MPLS and Segment Routing

A service provider runs OSPF with LDP for label distribution across the core. After a link flap on a P router, operators notice that transit traffic is temporarily black-holed: the OSPF adjacency comes up and the IGP starts forwarding over the restored link before LDP has re-established its session and exchanged labels, causing unlabeled packets to be dropped on the LSP. Which feature should be configured on the affected interface to prevent this behavior?

A service provider runs LDP across its core using default Cisco IOS behavior. A network engineer notices that during an IGP convergence event, some LSRs begin advertising a label mapping for a FEC to their upstream neighbors before the downstream label mapping for that same FEC has been received. This creates a brief window where an LSR advertises a label but has no valid downstream label to swap to. Which LDP label distribution control mode explains this behavior, and what is its key characteristic?

A service provider runs LDP across all core and edge routers. The network team wants to reduce the number of labels advertised in the core so that only loopback (/32) host routes used as BGP next-hops receive labels, while all other IGP prefixes are not assigned local labels. Which LDP feature should be configured to achieve this goal?

A network engineer is troubleshooting label allocation on an MPLS core running LDP. On a Cisco IOS XR router, the engineer observes that the same local label value is advertised to all LDP peers for a given IGP prefix, regardless of which interface the LDP session uses. A colleague expected each interface to allocate a distinct label for the prefix. Which statement correctly explains this behavior?

A service provider runs LDP with an IGP across their core. An intermittent physical link between routers P1 and P2 flaps several times per hour. Each flap causes the LDP session to tear down and all associated labels to be purged, triggering a full label re-exchange and brief traffic loss once the link recovers. The operator wants to keep the LDP session and its label bindings intact across short link outages, provided an alternate IGP path exists between the two LSRs. Which LDP feature should be configured to achieve this?

An SP engineer is provisioning an Ethernet-over-MPLS point-to-point pseudowire (VPWS) between PE1 and PE3, which are not directly connected and are separated by core router P2. LDP is enabled on all core-facing interfaces for transport label exchange. After configuring the xconnect on both PEs, the pseudowire remains down and no VC label is exchanged between PE1 and PE3. What must be configured to bring up the pseudowire?

A service provider runs LDP across its core. The egress PE (loopback 10.0.0.5/32) advertises a label mapping to its penultimate-hop router. The QoS team complains that on MPLS-transported voice traffic, the EXP-based marking is being lost right before packets reach the egress PE, causing inconsistent per-hop behavior on the final PE. They want the penultimate router to keep pushing a labeled packet to the egress PE so the EXP bits survive to the last hop, while still avoiding a double IP lookup penalty on the egress. Which label-signaling behavior should the egress PE advertise to achieve this?

A service provider deploys RSVP-TE tunnels across its core. The network team wants sub-50ms recovery if the next-hop router of a protected tunnel completely fails, not just a link. They configure MPLS TE Fast ReRoute (FRR) with a backup tunnel. Which backup tunnel design ensures traffic is protected against the failure of the immediate downstream P router?

A service provider is deploying RSVP-TE tunnels and wants a specific tunnel to traverse only links that are colored as 'high-bandwidth' (bit 0x01) while avoiding any link colored as 'satellite' (bit 0x04). The core links have their attribute-flags configured accordingly. Which tunnel configuration correctly enforces this path constraint using affinity and mask values?

A service provider has configured an MPLS TE tunnel from PE1 (headend) to PE3 (tailend) that traverses a lower-utilization path across the core. After bringing the tunnel up, the operator confirms it is signaled and operational, but a traceroute to prefixes reachable via PE3 shows traffic is still following the native IGP shortest path and not entering the tunnel. Which configuration on PE1 will cause the IGP to consider the TE tunnel as a valid path for reaching destinations downstream of the tailend?

An SP engineer configures an MPLS TE tunnel across a core where link IGP metrics and TE metrics were set independently. The IGP metric emphasizes hop count, while the TE metric was tuned to reflect actual link latency. The engineer wants the tunnel's CSPF path computation to prefer the lowest-latency path rather than the IGP-optimized path. Which configuration achieves this on the tunnel interface?

A service provider deploys an MPLS TE tunnel across a core where every physical link supports a 1500-byte IP MTU. Customers send 1500-byte IP packets with the DF bit set. After the TE tunnel is activated, users report that large TCP transfers fail while small pings succeed. What is the most likely cause and correct remedy?

A service provider runs RSVP-TE across its core. Two tunnels traverse a link with 100 Mbps of reservable bandwidth. Tunnel-A (already established) reserves 70 Mbps and was configured with setup priority 7 and hold priority 7. Tunnel-B is now being signaled with a 50 Mbps reservation, setup priority 3 and hold priority 3. The link cannot accommodate both reservations simultaneously. What happens when Tunnel-B is signaled?

A service provider engineer is configuring an RSVP-TE tunnel from PE1 to PE4 across an IS-IS core. The requirement is that the tunnel must traverse the router P2 (loopback 10.0.0.2) for regulatory reasons, but the path between the ingress and P2, and between P2 and the egress, should be computed dynamically by the ingress CSPF engine. Which explicit-path configuration approach satisfies this requirement?

A service provider is migrating from LDP to Segment Routing on an IS-IS enabled IOS XR core. An engineer configures a Prefix-SID index of 100 on the loopback of router PE1. The default SRGB across all routers is 16000-23999. When PE2 forwards traffic toward PE1's loopback, which MPLS label value will PE2 impose for the Prefix-SID?

A service provider is deploying Segment Routing over MPLS (SR-MPLS) with IS-IS as the IGP. During verification, an engineer notices that each router advertises Adjacency SIDs for its links. On router R3, an Adjacency SID value of 24012 is assigned to the link toward R4. The engineer expects this label to be usable network-wide for steering traffic. Which statement correctly describes the nature and scope of this Adjacency SID?

A service provider is deploying SR-MPLS across a multi-area IS-IS backbone. Two ABRs (PE-facing border routers R5 and R6) connect the access domain to the core. The design team wants ingress PEs to steer traffic toward either ABR interchangeably so that the loss of one ABR does not require a full recomputation of the SR label stack at the ingress node. Which segment routing mechanism should be configured on R5 and R6 to achieve this behavior?

A service provider is deploying SR-MPLS across its IS-IS backbone. All routers are Cisco IOS XR except one legacy node, R5, which was manually configured with an SRGB of 20000-28000, while every other router uses the default SRGB of 16000-23999. R5 advertises Prefix-SID index 100 for its loopback. A neighboring router, R4 (SRGB 16000-23999), needs to forward traffic toward R5's loopback. Which outgoing label will R4 impose, and why?

A service provider running Segment Routing over IS-IS observes brief traffic loss after a core link fails, even though TI-LFA is configured and provides a valid backup path. Packet captures show transient forwarding loops occurring a few hundred milliseconds after the failure, once IGP reconvergence completes and the PLR reverts from the backup path to the new primary path. Which feature should be enabled to eliminate these transient loops during the convergence transition?

A service provider runs Segment Routing over IS-IS on an IOS XR core. An SR-TE policy must force traffic across a specific parallel link between P2 and P3 that is not chosen by the IGP shortest path, without relying on the IGP to compute that hop. Which type of SID must the operator include in the policy's segment list to steer traffic onto that exact link?