Overview of Entropy Labels

The 7705 SAR supports MPLS entropy labels on RSVP-TE and SR-TE LSPs, as per RFC 6790. The entropy label provides greater granularity for load balancing on an LSR where load balancing is typically based on the MPLS label stack.

The ability of a node to receive and process an entropy label for an LSP is signaled using capability signaling (referred to as entropy label capability (ELC)). Entropy labels are supported on RSVP-TE and SR-TE tunnels.

Inserting an entropy label adds two labels in the MPLS label stack: the entropy label itself and the entropy label indicator (ELI).

The entropy label is inserted directly below the tunnel label and closest to the service payload that has advertised entropy label capability (which may be above the bottom of the stack). The value of the entropy label is calculated at the iLER and is based on a hash of the packet payload header content and other system parameters at ingress. For more information on hashing inputs, see the ‟Per-Flow Hashing” section in the 7705 SAR Interface Configuration Guide.

The ELI is inserted by the iLER. The ELI is a special-purpose MPLS label (value = 7) that indicates that the entropy label is the next label in the stack.

Entropy label capability is advertised at the tunnel level by the far-end node (eLER). This capability can be advertised for an RSVP-TE FEC or an SR-TE tunnel on IS-IS or OSPF. Capability signaling is not supported for point-to-multipoint LSPs, BGP tunnels, or LDP FECs. An LSR used for RSVP-TE and SR-TE tunnels will pass the entropy label capability signal from the downstream LSP segment to upstream peers. However, earlier releases that do not support entropy label functionality will pass the capability flag transparently, without altering the value.

The insertion of an entropy label by the upstream LER on a tunnel enabled for entropy label capability is enabled on a per-service basis. The entropy label is only inserted if the downstream peer has signaled entropy label support. The upstream LER only inserts a single entropy label, even if multiple LSP labels exist in a label stack.

The 7705 SAR supports the entropy label feature for the following services:

• Cpipe, Epipe, and Ipipe access to spoke SDP

• Cpipe, Epipe, and Ipipe spoke SDP to spoke SDP (vc-switching)

• VPLS SAP to VPLS spoke SDP or mesh SDP

• VPLS spoke SDP to VPLS spoke SDP

• VPRN for RSVP-TE

• R-VPLS

• IGP shortcut

• IS-IS for SR-TE

• OSPF for SR-TE

Entropy label capability on RSVP-TE LSPs is enabled on the eLER using the config>router>rsvp>entropy-label-capability command.

At the iLER, the insertion of the entropy label into the label stack is enabled using the entropy-label command under the service, mesh SDP, or spoke SDP context or under the config>router>isis (or ospf)>segment-routing context for SR-TE LSPs.

The entropy label requires the insertion of two additional labels in the label stack. In some cases, this may result in an unsupported label stack depth or large changes in the label stack depth during the lifetime of an LSP (for example, due to switching from a primary path with entropy label capability enabled to a secondary path for which the far end has not signaled entropy label capability).

The entropy-label command under the config>router>mpls and config>router>mpls>lsp contexts provides local control at the head end of an LSP over whether the entropy label is inserted on an LSP by overriding the entropy label capability signaled from the far-end LER, and control over how the additional label stack depth is accounted for. This allows the user to avoid entropy label insertion where there is a risk of the label stack depth becoming too great.

For entropy labels that are supported on LDP tunnels with remote-LFA protection (that is, for rsvp-shortcut), only loop-free alternate protect (lfa-protect) and LFA (lfa-only) are allowed.

Support of entropy labels over RSVP-TE and SR-TE tunnels are the only valid options, except when the 7705 SAR is the LER node with BGP labeled unicast (BGP-LU) tunnels. A 7705 SAR in an LER role can push and pop an entropy label for Epipe and VPLS services with a BGP-LU tunnel riding over an RSVP-TE LSP. Conversely, a 7705 SAR does not support being in an ABR or ASBR role with BGP-LU. Table: Summary of Entropy Label Support lists entropy label support on the 7705 SAR.