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 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, which is 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. The Local Path ID only has a meaning locally on the router generating it. Once generated, the Local Path ID MUST be preserved between VRFs, and Routing Information Bases. It, however, MUST NOT be exchanged or synchronized between routers. The value of 0 for a Local Path ID is reserved.

In this specification, we recommend making the Local Path ID made of two concatenated parts: < process_id | path_discriminator >. 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 and 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 path_discriminator managing process. The path_discriminator allows differentiation between different paths for a NLRI, coming from the same process (with the same process_id). This process originating the path is in charge of guaranteeing the uniqueness of the path_discriminator it produces for each path of its NLRIs. To ensure traceability in monitoring, processes importing a path (like BGP redistribution or VRF imports) SHOULD keep the same Local Path ID if provided by the source.

The Local Path ID is included in BMPv4 Route Monitoring messages as an optional TLV, called "Local Path ID TLV".

Type: set to TBD
Length: the length of the Local Path ID, in bytes (TBD static or dynamic)
Index: index of the NLRI in the BGP Update PDU as described by . The Index MUST refer to a single NLRI (no Group TLV).
local_path_id: the Local Path ID defined in , on Length bytes

An implementation enabled for Local Path ID usage MUST notify if a Local Path ID is unavailable (for any reason) by setting the value field to the reserved value of 0. This means such implementations SHOULD always include a Local Path ID TLV.