SRTE Overview¶

SRTE overview¶

SR-TE is the service that implements a policy architecture for SR networks.

An SR policy is an ordered list of SIDs located at a given headend node in a SR network. That SR policy matches traffic associated with a color number, and steers that traffic to a given endpoint, by appending the SID list in the packets.

On Virtual Service Router, the SR policies are locally defined, and permit a distributed control across the SR nodes. SR policies can be used by BGP: incoming BGP traffic whose next-hop matches a given SR endpoint can be colored, and steered with the corresponding SR policy.

The below figure illustrates an SR policy created on the A headend. Traffic flow marked with a defined color will be diverted to the G endpoint, by using the following SID list : <f,g>.

Diverting traffic from host01 to host02 using SR-TE service¶

An SR policy is made up of a list of candidate paths indexed by a preference number. Candidate paths forge an MPLS segment list which may be defined either explicitly or dynamically. The active candidate path with the biggest preference number will append its segment list to the matching traffic.

Explicit segment list¶

The following segment types are available when configuring an explicit segment list in the candidate path:

Type A: MPLS label for SR. Global labels from the IGP will be chosen.
Type F and Type H: IP addresses for link endpoints as local, remote pair. Based on the addresses provuided, and the TE information from the IGP, the local label adjacencies will forge an MPLS segment list.

Dynamic segment list¶

Dynamic candidate paths rely on the flex algorithm number defined in the IS-IS protocol. Each algorithm stands for an SPF algorithm with constraints, and computes a specific segment list for a given next hop, by using specific Flex algorithm SIDs. Those segment lists are updated in the associated SR candidate path. The following constraints are reused in the dynamic candidate path:

using the TE metric instead of the regular metric, as defined in RFC 5305.
using the TE minimum delay, as per RFC 8667.

Advantages¶

SR-TE can be applied to routed traffic, but also MPLS traffic, by associating a unique binding SID to each policy. In segment routing dataplanes, this SID stands for an MPLS label that has a local significance. When used by upstream traffic, the incoming MPLS traffic will apply the SR policy that owns the BSID identifier.

Also, to reduce the configuration footprint when diverting traffic for a mesh of endpoints with different colors, template policies can be forged. When enabled, incoming BGP colored updates will trigger the creation of policies and candidate paths will be dynamically linked to an IS-IS Flex Algorithm number.

Standards¶

Virtual Service Router provides the following features:

RFC 9256:: Segment Routing Policy Architecture