350-501 Networking Practice Test 8 — 15 Questions

Question 1

A service provider is migrating from OSPF to IS-IS in the core. During the migration, both protocols are running. Some prefixes are learned via both OSPF and IS-IS. The network uses BGP for external routes and MPLS for VPNs. The engineer notices that BGP routes are pointing to a next-hop that is reachable via OSPF but not via IS-IS, causing traffic to be dropped. What is the best course of action to ensure that during the migration, BGP uses only one IGP protocol?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Increase the administrative distance of IS-IS to be higher than OSPF. Increasing the administrative distance of IS-IS to be higher than OSPF (e.g., from 115 to 120) makes OSPF routes preferred over IS-IS routes in the routing table. Since BGP uses the IGP route to resolve its next-hop, this ensures BGP always selects the OSPF path, preventing traffic drops when the next-hop is only reachable via OSPF. This is a simple, protocol-agnostic way to control IGP preference during a migration without altering route redistribution or tunneling.

Answer

Use BGP next-hop tracking to prefer IS-IS routes

Answer

Configure a route-map to selectively distribute routes

Answer

Implement MPLS TE tunnels for BGP next-hops

Question 2

A service provider's network uses MPLS TE with Fast Reroute (FRR) for link protection. After a fiber cut on a core link, the traffic is rerouted via a backup tunnel, but the backup tunnel's path suffers from high latency. The customer complains about poor voice quality. What can be done to avoid this scenario in future deployments?

Accepted Answer

Configure backup tunnels with explicit paths that avoid high-latency links. Configuring backup tunnels with explicit paths that avoid high-latency links ensures that when MPLS TE Fast Reroute (FRR) activates, the backup path meets the latency requirements for real-time traffic like voice. By specifying explicit routes (e.g., via the 'path-option' command with explicit-path objects), the operator can exclude links known to have high propagation delay or congestion, thus preventing the poor voice quality experienced after the fiber cut.

Answer

Increase the bandwidth reservations on the backup tunnel

Answer

Implement BGP PIC for edge routers

Answer

Use node protection instead of link protection

Question 3

A service provider is deploying multicast service for IPTV using PIM-SM with a single RP. During high traffic periods, the RP becomes overloaded. What is the most scalable solution to distribute the load across multiple RPs?

Accepted Answer

Use Anycast-RP. Anycast-RP allows multiple RPs to share the same IP address, enabling load distribution and redundancy. In PIM-SM, sources register with the nearest RP via unicast routing, and receivers join toward the same Anycast-RP address, which is routed to the closest physical RP. This distributes the registration and join processing load across multiple RPs without requiring protocol changes.

Answer

Use Bidirectional PIM

Answer

Increase the RP's memory and CPU

Answer

Use PIM-DM

Question 4

A service provider's network core runs IS-IS as the IGP. After adding a new router, some routers have incomplete LSP databases. The new router's interfaces are up, and IS-IS adjacency is up with neighbors. What is the cause? The network has a mix of interface MTUs, with some links having MTU 1500 and others 4470.

Accepted Answer

MTU mismatch on the interface. An MTU mismatch on the interface causes incomplete LSP databases. IS-IS uses a maximum LSP size derived from the interface MTU minus the IS-IS header overhead (typically 3 bytes for the LSP header). When a router with a smaller MTU (e.g., 1500) receives an LSP that was generated on a larger MTU link (e.g., 4470), the LSP may be too large to be stored or processed, leading to fragmentation or rejection. This results in an incomplete LSP database on some routers, even though adjacencies are up.

Answer

IS-IS overload bit set on the new router

Answer

IS-IS LSP flooding is inhibited on the new router

Answer

The new router's system-id is a duplicate

Question 5

A network engineer is troubleshooting an OSPF adjacency failure between two directly connected routers, R1 and R2. Both routers are configured with the same OSPF process ID and area. The engineer verifies that the interfaces are up and IP connectivity exists. Which configuration mismatch is most likely causing the adjacency to fail?

