CR-LSP Backup

CR-LSP backup provides end-to-end protection for an MPLS TE tunnel. If the ingress node detects a failure of the primary CR-LSP, it switches traffic to a backup CR-LSP. After the primary CR-LSP recovers, traffic switches back to the primary CR-LSP.

Concepts

CR-LSP backup functions include hot standby, ordinary backup, and the best-effort path:

Hot standby: A hot-standby CR-LSP is set up immediately after the primary CR-LSP is set up. When the primary CR-LSP fails, traffic switches to the hot-standby CR-LSP.
Ordinary backup: An ordinary backup CR-LSP can be set up only after a primary CR-LSP fails. The ordinary backup CR-LSP takes over traffic when the primary CR-LSP fails.
Best-effort path: If both the primary and backup CR-LSPs fail, a best-effort path is set up and takes over traffic.

In Figure 1, the primary CR-LSP is set up over the path PE1 -> P1 -> P2 -> PE2, and the backup CR-LSP is set up over the path PE1 -> P3 -> PE2. When both CR-LSPs fail, PE1 sets up a best-effort path PE1 -> P4 -> PE2 to take over traffic.

Figure 1 Best-effort path

A best-effort path has no bandwidth reserved for traffic, but has an affinity and a hop limit configured to control the nodes it passes.

Implementation

The process of CR-LSP backup is as follows:

CR-LSP backup deployment

Determine the paths, bandwidth values, and deployment modes. Table 1 lists CR-LSP backup deployment items.

**Table 1** CR-LSP backup deployment
Item	Hot Standby	Ordinary Backup	Best-Effort Path
Path	Determine whether the paths of primary and hot-standby CR-LSPs partially overlap. A hot-standby CR-LSP can be established over an explicit path. A hot-standby CR-LSP supports the following attributes: Explicit path Affinity attribute Hop limit Path overlapping	The path of an ordinary CR-LSP can partially overlap the path of the primary CR-LSP, no matter whether the ordinary CR-LSP is set up along an explicit or implicit path. An ordinary backup CR-LSP supports the following attributes: Explicit path Affinity attribute Hop limit	A best-effort path is automatically calculated by the ingress node. A best-effort path supports the following attributes: Affinity attribute Hop limit
Bandwidth	A hot-standby CR-LSP has the same bandwidth as a primary CR-LSP by default. Dynamic bandwidth protection can ensure that a hot-standby CR-LSP does not use additional bandwidth when it is not transmitting traffic.	An ordinary backup CR-LSP has the same bandwidth as a primary CR-LSP.	A best-effort path is only a protection path that does not have reserved bandwidth.
Deployment mode	Can be established without attribute templates.	Can be established without attribute templates.	Can be established without attribute templates.
Deployment mode	Can be established using attribute templates.	Can be established using attribute templates.	Automatically established and does not support attribute templates.
Configuration combination	If a hot-standby CR-LSP is established without an attribute template, the hot-standby CR-LSP can be used together with a best-effort path to protect the primary CR-LSP. If a hot-standby CR-LSP is established using an attribute template, the hot-standby CR-LSP can be used together with an ordinary backup CR-LSP and a best-effort path to protect the primary CR-LSP.	If an ordinary CR-LSP is established without an attribute template, the ordinary CR-LSP can only be used alone to protect the primary CR-LSP. If an ordinary CR-LSP is established using an attribute template, the ordinary backup CR-LSP can be used together with a hot-standby backup CR-LSP and a best-effort path to protect the primary CR-LSP.	-

**Table 2** CR-LSP backup modes
Backup Mode	Description	Advantage	Shortcoming
Hot standby	A hot-standby CR-LSP is set up over a separate path immediately after a primary CR-LSP is set up.	A rapid traffic switchover can be performed.	If dynamic bandwidth adjustment is disabled, additional bandwidth needs to be reserved for a hot-standby CR-LSP.
Ordinary backup	The system attempts to set up an ordinary backup CR-LSP if a primary CR-LSP fails.	No additional bandwidth is needed.	Ordinary backup performs a traffic switchover slower than hot standby.
Best-effort path	The system establishes a best-effort path over an available path if both the primary and backup CR-LSPs fail.	Establishing a best-effort path is easy and a few constraints are needed.	Some quality of service (QoS) requirements cannot be met.

