Cisco Certified Network Professional (CCNP) Service Provider — Practice Set 1

Question 1

In a traditional service provider network hierarchy, which layer is responsible for aggregating traffic from multiple access nodes and forwarding it toward the core?

Accepted Answer

The aggregation layer (also called the distribution layer) sits between the access and core layers. It collects traffic from multiple access nodes such as DSLAMs, OLTs, and DSLAM concentrators and forwards the aggregated traffic toward the high-speed core. The access layer connects customer equipment; the core provides fast backbone transport; the peering layer handles inter-AS connectivity at IXPs.

Question 2

What is a key architectural advantage of the IOS XR modular operating system compared to monolithic router operating systems?

Accepted Answer

IOS XR is built on a microkernel (QNX-based) architecture where each routing protocol and system process runs in its own protected memory space. If one process (such as BGP or ISIS) crashes, it can be restarted independently without affecting other processes or requiring a system reboot. This provides far greater high-availability characteristics than monolithic OS designs where a single fault can crash the entire system.

Question 3

A service provider is deploying new routers and must choose between the Cisco ASR 9000 and NCS 5500 platforms. Which statement BEST describes the primary design roles of these two platforms in a SP network?

Accepted Answer

The ASR 9000 series is a modular, services-oriented platform typically deployed at the PE (Provider Edge) or aggregation layer, offering rich feature support for BNG subscriber management, L2VPN, and advanced MPLS. The NCS 5500 uses merchant silicon (Broadcom Jericho family) and is designed for high-density, high-throughput backbone/core deployments where forwarding scale and low power-per-bit matter more than deep service features. Both run IOS XR.

Question 4

A service provider requires that during a Route Processor (RP) switchover event on an IOS XR router, BGP sessions must not be reset and the forwarding plane must continue to forward packets using pre-installed FIB entries. Which IOS XR high-availability feature satisfies BOTH of these requirements?

Accepted Answer

IOS XR Non-Stop Routing (NSR) synchronizes full routing protocol state (including BGP TCP sessions, ISIS adjacency state, and OSPF neighbor state) between the active and standby RPs. When a switchover occurs, the standby RP takes over with all sessions intact — neighboring routers see no interruption. NSF (Non-Stop Forwarding) combined with Graceful Restart keeps forwarding intact but relies on peer cooperation and does reset protocol adjacencies. NSR requires no peer support and is therefore more robust.

Question 5

A carrier Ethernet service provider is offering a multipoint-to-multipoint service that connects multiple customer sites in a full-mesh topology over a shared Ethernet infrastructure. Which MEF service type does this describe?

Accepted Answer

MEF defines three primary Ethernet Virtual Connection (EVC) types. E-LINE provides point-to-point connectivity between two UNIs, equivalent to a leased-line replacement. E-LAN provides multipoint-to-multipoint connectivity where all UNIs can communicate with each other in a full mesh — suitable for LAN extension across multiple sites. E-TREE provides rooted-multipoint connectivity where leaf nodes can only communicate with the root, not with other leaves — useful for hub-and-spoke topologies such as broadband aggregation.

Question 6

Which IOS XR BGP configuration correctly establishes an iBGP session with a neighbor at 192.168.1.1 within AS 65000 and activates the IPv4 unicast address family?

Accepted Answer

In IOS XR, the BGP neighbor configuration uses a hierarchical syntax where the neighbor IP address is a sub-context and attributes (remote-as, address-family) are configured beneath it. The correct syntax is 'router bgp ' → 'neighbor ' → 'remote-as ' and 'address-family ipv4 unicast' within the neighbor stanza. Option A uses IOS (classic) syntax where 'activate' is needed in the address-family block. Option C uses invalid 'ip bgp' command. Option D configures eBGP (remote-as 65001 differs from local AS 65000).

Question 7

A BGP engineer needs to influence outbound traffic from AS 65100 so that all traffic destined for prefix 203.0.113.0/24 exits via a specific eBGP peer. The engineer decides to use LOCAL_PREF on the routes received from that peer. Which statement about LOCAL_PREF is correct?

Accepted Answer

LOCAL_PREF is a well-known discretionary BGP attribute that is exchanged only among iBGP peers within the same autonomous system. It is used to indicate the preferred exit point for traffic leaving the AS — the path with the highest LOCAL_PREF value is preferred. It is stripped from eBGP updates by Cisco IOS XR by default, so it cannot directly influence neighboring ASes. To influence inbound traffic from other ASes, operators use MED (MULTI_EXIT_DISC) or AS_PATH prepending instead.