Accepted Answer

OSPF network type mismatch between the interfaces. An OSPF network type mismatch, such as one interface configured as point-to-point and the other as broadcast, prevents OSPF adjacency from forming. Options A, B, and C are incorrect: An MTU mismatch (A) usually allows adjacency to form but causes problems during LSA exchange; a passive interface (B) stops the router from sending hellos, so it wouldn't see the neighbor at all, but the question implies both interfaces are operational; an area ID mismatch (C) would prevent adjacency, but the scenario specifies that both routers are configured in the same area. Therefore, the most likely cause of the adjacency failure when interfaces are up and IP connectivity exists is a network type mismatch.

Answer

MTU mismatch between the interfaces

Answer

Passive interface configuration on one router

Answer

Area ID mismatch on the interfaces

Question 6

An SP engineer is designing a BGP-based MPLS L3VPN service. The PE routers are fully meshed via iBGP for VPNv4 routes, and an RR is deployed to reduce sessions. The engineer notices that all PE routers are receiving duplicate routes from different PEs, causing suboptimal path selection. Which BGP feature should be enabled to ensure proper load balancing across multiple equal-cost paths?

Accepted Answer

Enable BGP multipath on the PE routers. BGP multipath on PE routers enables the router to install multiple equal-cost paths into the routing table, allowing load balancing across them. In an MPLS L3VPN iBGP full mesh with an RR, duplicate routes are received from different PEs; BGP multipath selects and uses up to the configured number of equal-cost paths. Option A (BGP additional paths) allows advertising multiple paths but does not directly install them. Option B (second RR) reduces sessions but does not affect duplicate routes. Option C (ignoring interior cost) modifies best-path selection but does not enable multipath.

Answer

Enable BGP additional paths on the RR

Answer

Deploy a second RR to reduce the number of iBGP sessions

Answer

Configure the BGP best path selection algorithm to ignore interior cost

Question 7

Which TWO tasks are required when implementing segment routing in an MPLS network?

Accepted Answer

Configure the IGP (OSPF or IS-IS) with segment routing extensions. Options C and D are correct. Configuring the IGP (OSPF or IS-IS) with segment routing extensions is essential to advertise prefix-SIDs and adjacency-SIDs, enabling segment routing control plane. Enabling MPLS on all interfaces that participate in segment routing forwarding is required to enable MPLS forwarding for the SIDs. Option A (RSVP-TE) is not required because segment routing uses source-routed paths instead of RSVP-TE LSPs. Option B (CEF) is generally enabled by default on Cisco routers and is a prerequisite for MPLS forwarding, but it is not a task specific to implementing segment routing. Option E (LDP) is not required because segment routing replaces LDP with SID-based label distribution.

Answer

Configure RSVP-TE to establish LSPs

Answer

Enable CEF on all routers

Answer

Enable LDP on all routers and interfaces

Question 8

Based on the exhibit, what is preventing the BGP session from establishing?

Accepted Answer

The TTL security check is blocking the connection. The error message indicates the TTL security check is rejecting the connection because the incoming TTL is not 255 (as expected for an eBGP multihop session). The remote AS is shown as 65000, so it is eBGP. The neighbor is reached via a single-hop but the TTL check expects TTL=255, but the router sees a TTL less than that. Option A is wrong because no MD5 password mismatch is indicated; the error explicitly mentions TTL security. Option B is wrong because the update-source is not the issue; the TTL check is the direct cause. Option C is wrong because the error message does not mention prefix limits; it clearly states TTL security.

