertised. If ISIS_TOPO_USEABLE is FALSE then the topology specific neighbor is NOT advertised.

To allow for a non-disruptive transition to the use of BFD some amount of time should be allowed before bringing down an UP adjacency on a BFD enabled interface when the value of ISIS_BFD_REQUIRED becomes TRUE as a result of the introduction of the BFD TLV or the modification (by adding a new supported MTID/NLPID) of an existing BFD TLV in a neighbor's IIH. A simple way to do this is to not update the adjacency hold-time when receiving such an IIH from a neighbor with whom we have an UP adjacency until ISIS_NEIGHBOR_USEABLE becomes TRUE. If the value of ISIS_BFD_REQUIRED becomes FALSE as a result of the removal the BFD TLV or the modification (by removing a supported MTID/NLPID) of an existing BFD TLV in a neighbor's IIH then BFD session establishment is no longer required to maintain the adjacency or transition the adjacency to the UP state. If a BFD session is administratively shut down and the BFD session state change impacts the value of ISIS_NEIGHBOR_USEABLE, then IS-IS SHOULD allow time for the corresponding MTID/NLPID to be removed from the neighbor's BFD TLV by not updating the adjacency hold time until ISIS_BFD_REQUIRED becomes FALSE. Note that while this allows a non-disruptive transition, it still enforces consistency between the administrative state of the BFD session and the MTID/NLPID(s) advertised in the BFD TLV. This is necessary to provide consistent behavior regardless of whether the BFD AdminDown state is introduced before or after an IS-IS adjacency UP state has been achieved.

It is worth considering what if anything should be done when IS-IS is gracefully restarting . In cases where BFD shares fate with the control plane, it can be expected that BFD session failure may occur in conjunction with the control plane restart. In such cases premature abort of IS-IS graceful restart as a result of BFD session failure is undesirable. Therefore, some mechanism to ignore the BFD session failure for a limited period of time would be beneficial. How this is implemented is beyond the scope of this document. Consult for further details.

The BFD enabled TLV is formatted as shown below. The TLV SHALL only be included in an IS-IS IIH PDU and only when BFD is enabled for one or more supported MTID/protocols on the interface over which the IIH is being sent. The NLPIDs encoded in the TLV are defined in

The TLV defined within this document describes an addition to the IS-IS Hello protocol and does not impact the security mechanism of the IS-IS protocol.

The following IS-IS TLV type is defined by this draft.

Please update the IS-IS TLV Codepoint Registry accordingly. Note to RFC Editor: this section may be removed on publication as an RFC.

The authors wish to thank Jeffrey Haas, Matthew Jones, Dave Katz, Jonathan Moon, Stefano Previdi, Mike Shand, Michael Shiplett and David Ward, for various input on this document.