Question 8

An IOS XR router is configured with the following IS-IS stanza. What does the 'net 49.0001.0100.0000.0001.00' statement define?

router isis CORE
 net 49.0001.0100.0000.0001.00
 is-type level-2-only
 interface GigabitEthernet0/0/0/0
  point-to-point
  address-family ipv4 unicast

Accepted Answer

In IS-IS, the Network Entity Title (NET) is an NSAP address that identifies the router within the IS-IS domain. The structure is: Area ID (variable length, here 49.0001), System ID (always 6 bytes, here 0100.0000.0001 — often derived from the loopback IP), and SEL byte (always 00 for a router). The area ID 49 indicates a private address space. The system ID must be unique within the IS-IS area. The SEL byte 00 designates this as a Network Entity Title rather than a transport connection.

Question 9

A network engineer configures BFD on an IOS XR router to detect BGP peer failures. The configuration includes 'bfd minimum-interval 300 multiplier 3' under the BGP neighbor. What does this configuration mean?

Accepted Answer

BFD timing parameters use milliseconds for the minimum-interval. The 'minimum-interval 300' sets the desired BFD hello packet interval to 300 ms (the actual negotiated value will be the higher of the two peers' configured minimums). The 'multiplier 3' means the session is declared down after 3 consecutive hellos are missed. Therefore the failure detection time is 300 ms × 3 = 900 ms (under 1 second), which is significantly faster than BGP hold-timer-based failure detection (default 90 seconds).

Question 10

In IOS XR QoS for MPLS networks, which field in the MPLS shim header is used to carry QoS markings, and what is the maximum number of classes it can distinguish?

Accepted Answer

The MPLS label stack entry contains a 3-bit EXP (Experimental) field, now officially called the Traffic Class (TC) field per RFC 5462. Because it is 3 bits wide it supports 8 values (0–7), enabling differentiation of up to 8 QoS classes across the MPLS core. In IOS XR, class-maps match on 'mpls experimental topmost' and policy-maps set or police based on EXP values. This allows end-to-end QoS to be enforced at label-switched routers without inspecting the inner IP header.

Question 11

A service provider needs to apply a BGP route policy in IOS XR that sets LOCAL_PREF to 200 for all routes matching a defined prefix-set named 'CUSTOMER_PREFIXES'. Which IOS XR route-policy syntax is correct?

Accepted Answer

IOS XR uses its own Route Policy Language (RPL), which is distinct from IOS route-maps. RPL uses a structured if-then-endif syntax where conditions reference named objects such as prefix-sets. The correct construct is 'if destination in then' followed by actions and 'endif', enclosed in 'route-policy ' and 'end-policy'. Option A uses IOS route-map syntax (not valid in IOS XR RPL). Option C is a classic IOS route-map (not IOS XR). Option D uses invalid RPL syntax.

Question 12

A service provider runs PIM-SM for IPv4 multicast in their core network. A customer reports that multicast traffic for group 239.1.1.1 is reaching some PE routers but not others. The engineer checks the RP and confirms the source is registering correctly. Which IOS XR command would BEST help verify whether the affected PE has a valid multicast routing tree entry for this group?

Accepted Answer

In IOS XR, 'show pim topology <group>' displays the PIM (S,G) and (*,G) state entries for a specific multicast group, including the upstream interface, downstream interfaces, and join/prune state. This directly reveals whether the affected PE has a valid tree entry for the group. 'show mrib route' shows the Multicast Routing Information Base entries but is a lower-level view. 'show ip route' is not a valid IOS XR command (IOS XR uses 'show route'). 'show multicast-routing' shows the MFIB (forwarding) table but 'show pim topology' is the most targeted command for debugging join state.

Question 13

An IOS XR BGP engineer needs to apply different route policies for inbound and outbound directions on eBGP peer 198.51.100.1. Which IOS XR configuration correctly attaches the policies 'IN-POLICY' and 'OUT-POLICY' to the neighbor?

Accepted Answer

In IOS XR, route-policy attachment to BGP neighbors must be done within the address-family sub-context of the neighbor stanza. The correct hierarchy is: router bgp → neighbor → address-family ipv4 unicast → route-policy in/out. Option C incorrectly places the route-policy attachment at the neighbor level outside the address-family context. Option B uses IOS route-map syntax (not valid in IOS XR). Option D places neighbor policy under the global address-family context rather than the specific neighbor stanza.