Backup CR-LSP setup

Multiple CR-LSP backup methods may be supported for a tunnel. The ingress node uses these methods in turn until a CR-LSP is successfully established.
The rules for establishing a backup CR-LSP are as follows:
1. If new tunnel configuration is committed or a tunnel goes Down, the ingress node attempts to establish a hot-standby CR-LSP, an ordinary backup CR-LSP, and a best-effort path in turn, until a CR-LSP is successfully established.
2. A maximum of three CR-LSP attribute templates can be configured for hot-standby CR-LSPs or ordinary backup CR-LSPs. These templates are prioritized. The ingress node tries these templates in descending order of priority until a CR-LSP is successfully established.
3. If the status of a CR-LSP established using a lower-priority attribute template changes, the ingress node attempts to establish a CR-LSP using a higher-priority attribute template. The make-before-break mechanism ensures nonstop traffic forwarding when a new CR-LSP is being established.
4. If a stable CR-LSP has been established using any of the attribute templates, you can lock the used attribute template. After the attribute template is locked, the ingress node will not use a higher-priority attribute template to establish a CR-LSP. This locking function prevents unnecessary traffic switchovers and lowers system costs.
Currently, switches support the following backup modes and you can choose one as required.
- Hot standby (manually configured)
- Hot standby (manually configured) and best-effort path
- Hot standby (configured using a TE attribute template)
- Hot standby (configured using a TE attribute template) and ordinary backup (configured using a TE attribute template)
- Hot standby (configured using a TE attribute template) and best-effort path
- Hot standby (configured using a TE attribute template), ordinary backup (configured using a TE attribute template), and best-effort path
- Ordinary backup (manually configured)
- Ordinary backup (configured using a TE attribute template)
- Ordinary backup (configured using a TE attribute template) and best-effort path
- Best-effort path
Backup CR-LSP attribute modification

If attributes of a backup CR-LSP are modified, the ingress node uses the make-before-break mechanism to reestablish the backup CR-LSP with the updated attributes. After that backup CR-LSP has been successfully reestablished, traffic on the original backup CR-LSP (if it is transmitting traffic) switches to this new backup CR-LSP, and then the original backup CR-LSP is torn down.
Fault detection
CR-LSP backup supports the following fault detection functions:
- Default error signaling mechanism of RSVP-TE: The fault detection speed is relatively slow.
- Bidirectional forwarding detection (BFD) for CR-LSP: This function is recommended because it implements fast fault detection.
Traffic switchover

After the primary CR-LSP fails, the ingress node attempts to switch traffic from the primary CR-LSP to a hot-standby CR-LSP. If the hot-standby CR-LSP is unavailable, the ingress node attempts to switch traffic to an ordinary backup CR-LSP. If the ordinary backup CR-LSP is unavailable, the ingress attempts to switch traffic to a best-effort path.
Traffic switchback

Traffic switches back to a path based on priorities of the available CR-LSPs. Traffic will first switch to the primary CR-LSP. If the primary CR-LSP is unavailable, traffic will switch to the hot-standby CR-LSP. The ordinary CR-LSP has the lowest priority.

Dynamic Bandwidth Protection for Hot-standby CR-LSPs

Hot-standby CR-LSPs support dynamic bandwidth protection. The dynamic bandwidth protection function allows a hot-standby CR-LSP to obtain bandwidth resources only after the hot-standby CR-LSP takes over traffic from a faulty primary CR-LSP. This function improves bandwidth efficiency and reduces network costs.

Dynamic bandwidth protection ensures that the hot-standby CR-LSP does not use bandwidth when the primary CR-LSP is transmitting traffic. The dynamic bandwidth protection process is as follows:

If the primary CR-LSP fails, traffic immediately switches to the hot-standby CR-LSP with 0 bit/s bandwidth. The ingress node uses the make-before-break mechanism to establish a hot-standby CR-LSP.
After the new hot-standby CR-LSP has been successfully established, the ingress node switches traffic to this CR-LSP and tears down the hot-standby CR-LSP with 0 bit/s bandwidth.
After the primary CR-LSP recovers, traffic switches back to the primary CR-LSP. The hot-standby CR-LSP then releases the bandwidth, and the ingress node establishes another hot-standby CR-LSP with 0 bit/s bandwidth.

Path Overlapping for a Hot-standby CR-LSP

The path overlapping function can be configured for hot-standby CR-LSPs. This function allows a hot-standby CR-LSP to use some links of a primary CR-LSP. After the hot-standby CR-LSP is established, it can protect traffic on the primary CR-LSP.