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Which access technology is commonly used in service provider networks to deliver high-speed internet over existing telephone lines?
DSL uses telephone lines for broadband.
Why this answer
DSL (Digital Subscriber Line) uses existing telephone lines to provide high-speed internet access.
Refer to the exhibit. An engineer configures MPLS LDP on a router. The router has two interfaces with IP addresses 10.0.0.1/30 and 10.0.0.5/30. The engineer notices that LDP sessions are not established. The OSPF neighbor adjacencies are up. What is the most likely cause?
Correct. LDP requires explicit interface-level activation using the `mpls ip` command. Even if LDP is enabled globally, sessions will not form on interfaces where `mpls ip` is missing.
Why this answer
D is correct because MPLS LDP requires explicit interface-level activation under the `mpls ip` command. Even if OSPF adjacencies are up and the global LDP process is configured, LDP will not form sessions on interfaces where `mpls ip` is missing. The engineer likely enabled LDP globally but forgot to enable it on the specific interfaces, which is a common oversight.
Exam trap
Cisco often tests the distinction between global LDP configuration and interface-level activation, leading candidates to assume that enabling LDP globally is sufficient for session establishment.
How to eliminate wrong answers
Option A is wrong because OSPF network statements are unrelated to LDP session establishment; OSPF adjacencies are already up, proving the interfaces are correctly covered. Option B is wrong because an MPLS MTU of 1500 is standard and does not prevent LDP session establishment; MTU issues typically cause label-switched path (LSP) problems, not LDP hello or session failures. Option C is wrong because a small label range would cause label allocation failures, not prevent LDP sessions from forming; LDP sessions use TCP port 646 and are independent of label range size.
Which TWO are characteristics of Segment Routing?
SR specifies path in the packet header.
Why this answer
Segment Routing (SR) is a source-routing paradigm where the ingress node encodes a list of segments (instructions) into the packet header, guiding the packet through the network without intermediate routers needing to maintain per-flow state. Option B is correct because SR inherently provides source routing capability: the source node determines the path by stacking segment identifiers (SIDs), and transit nodes simply forward based on the top SID.
Exam trap
Cisco often tests the misconception that Segment Routing requires a centralized controller (like SDN) or RSVP-like signaling, when in fact it can run purely with IGP-based distributed control and source-routed packet headers.
An SP engineer notices that BGP routes from a CE are not being installed in the VRF routing table, although the BGP session is established. The VRF configuration includes route-target import 100:1. The CE is sending routes with RT 100:1. What is the most likely cause?
If an import map is configured, only routes matching the map are imported.
Why this answer
Even though the BGP session is up and the CE is sending routes with the correct route-target (RT 100:1), the VRF may have an import map applied. An import map filters which received VPN routes are actually installed into the VRF routing table. If the route does not match the conditions defined in that import map, it will be accepted by BGP but not installed, which explains the symptom.
Exam trap
Cisco often tests the distinction between a BGP session being up and routes being installed in the VRF, leading candidates to overlook the import map as a filtering mechanism that can block routes even when the RT matches.
How to eliminate wrong answers
Option A is wrong because a BGP route update delay (e.g., bgp update-delay) affects the initial convergence of the BGP table, not the permanent filtering of routes based on RT or import maps. Option C is wrong because the VRF name is a local label and does not affect route import; the import is controlled by RT matching, not the VRF name. Option D is wrong because the Route Distinguisher (RD) is used to make routes unique across VRFs, but it is not required for route installation; a missing RD would prevent the VRF from being configured properly, but the question states the VRF is configured with RT import, implying RD is present.
A large service provider operates an MPLS L3VPN network with multiple Route Reflectors (RRs) in the core. The network uses BGP as the control plane for both IPv4 unicast and VPNv4 routes. Recently, one of the RRs started flapping, causing route withdrawals to many clients. The network architect wants to improve stability. The RRs are fully meshed with each other and clients are configured as route-reflector clients. The RRs have both IPv4 and VPNv4 address families enabled. Which action should be taken to minimize the impact of an RR failure?
This provides redundancy; clients receive routes from multiple RRs, and if one RR fails, routes are still available via the other.
Why this answer
Configuring client-to-client reflection on the RRs and ensuring each PE is a client of at least two RRs provides redundancy; if one RR fails, routes are still available via the other RR. BGP PIC helps fast failover but does not prevent route withdrawal impact; add-paths increases paths but not redundancy; same cluster ID reduces redundancy.
Refer to the exhibit. Based on the exhibit, what is the most likely reason for no label bindings?
Label filtering on the remote peer can prevent advertisement of labels.
Why this answer
The exhibit shows that the local router has not received any label bindings from the remote peer (as evidenced by the empty 'show mpls ldp bindings' output), which indicates that the remote peer is not advertising labels. This is typically due to an outbound label filtering policy applied on the remote peer, such as 'mpls ldp neighbor [neighbor-id] label-filter' or 'mpls ldp label-filter' that rejects label advertisements to the local router. The local router's LDP session may be established (based on 'show mpls ldp neighbor'), but the remote peer is actively preventing label distribution.
Exam trap
Cisco often tests the distinction between LDP session establishment and actual label advertisement. Candidates may mistakenly attribute missing label bindings to local IGP routes or LDP session issues, but the root cause can be label filtering applied on the remote peer, which prevents the advertisement of labels altogether.
How to eliminate wrong answers
Option A is wrong because the local router has not disabled label advertisements; the 'show mpls ldp bindings' output shows that the local router is actively receiving label bindings from the remote peer, which would not happen if label advertisements were disabled globally or per interface. Option B is wrong because the IGP routes are present in the routing table; the 'show ip route' output would show the routes for the prefixes listed in the bindings, and LDP relies on IGP reachability to establish sessions and exchange labels, so missing IGP routes would prevent LDP from even establishing a session. Option C is wrong because the LDP session is fully established; the 'show mpls ldp neighbor' output would show the session state as 'Operational' and the 'show mpls ldp bindings' output confirms that label bindings have been exchanged, which requires a fully established LDP session.
An SP uses Cisco ASR 9000 routers with IOS XR. Which statement correctly describes the commit/rollback configuration model in IOS XR?
This is the commit/rollback model.
Why this answer
IOS XR requires a 'commit' to apply changes; it supports rollback to previous committed configurations, allowing safe changes.
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