Question 14

A service provider uses IS-IS with segment routing enabled. An engineer notices that IS-IS adjacencies on a point-to-point link are not forming. The interface is configured with 'address-family ipv4 unicast' but no adjacency appears in 'show isis adjacency'. What is the most likely cause?

Accepted Answer

In IOS XR IS-IS configuration, the link type must be explicitly specified under the interface stanza. For point-to-point links, the 'point-to-point' keyword is required under the interface within the router isis context. Without it, IOS XR defaults to treating the interface as a broadcast network, which requires a DIS election and uses different hello PDUs. On a true point-to-point link this mismatch prevents adjacency formation. BFD is optional for adjacency formation, and segment routing configuration does not block IS-IS adjacency establishment.

Question 15

What is the function of the Label Information Base (LIB) in an MPLS LDP-enabled router?

Accepted Answer

The Label Information Base (LIB) in LDP is analogous to the RIB in IP routing — it stores all label bindings received from all LDP neighbors, including bindings that are not currently selected for forwarding. The Label Forwarding Information Base (LFIB), analogous to the FIB, stores only the active (best) forwarding entries actually used to switch packets. On IOS XR, 'show mpls ldp bindings' displays the LIB contents, while 'show mpls forwarding' displays the LFIB.

Question 16

In an MPLS L3VPN deployment on IOS XR, a PE router has the following VRF configuration:

vrf CUSTOMER_A
 rd 65000:100
 address-family ipv4 unicast
  import route-target 65000:100
  export route-target 65000:100

What is the purpose of the Route Distinguisher (RD) 65000:100?

Accepted Answer

The Route Distinguisher (RD) is an 8-byte value prepended to customer IPv4 prefixes to create globally unique VPN-IPv4 (RFC 4364) addresses in the MP-BGP VPNv4 address family. This allows multiple customers with overlapping IPv4 address spaces (e.g., both using 10.0.0.0/8) to coexist in the same PE's BGP table without conflict. The Route Target (RT) — configured as import/export route-target — controls which VRFs import which routes. The RD itself does not control route distribution.

Question 17

In Segment Routing MPLS (SR-MPLS) on IOS XR, what is the difference between a Prefix-SID and an Adjacency-SID?

Accepted Answer

In SR-MPLS, a Prefix-SID is associated with a router's loopback address and represents a path to that router via the shortest IGP path. The MPLS label for a Prefix-SID is computed as SRGB-base + index (e.g., SRGB base 16000 + index 1 = label 16001) and is consistent across all routers in the SR domain that share the same SRGB. An Adjacency-SID represents a specific link (adjacency) and is allocated dynamically from the local label space — it is locally significant and used to explicitly steer traffic over a specific link rather than the IGP-best-path.

Question 18

An engineer runs 'show bgp vpnv4 unicast all' on a PE router and sees VPN routes with next-hop addresses pointing to remote PE loopbacks but the routes are not being installed into the customer VRF. The VRF has correct import route-targets. What is the most likely cause?

Accepted Answer

In MPLS L3VPN, VPNv4 routes received via MP-BGP are installed into the VRF only if the BGP next-hop (the remote PE's loopback address) is resolvable with an MPLS label in the local router's LFIB. If LDP or SR-MPLS has not established a label-switched path to the remote PE loopback, the BGP route will appear in the VPNv4 BGP table but will not be installed into the VRF routing table — it will be marked as 'not in VRF' or 'no label'. The RD does not need to match between PEs; it only needs to be unique. iBGP is the correct and standard way to distribute VPNv4 routes between PEs via a Route Reflector.

Question 19

An IOS XR router has LDP graceful restart configured. During a planned RP switchover, what mechanism does LDP graceful restart use to maintain MPLS forwarding continuity?

Accepted Answer

LDP Graceful Restart (RFC 3478) works by having the restarting LSR notify its LDP neighbors (via the Fault Tolerant TLV in the LDP Initialization message) that it supports graceful restart. When the router restarts its LDP process (or during an RP switchover with NSF), neighbors hold their LDP session state and MPLS forwarding entries for a configurable 'recovery time' rather than withdrawing labels. The restarting router then re-establishes LDP sessions and resynchronizes its LIB within this window, maintaining uninterrupted MPLS forwarding throughout the process.

