Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 166)

142 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Link protection for FRR
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Link protection for FRR
A Label Switched Path (LSP) set up across an MPLS network is used to switch traffic across MPLS
network. The path used by a LSP across the network is based upon network resources or any other
traffic engineering constraints provided by the user. Based on TE-constraints, the ingress MPLS
router computes the path to be taken by LSP and signals it using RSVP protocol.
By nature, nodes and links in a MPLS networks are prone to failure. It is likely that the link or the
nodes through which LSP is traversing can fail. In the event of a failure of a node or link, RSVP
protocol has mechanisms that informs the ingress node about the failure the to ingress node. On
receipt of failure message for LSP across the path, the ingress router re-signals the LSP using a
new path.
Due to messaging and other network delays, the ingress router cannot respond fast enough to
minimalize the loss of traffic. Traffic is lost from the moment the failure occurs and until the new
path is setup for the LSP, which is quite large in quantum for service provider networks.
In order to avoid loss of traffic, Fast Reroute (FRR) protects the LSP and allows a broken LSP to be
repaired immediately at the point of failure. Point of failure is termed as “Point of local repair
(PLR)”, where the LSP can be repaired locally without intimating or waiting for the ingress router.
PLR is the MPLS router which detects the failure and redirects the traffic appropriately to its
backup path with minimal loss.
Typically at the PLR, two type of protection can be provided to LSP:
Link Protection: In this protection, the backup is selected in such a way that it avoids the failed link
which was used earlier by the LSP. Traffic merges back to main stream from the backup on the very
next MPLS router. Refer to following
Figure 21 illustrating link protection provided at R2 to LSP
ingressing from R1 to R4.
FIGURE 21 Link protection