SR-TP Tunnels

What Are SR-TP Tunnels?

Segment Routing Transport Profile (SR-TP) is a tunneling technology for Transport Profile (TP) that utilizes Segment Routing (SR) as its control protocol. It allows the source node to encapsulate a sequence of segments representing a path in the network, indicating a specific forwarding route. Unlike traditional tunnels, SR tunnels do not necessitate transit nodes to uphold path states. This enhances the flexibility of tunnel path adjustment and improves network programmability.

Principles of SR-TP Tunnels

• Topology Collection and Label Allocation: After enabling IS-IS and SR on devices and establishing a neighbor relationship between each pair of directly connected devices, a controller allocates a local adjacency label to each link on the network. These adjacency labels are propagated throughout the network using the IS-IS SR extension. If BGP-LS is deployed between devices and the controller, devices run BGP-LS to collect topology information with SR labels and then report this information to the controller.

• Tunnel creation: Using SR-TP tunnel attributes, the controller acts as a path computation element to compute a path similar to TP path computation. It then combines the adjacency labels of the entire path to generate a label stack, which represents the path computation result. Via the Network Configuration Protocol and Path Computation Element Communication Protocol, the controller delivers the tunnel configuration and label stack to the specified forwarder. The forwarder then establishes an SR-TP tunnel accordingly.

• Data Forwarding: The forwarder conducts label operations on the incoming packet based on the label stack associated with the SR-TP tunnel and determines the outbound interface based on the top label. This process enables the packet to be forwarded to the destination incrementally. Upon receiving the packet, each transit node (P node) searches its adjacency label table for the appropriate outbound interface, removes the outer label, and forwards the packet to the next hop. The tunnel is terminated when the outer label of the packet received by the sink node matches the path SID allocated by the sink node itself.

Protection Mechanism for SR-TP Tunnels

SR-TP tunnels inherit the APS and OAM mechanisms from MPLS-TP tunnels. The following explains how the protection mechanism for SR-TP tunnels functions.

SR-TP APS utilizes the SR-TP OAM detection mechanism to monitor the status of the working and protection tunnels. Specifically, the source node of a tunnel periodically sends OAM detection packets to the sink node. If the sink node does not receive such packets within a specified period, it considers a link signal failure to have occurred in the tunnel and triggers APS. Additionally, the sink node notifies the APS module on the source node of the failure to initiate service switching on the source node, thereby implementing service protection. SR-TP APS 1:1 protection switching is activated when any of the following conditions are met:

• The working link fails.

• Board hardware fails.

• A cold reset is performed on a board.

• A switching command is manually issued.

• The bit error rate of an Ethernet interface exceeds the preset threshold.

There is an option: SR-TP tunnel rerouting improves the reliability of tunnels for critical services. As an enhancement to SR-TP APS, SR-TP tunnel rerouting requires configuring an active route and a standby route on a device. Information about both routes is stored in the forwarding table. If the active route fails, the device switches traffic to the standby route based on forwarding entries. SR-TP tunnels support the following pinning modes:

• Hard pinning: In this mode, tunnel rerouting is not performed under any circumstance.

• Soft pinning: In this mode, the tunnel path does not proactively respond to topology changes, and the original path is restored only after the associated tunnel fault is rectified through re-routing.

• No pinning: In this mode, path re-computation is performed based on the latest BGP topology after the original path fails.

SR-TP APS can be deployed for tunnels carrying critical services, and the working and protection SR-TP tunnels can be configured to operate in hard pinning and soft pinning modes, respectively.