Answer

The BGP MD5 password is mismatched

Answer

The BGP update-source is misconfigured

Answer

The neighbor has reached its maximum prefix limit

Question 9

A service provider is experiencing intermittent multicast issues in their core network. They use PIM-SM with a static RP at 10.1.1.1. The multicast traffic originates from a source connected to PE1 and is received by customers connected to PE2. Recently, after a firewall upgrade between the PE routers and the core, some multicast streams stopped working, while others continue. The network team notices that 'show ip mroute' on PE2 shows the (*, G) entry but not the (S, G) entry for the affected groups. The RP is reachable via OSPF. The firewall logs show no dropped packets for known multicast addresses. Which action should the engineer take to restore full multicast forwarding?

Accepted Answer

Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic. The firewall upgrade likely blocked PIM register messages from PE1 to the RP, preventing the RP from learning about the source. By configuring a static RP on the customer-facing interface or using a different RP that can receive registration, the (S,G) state can be built. Option A is wrong because PIM-SM is correct for sparse-mode groups. Option C is wrong because adjusting timers would not fix the absence of (S,G). Option D is wrong because Auto-RP would add complexity and might be blocked by the firewall as well.

Answer

Increase the PIM register suppression interval on the source's first-hop router

Answer

Change the multicast mode from PIM-SM to PIM-DM on all interfaces

Answer

Enable Auto-RP on the network to dynamically learn the RP

Question 10

An SP engineer is configuring IS-IS on a router and specifies a NET address of 49.0001.1921.6800.1001.00. What does the '00' at the end represent?

Accepted Answer

SEL (NSAP Selector). The last two octets of an IS-IS NET address represent the SEL (NSAP Selector), which is always 00 for a router.

Answer

AFI

Answer

System ID

Answer

Area ID

Question 11

In an OSPF network, an engineer wants to minimize the size of the routing table in a non-backbone area while still allowing the area to learn external routes from other ASs. Which OSPF area type should be configured?

Accepted Answer

NSSA. A Not-So-Stubby Area (NSSA) allows importing external routes as Type 7 LSAs while still blocking Type 5 LSAs from other areas.

Answer

Stub area

Answer

Totally stubby area

Answer

Backbone area 0

Question 12

An SP network uses MPLS to provide IPv6 connectivity over an IPv4 backbone using 6PE. Which BGP extension is required on the provider edge routers to exchange IPv6 prefixes with an IPv4 next-hop?

Accepted Answer

BGP multiprotocol extensions (MP-BGP) with IPv6 address family. 6PE uses BGP multiprotocol extensions (MP-BGP) with an AFI of 2 (IPv6) and SAFI of 1 (unicast), and the next-hop is encoded as an IPv4 address using the 'next-hop' attribute.

Answer

BGP route reflection

Answer

BGP confederation

Answer

BGP prefix-independent convergence (PIC)

Question 13

In IS-IS, which type of LSP is used to advertise prefix information for IPv4 and IPv6 in a modern SP network that supports traffic engineering?

Accepted Answer

TLV 135 (Extended IP Reachability). TLV 135 for IPv4 (extended IP reachability) and TLV 236 for IPv6 (IPv6 reachability) are used in IS-IS for prefix advertisement.

Answer

TLV 2 (IS Neighbors)

Answer

TLV 1 (Area Addresses)

Answer

TLV 22 (IS Extended Neighbors)

Question 14

An OSPF network is configured with RSVP-TE. Which OSPF extension is used to advertise TE link attributes such as bandwidth and administrative group?

Accepted Answer

OSPF Type 10 Opaque LSA. OSPF TE extensions use Opaque LSAs (Type 9, 10, 11) to carry TE information. Type 10 (area-local) is commonly used for TE.

Answer

OSPF Type 1 LSA

Answer

OSPF Type 5 LSA

Answer

OSPF Type 7 LSA

Question 15

In BGP route reflection, what is the purpose of the ORIGINATOR_ID attribute?

Accepted Answer

To prevent routing loops by ensuring that a route is not reflected back to its originator. ORIGINATOR_ID is a non-transitive optional attribute that identifies the router that originated the route into the iBGP domain, used to prevent loops.

Answer

To indicate the next-hop IP address

Answer

To identify the route reflector

Answer

To carry the community value