The PCE path profile defined in draft-alvarez-pce-path-profiles is used to request path diversity or a disjoint for two or more LSPs originating on the same or different PE routers. It is also used to request that paths of two unidirectional LSPs between the same two routers use the same TE links. This is referred to as the bidirectionality constraint.
path diversity, node-disjoint, link-disjoint
path bidirectionality, symmetric reverse route preferred, symmetric reverse route required
maximum path IGP metric (cost)
maximum path TE metric
maximum hop count
IGP metric (cost)
TE metric
hops (span)
The CSPF algorithm will optimize this objective. If a constraint is provided for the same metric, then the CSPF algorithm makes sure that the selected path achieves a lower or equal value to the bound specified in the constraint.
For hop-count metrics, if a constraint is sent in a METRIC object, and is also specified in a PCE profile referenced by the LSP, the constraint in the METRIC object is used.
For IGP and TE metrics, if an objective is sent in a METRIC object, and is also specified in a PCE profile referenced by the LSP, the objective in the Path Profile is used.
The constraints in the Bandwidth object and the LSPA object, specifically the include/exclude admin-group constraints and setup and hold priorities, are not supported in the PCE profile.
In order to indicate the path diversity and bidirectionality constraints to the PCE, the user must configure the profile ID and path group ID of the PCE path that the LSP belongs to. The CLI for this is described in the Configuring and Operating SR-TE section. The path group ID does not need to be defined in the PCE as part of the path profile configuration, and identifies implicitly the set of paths which must have the path diversity constraint applied.
The user can only associate a single path group ID with a specific PCE path profile ID for a given LSP. However, the same path group ID can be associated with multiple PCE profile IDs for the same LSP.
The path profiles are inferred using the path ID in the path request by the PCC. When the PE router acting as a PCC wants to request path diversity from a set of other LSPs belonging to a path group ID value, it adds a new path profile object into the PCReq message. The object contains the path profile ID and the path group ID as an extended ID field. In other words, the diversity metric is carried in an opaque way from PCC to PCE.
The bidirectionality constraint operates the same way as the diversity constraint. The user can configure a PCE profile with both the path diversity and bidirectionality constraints. PCE will check if there is an LSP in the reverse direction which belongs to the same path group ID as an originating LSP it is computing the path for, and will enforce the constraint.
In order for the PCE to be aware of the path diversity and bidirectionality constraints for an LSP that is delegated but for which there is no prior state in the NRC-P LSP database, the path profile object is included in the PCRpt message with the P-flag set in the common header to indicate that the object must be processed.
Table 1 describes the new objects introduced in the PCE path profile.
|
TLV, Object, or Message |
Contained in Object |
Contained in Message |
|---|---|---|
|
PATH-PROFILE-CAPABILITY TLV |
OPEN |
OPEN |
|
PATH-PROFILE Object |
— |
PCReq, PCRpt, PCInitiate |
A path profile object can contain multiple TLVs containing each profile-id and extend-id, and should be processed properly. If multiple path profile objects are received, the first object is interpreted and the others are ignored. The PCC and the PCE support all PCEP capability TLVs defined in this chapter and will always advertise them. If the OPEN object received from a PCEP speaker does not contain one or more of the capabilities, the PCE or PCC will not use them during that PCEP session.