CR-LSP Backup

On one tunnel, a CR-LSP used to protect the primary CR-LSP is called a backup CR-LSP.

A backup CR-LSP protects traffic on important CR-LSPs. If a primary CR-LSP fails, traffic switches to a backup CR-LSP.

If the ingress detects that a primary CR-LSP is unavailable, the ingress switches traffic to a backup CR-LSP. After the primary CR-LSP recovers, traffic switches back. Traffic on the primary CR-LSP is protected.

CR-LSP backup is performed in either of the following modes:

Hot standby: A backup CR-LSP is set up immediately after a primary CR-LSP is set up. If the primary CR-LSP fails, traffic switches to the backup CR-LSP. If the primary CR-LSP recovers, traffic switches back to the primary CR-LSP. Hot standby CR-LSPs support best-effort paths.

Ordinary backup: A backup CR-LSP is set up after a primary CR-LSP fails. If the primary CR-LSP fails, traffic switches to the backup CR-LSP. If the primary CR-LSP recovers, traffic switches back to the primary CR-LSP.

For details about the differences between hot standby and ordinary CR-LSPs, see the Table 1.

**Table 1** Differences between hot-standby and ordinary CR-LSPs
Item	Hot Standby	Ordinary Backup
When a backup CR-LSP is established	Created immediately after the primary CR-LSP is established.	Created only after the primary CR-LSP fails.
Path overlapping	You can specify whether path overlapping is allowed using the configuration. If an explicit path is allowed for a backup CR-LSP, the backup CR-LSP can be set up over an explicit path.	Allowed path overlapping in any case.
Whether a best-effort path is supported	Supported	Not supported

Best-effort path

The hot standby function supports the establishment of best-effort paths. If both the primary and hot-standby CR-LSPs fail, a best-effort path is established and takes over traffic.

In Figure 1, the path of the primary CR-LSP is PE1→P1→PE2, and the path of the backup CR-LSP is PE1→P2→PE2. If both the primary and backup CR-LSPs both fail, the router triggers the setup of a best-effort path PE1→P2→P1→PE2.

Figure 1 Schematic diagram for a best-effort path

A best-effort path does not provide reserved bandwidth for traffic. The affinity attribute and hop limit are configured as needed.

Hot standby CR-LSP Switchover and Revertive Switchover Policy

Traffic can switch to a hot standby CR-LSP in automatic or manual mode:

Automatic switchover: Traffic switches to a hot standby CR-LSP from a primary CR-LSP when the primary CR-LSP goes Down. If the primary CR-LSP goes Up again, traffic automatically switches back to the primary CR-LSP. This is the default setting. You can determine whether to switch traffic back to the primary CR-LSP and set a revertive switchover delay time.
Manual switchover: You can manually trigger a traffic switchover to solve specific problems. For example, when some devices on the primary CR-LSP need to be upgraded or some parameters of the primary CR-LSP need to be adjusted, you can manually switch traffic to the backup CR-LSP. After the required operations are complete, you can manually switch traffic back to the primary CR-LSP.

Path Overlapping

The path overlapping function can be configured for hot standby CR-LSPs. It means that the paths of a hot standby CR-LSP and primary CR-LSP can overlap while being disjointed as much as possible. This ensures that the hot standby CR-LSP provides maximum protection to the primary CR-LSP.

Comparison Between CR-LSP Backup and Other Features

1. Differences between CR-LSP backup and TE FRR

CR-LSP backup provides end-to-end path protection for an entire CR-LSP.
Fast reroute (FRR) is a partial protection mechanism used to protect a link or node on a CR-LSP. It is a temporary protection measure that can respond to faults rapidly, but it has strict requirements on the switching time.

2. CR-LSP hot standby and TE FRR are used together.

TE FRR is a temporary local protection mechanism used when the ingress does not detect a fault. It is a supplement to CR-LSP hot-standby. Once the ingress of the primary CR-LSP detects the fault, it switches traffic to the hot standby CR-LSP.

If the protected link or node is faulty, the Point of Local Repair (PLR) location varies according to the fault location. Accordingly, the switching sequence of CR-LSP hot standby and TE FRR is different.

If the PLR is the ingress of the primary CR-LSP, it can fast detect the fault and immediately switch traffic to the hot standby CR-LSP instead of entering the TE FRR process.
If the PLR is a transit node along the primary CR-LSP, the ingress of the primary CR-LSP may fail to detect the fault in time. To ensure uninterrupted traffic forwarding, the PLR switches traffic to the TE FRR bypass tunnel before transmitting the fault information to the ingress of the primary CR-LSP through RSVP signaling, triggering the ingress to switch traffic to the hot standby CR-LSP.
In this scenario, the ingress cannot quickly detect the fault due to slow RSVP signaling transmission. As a result, traffic is always forwarded over the TE FRR bypass tunnel. To speed up switching of traffic to the hot standby CR-LSP by the ingress, you can enable CSPF fast switching. After this function is enabled, an IGP notifies the ingress of the primary CR-LSP when its topology changes. After detecting the fault, the ingress of the primary CR-LSP does not wait for RSVP signaling but switches traffic to the hot standby CR-LSP in advance.

If the hot-standby CR-LSP is down, the ingress keeps attempting to reestablish a hot standby CR-LSP.

3. CR-LSP ordinary backup and TE FRR association.

The association is disabled.

If a protected link or node fails, a PLR switches traffic to a bypass tunnel. Only after both the primary and bypass CR-LSPs fail, the ingress of the primary CR-LSP attempts to establish an ordinary backup CR-LSP and switches traffic to this CR-LSP.
The association is enabled (FRR in Use).

If a protected link or node fails, a PLR switches traffic to a bypass tunnel. If the PLR is the ingress of the primary CR-LSP, the PLR attempts to establish an ordinary backup CR-LSP. If the ordinary backup CR-LSP is successfully established, the PLR switches traffic to the new CR-LSP. If the PLR is a transit node on the primary CR-LSP, the PLR advertises the fault to the ingress of the primary CR-LSP through RSVP signaling, the ingress attempts to establish an ordinary backup CR-LSP. If the ordinary backup CR-LSP is successfully established, the ingress switches traffic to the new CR-LSP.

If the ordinary backup CR-LSP fails to be established, traffic keeps traveling through the bypass CR-LSP.