Configuring Session Protection; Configuring Label Distribution Protocol- Interior Gateway Protocol (Igp) Synchronization - Cisco NCS 5500 Series Configuration Manual

Implementing MPLS Label Distribution Protocol

Configuring Session Protection

When a new link or node comes up after a link failure, IP converges earlier and much faster than MPLS LDP
and may result in MPLS traffic loss until the MPLS convergence. If a link flaps, the LDP session also flaps
due to loss of link discovery. LDP session protection minimizes traffic loss, provides faster convergence, and
protects existing LDP (link) sessions. When session protection is enabled for a peer, LDP starts sending
targeted hello (directed discovery) in addition to basic discovery link hellos. When the direct link goes down,
the targeted hellos can still be forwarded to the peer LSR over an alternative path as long as there is one. So,
the LDP session stays up after the link goes down.
You can configure LDP session protection to automatically protect sessions with all or a given set of peers
(as specified by peer-acl). When configured, LDP initiates backup targeted hellos automatically for neighbors
for which primary link adjacencies already exist. These backup targeted hellos maintain LDP sessions when
primary link adjacencies go down.
Configuration Example
This example shows how to configure LDP session protection for peers specified by the access control list
peer-acl-1 for a maximum duration of 60 seconds.
RP/0/RP0/CPU0:Router(config)# mpls ldp
RP/0/RP0/CPU0:Router(config-ldp)# session protection for peer-acl-1 duration 60
RP/0/RP0/CPU0:Router(config-ldp)# commit
Configuring Label Distribution Protocol- Interior Gateway Protocol (IGP)
Synchronization
Lack of synchronization between LDP and Interior Gateway Protocol (IGP) can cause MPLS traffic loss.
Upon link up, for example, IGP can advertise and use a link before LDP convergence has occurred or, a link
may continue to be used in IGP after an LDP session goes down.
LDP IGP synchronization coordinates LDP and IGP so that IGP advertises links with regular metrics only
when MPLS LDP is converged on that link. LDP considers a link converged when at least one LDP session
is up and running on the link for which LDP has sent its applicable label bindings and received at least one
label binding from the peer. LDP communicates this information to IGP upon link up or session down events
and IGP acts accordingly, depending on sync state.
LDP-IGP synchronization is supported for both OSPF and ISIS protocols and is configured under the
corresponding IGP protocol configuration mode. Under certain circumstances, it might be required to delay
declaration of re-synchronization to a configurable interval. LDP provides a configuration option to delay
declaring synchronization up for up to 60 seconds. LDP communicates this information to IGP upon linkup
or session down events.
Configuring LDP IGP Synchronization: Open Shortest Path First (OSPF) Example
This example shows how to configure LDP-IGP synchronization for an OSPF instance. The synchronization
delay is configured as 30 seconds.
RP/0/RP0/CPU0:Router(config)# router ospf 100
RP/0/RP0/CPU0:Router(config-ospf)# mpls ldp sync
RP/0/RP0/CPU0:Router(config-ospf)# mpls ldp igp sync delay 30
RP/0/RP0/CPU0:Router(config-ospf)# commit
MPLS Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
Configuring Session Protection
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