SRv6 binding SID procedures

The router supports one binding SID per SRv6 policy. The application of binding SIDs for SRv6 policies is similar to those for SR policies with an MPLS data plane. The binding SID is programmed using the function type End.B6.Encaps.Red (as defined in RFC 8986).

Binding SIDs for local static SRv6 policies

If the head end is local, the router attempts to program the SR policy. The binding SID is mandatory and the configuration consists of:

a locator name
a function type
a function value (optional)

Only one binding SID can be configured per SRv6 policy. The router performs the following checks and actions:

The locator name must be configured.
The function type must be configured. Only end-b6-encaps-red is supported.
Note: The procedures defined in RFC 8986 for the End.B6.Encaps.Red function mean that a maximum of two SRv6 policies can be concatenated to provide an end-to-end path using binding SIDs with this function.
A function value can optionally be configured. If a value is configured, it is considered static. If no value is configured, the router tries to allocate a dynamic value for the function within the named locator.
The binding SID function value uses the rules for the named locator:
- The named locator has a static and a dynamic range.
  
  If a static value is configured, the router checks and allocates within the static range for the locator. If no static value is configured, the router allocates a value within the dynamic range.
- The named locator uses a reserved global function block from which statically and dynamically allocated function values of all locators can be drawn.
  
  The router checks and allocates the value within the reserved global function block.
- All above checks fail.
  
  The SRv6 policy is not programmed. The router retries the checks at a later time. The allocation scheme for the binding SID function values is first-come-first-served.

Binding SIDs for non-local static SRv6 policies

If the head end is not local, the router advertises the SRv6 policy in BGP toward the head end router.

The binding SID is a 128-bit IPv6 address. The router performs no local checks on the binding SID value. The router advertises the binding SID in BGP using the SRv6 binding SID sub-TLV, see BGP SRv6 policies. The SRv6 endpoint behavior sub-TLV is not included.

Receiving and programing BGP SRv6 policy routes

In general, the behavior for received SRv6 policy routes is similar to IPv4 SR-MPLS policies. For an SRv6 policy imported through BGP, the binding SID type is the indicator of the type of the SR policy. If the binding SID is an SRv6 binding SID, the router performs a longest prefix match against the route.

Note: The longest prefix match fails for SIDs with non-zero argument bits. SR OS requires that the argument bits are set to zero in the incoming SRv6 policy binding SID.

If there is no match, the SRv6 policy (candidate path) is invalid.

If there is a match, the router performs the following checks on the locator and the function value:

If the function value collides with an already allocated function value within the static range for the locator matching the binding SID (or within the static part of a reserved global function block for the locator, if this is configured), the programming fails. If there is no collision with an already allocated static function value (regardless of the function type), the router allocates the function value to the End.B6.Encaps.Red function for the binding SID. The allocation is on a first-come-first-served basis.
If a received SRv6 policy includes a SID structure TLV associated with the binding SID sub-TLV or the SRv6 segment sub-TLV, the router ignores it.
If a received SRv6 policy route contains more than one SRv6 binding SID TLV, the router treats the path as invalid.

If the binding SID is valid, the router programs the SRv6 policy. The binding SID is programmed as a full /128 address.