•
When QoS EXP remarking on an interface is enabled, the EXP value is used to determine the egress
tunnel interface, not the incoming EXP value.
•
Egress-side remarking does not affect PBTS tunnel selection.
Path Protection
Path protection provides an end-to-end failure recovery mechanism (that is, a full path protection) for MPLS-TE
tunnels. A secondary Label Switched Path (LSP) is established, in advance, to provide failure protection for
the protected LSP that is carrying a tunnel's TE traffic. When there is a failure on the protected LSP, the source
router immediately enables the secondary LSP to temporarily carry the tunnel's traffic. If there is a failure on
the secondary LSP, the tunnel no longer has path protection until the failure along the secondary path is
cleared. Path protection can be used within a single area (OSPF or IS-IS), external BGP [eBGP], and static
routes.
The failure detection mechanisms triggers a switchover to a secondary tunnel by:
•
Path error or resv-tear from Resource Reservation Protocol (RSVP) signaling
•
Notification from the Bidirectional Forwarding Detection (BFD) protocol that a neighbor is lost
•
Notification from the Interior Gateway Protocol (IGP) that the adjacency is down
•
Local teardown of the protected tunnel's LSP due to preemption in order to signal higher priority LSPs,
a Packet over SONET (POS) alarm, online insertion and removal (OIR), and so on
An alternate recovery mechanism is Fast Reroute (FRR), which protects MPLS-TE LSPs only from link and
node failures, by locally repairing the LSPs at the point of failure. Co-existence of FRR and path protection
is supported; this means FRR and path-protection can be configured on the same tunnel at the same time.
Although not as fast as link or node protection, presignaling a secondary LSP is faster than configuring a
secondary primary path option, or allowing the tunnel's source router to dynamically recalculate a path. The
actual recovery time is topology-dependent, and affected by delay factors such as propagation delay or switch
fabric latency.
Related Topics
Configure Tunnels for Path Protection: Example, on page 352
Pre-requisites for Path Protection
These are the pre-requisites for enabling path protection:
•
Ensure that your network supports MPLS-TE, Cisco Express Forwarding, and Intermediate
System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF).
•
Enable MPLS.
•
Configure TE on the routers.
•
Configure a TE tunnel with a dynamic path option by using the path-option command with the
dynamic keyword.
Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, Release 5.1.x
188
Implementing MPLS Traffic Engineering
Path Protection
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