BMP Local Path-ID

When using VRFs and/or BGP Multipath, multiple paths to the same destination may be shared among various routing information bases. From a collection perspective, tracking the identity of a path thus requires some form of modeling, which is subject to inaccuracy. This aspect is exacerbated as path attributes may be modified in the process. This is especially problematic when a PE using BGP multipath in VPN instances exports multiple paths for the same destination into the default VRF, which were learned from different peers. While BGP ADD-PATH provides a way to identify paths in BGP multi-path scenarios, the scope of the ADD-PATH path-id is local to a single BGP peering session, and thus cannot be used to distinguish paths received over multiple sessions. This document introduces a way to identify paths globally within a router, allowing operators to not resort to modeling when monitoring BGP paths on a router. In , we introduce the concept of Local Path ID, which is an identifier of a path for a given NLRI, preserved through the import/export operations performed onto them. In , we introduce a BMPv4 TLV allowing to communicate the value of a Local Path ID on a BMP session.

In this section, we define an identifier called Local Path ID, which allows to uniquely identify a path for a given NLRI on a router. According to this specification, a path to be advertised by BMP is provided with an associated Local Path ID. The Local Path ID is an opaque numerical value with a few properties guaranteeing its utility. The exact approach to generate a Local Path ID is however left for the implementation.

The Local Path ID of each path MUST be unique for a given NLRI. We scope the identifier space to each NLRI to keep it a small value. Indeed, most Internet routers have at most a few tens of paths for a given NLRI. While we put a minimum scope (the NLRI) for the identifier space, an implementor may decide to use a broader space for this unicity, as long as Local Path IDs are still unique for a given NLRI. For example, Local Path IDs can be unique accross VRFs, even though they will have to be larger, as this does not violate the rule. The Local Path ID only has a meaning local to the router generating it. Once generated, the Local Path ID MUST be preserved between VRFs, and Routing Information Bases. It is however not intended to be exchanged or synchronized between routers. To ensure traceability in monitoring, the Local Path ID SHOULD be transmitted when paths are redistributed between processes. If the Local Path ID is not transmitted then the process receiving the path SHOULD allocate one. The value of 0 for a Local Path ID is reserved.

In this section, we give general recommendations for the Local Path ID generation. These recommendations may or may not be applicable depending on the platform, the implementation of BGP, etc. The actual generation process of the Local Path ID does not matter as long as the the properties defined in are respected. We recommend having the Local Path ID made of three concatenated parts: < process_id | vrf_id | path_discriminator >. The path_discriminator allows for differentiation between paths for a given NLRI, coming from the same table and process (with the same vrf_id and process_id). The process originating the path is in charge of guaranteeing the uniqueness of the path_discriminator it produces for each path of its NLRIs. The vrf_id represents a unique identifier for the VRF in which the path to the NLRI is contained. It allows leveraging the already existing routing table structures of most BGP implementations by having to guarantee the uniqueness of the path_discriminator only within the table. The process_id is the identifier of the process which produced, originated, or received a path. The process_id allows differentiation between path IDs generated in BGP from path IDs generated in other processes like an IGP. Redistributed IGP paths will then have a different Local Path ID no matter if BGP or another IGP has chosen the same path_discriminator value. Using the process_id avoids requiring interprocess synchronization of path_discriminators or the use of a Local Path ID management process.

The Local Path ID is to be included in BMPv4 Route Monitoring messages as a BMPv4 TLV, called "Local Path ID TLV". This TLV can carry multiple types of data which are discriminated by the "TLV Sub-Type" field. The "TLV Sub-Type" field can take the values given in . When a Local Path ID is allocated for a path, the router includes it using the "TLV Sub-Type" value 0x00 (Local Path ID Value). When a Local Path ID is not allocated, the router includes a "TLV Sub-Type" value 0x01 (Unavailability Reason Code) that describes why the Local Path ID is not available for the path. In practice, this means that a Local Path ID enabled router SHOULD always associate a Path ID TLV with a path advertised in BMP. On the collector side, a path should be identified using the Local Path ID Value when provided, and only resort to network modeling when the Local Path ID is not available.

The encoding of the "Local Path ID TLV" is illustrated in .

Type:: set to &IANA_LPID_TLV;
Length:: the length of the "TLV Sub-Type" field + the length of the "Value" field, in bytes
Index:: index of the NLRI in the BGP Update PDU as described by . The Index MUST refer to a single NLRI (no Group TLV).
Sub-Type:: a one byte TLV Sub-Type, as listed in
Value:: the value corresponding to the TLV Sub-Type as described in

The defines the list of TLV Sub-Types and references the section defining their associated values. Local Path ID TLV Sub-Types

Code	Name	Length	Section
0x00	Local Path ID Value	Variable
0x01	Unavailability Reason Code	2 bytes

When the Local Path ID is available for a path, the router exports the Local Path ID using a "Local Path ID TLV" with "TLV Sub-Type" = 0x00. The Local Path ID Value contains the raw bytes of the generated Local Path ID (). The Length field is thus set to the length, in bytes, of the Local Path ID, plus one (for the TLV Sub-Type field).

An implementation enabled for Local Path ID usage MUST notify if a Local Path ID is unavailable (for any reason) by using the "TLV Sub-Type" = 0x01. When "TLV Sub-Type" is 0x01, the TLV Value is a 2-byte error code from . The Length field of the "Local Path ID TLV" is thus set to 3 in this case. Local Path ID Unavailability Reason Codes

Code	Reason	Description
0x00	Unknown Reason	A unknown error occurred during the allocation of the Local Path ID
0x01	Origin process did not provide a Local Path ID.	A path is imported from another process, the latter does not provide a Local Path ID for the imported path and the receiving process did not allocate one either
0x02	All Local Path ID have been allocated.	A Local Path ID could not be allocated for a path because the entire range of Local Path IDs is in use

This document requests that IANA assigns the following new parameters to the "BMP Route Monitoring TLVs" registry

Type = &IANA_LPID_TLV;: Local Path ID TLV type. The value of this TLV is defined in

This document also requests the definition of a "Local Path ID Reason Codes" registry in the "BMP Parameters" namespace, seeded as follows:

Type = 0: Unknown Reason. Set to 0 when an unknown error occurred during the allocation of the Local Path ID.
Type = 1: Origin Process did not provide a Local Path ID. Set to 1 when a path is imported from another process, the latter does not provide a Local Path ID for the imported path and the receiving process did not allocate one either.
Type = 2: All Local Path ID have been allocated. Set to 2 when a Local Path ID could not be allocated for a path because the entire range of Local Path IDs is in use.