Question 20

In a VPLS (Virtual Private LAN Service) deployment, what is the purpose of split-horizon forwarding?

Accepted Answer

In a full-mesh VPLS topology, all PE routers are connected to each other via pseudowires. The split-horizon rule states that a PE must not forward traffic received on a pseudowire out to other pseudowires in the same VPLS instance — only to attachment circuits (customer-facing ports). Without split-horizon, a broadcast frame entering on one pseudowire would be replicated to all other pseudowires, which would then reflect it further, creating a broadcast storm loop. Split-horizon is applied automatically in BGP-based and LDP-based VPLS implementations.

Question 21

An engineer configures an MPLS pseudowire on IOS XR to connect two customer sites over an MPLS core. Which configuration snippet correctly defines an L2VPN xconnect for an MPLS-encapsulated pseudowire?

Accepted Answer

IOS XR uses the 'l2vpn' configuration hierarchy for pseudowires. The correct structure is: l2vpn → xconnect group → p2p → interface (attachment circuit) and neighbor ipv4 pw-id with encapsulation mpls specified under the neighbor. This differs from the IOS (classic) syntax shown in options B and D which use the legacy 'xconnect' interface sub-command with pseudowire-class. Option C is close but uses the older 'neighbor' syntax without 'ipv4' keyword required in current IOS XR.

Question 22

A service provider uses RADIUS Change of Authorization (CoA) with their BNG platform. Which RADIUS CoA operation would be used to re-apply updated service policy to an already-authenticated subscriber session without terminating the session?

Accepted Answer

RADIUS Change of Authorization (CoA), defined in RFC 5176, allows the RADIUS server (or a policy server such as Cisco ISE or a PCRF) to dynamically modify active subscriber sessions on the BNG. A CoA-Request message can update session attributes such as QoS policy, ACL, bandwidth profile, or service activation without disconnecting the subscriber. The Disconnect-Message (DM) terminates the session outright. CoA is critical for dynamic policy enforcement in subscriber management scenarios such as upgrading bandwidth tiers or applying parental controls.

Question 23

A carrier Ethernet engineer needs to configure OAM for a customer E-LINE service to enable end-to-end connectivity fault management. Which OAM standard operates at the Ethernet service layer (not the access link layer) and supports Continuity Check Messages (CCMs) between service endpoints?

Accepted Answer

ITU-T Y.1731 and IEEE 802.1ag (Connectivity Fault Management, CFM) define Ethernet service-layer OAM. They introduce Maintenance Domains (MD), Maintenance Associations (MA), and Maintenance End Points (MEP) that operate at different hierarchical levels of the network. Continuity Check Messages (CCMs) are sent periodically between MEPs to verify end-to-end service connectivity — analogous to keepalives. IEEE 802.3ah (EFM OAM) only operates between two directly connected devices on a single link. BFD and TWAMP are IP/UDP-based measurement tools, not Ethernet CFM protocols.

Question 24

An IOS XR router is being configured for model-driven telemetry. Which three components must be configured to stream operational data to an external collector?

Accepted Answer

IOS XR model-driven telemetry (MDT) requires three configuration elements: (1) a sensor-group that specifies one or more YANG sensor paths (e.g., Cisco-IOS-XR-infra-statsd-oper:infra-statistics/interfaces/interface/latest/generic-counters); (2) a destination-group that defines the external collector's IP address, port, and encoding/transport (gRPC/gRPC-TLS/TCP with GPB or JSON encoding); and (3) a subscription that associates the sensor-group with the destination-group and specifies the sample interval in milliseconds. This is configured under the 'telemetry model-driven' hierarchy in IOS XR.

Question 25

A service provider is evaluating Cisco Crosswork for their SP network. Which statement BEST describes the primary function of Cisco Crosswork Network Automation?

Accepted Answer

Cisco Crosswork Network Automation is a software platform designed for SP network operations. It integrates multiple applications including Crosswork Health Insights (telemetry-based monitoring), Crosswork Change Automation (automated playbook execution), Crosswork Optimization Engine (SR-TE traffic engineering), and Network Services Orchestrator (NSO) for device configuration management. It uses NETCONF/YANG, gRPC telemetry, and REST APIs to interact with IOS XR devices. Crosswork acts as a management and orchestration layer, not as a network device or routing protocol replacement.

350-501 SPCOR — Cisco Certified Network Professional (CCNP) Service Provider