350-501 Networking Practice Test 7 — 15 Questions

Question 1

Which THREE of the following are characteristics of Segment Routing (SR-MPLS) compared to traditional MPLS with LDP?

Accepted Answer

The IGP (IS-IS or OSPF) is used to advertise prefix SIDs.. In Segment Routing (SR-MPLS), the IGP (IS-IS or OSPF) is extended to advertise prefix Segment Identifiers (SIDs), which are used to build MPLS labels directly from the IGP database. This eliminates the need for a separate label distribution protocol like LDP, as the IGP itself carries the label information within its link-state advertisements.

Answer

It can only be used for transit, not for egress routers.

Answer

Labels are distributed via multicast to all routers.

Question 2

Which THREE are valid reasons for using the 'ipv6 unicast-routing' command on a Cisco router?

Accepted Answer

It enables global IPv6 routing.. The 'ipv6 unicast-routing' command globally enables IPv6 unicast routing on the router, which is necessary for the router to forward IPv6 packets between interfaces. Without this command, the router will not process or forward IPv6 unicast traffic, even if IPv6 addresses are configured on interfaces.

Answer

It enables IPv6 on all interfaces.

Answer

It enables IPv6 multicast routing.

Question 3

Refer to the exhibit. Which of the following is true about the BGP table?

Accepted Answer

The prefix 192.168.3.0/24 has two paths, with the best path selected based on some attribute.. The exhibit shows two paths for prefix 192.168.3.0/24: one via 10.4.4.4 with AS path 300 400 and one via 10.5.5.5 with AS path 300 500. BGP selects the best path based on a sequence of attributes (e.g., weight, local preference, AS path length, origin code, MED, etc.). Option B is correct because the prefix has two paths and BGP will choose one as best based on these attributes.

Answer

The route to 192.168.3.0/24 with path 300 400 has an origin of IGP.

Answer

The prefix 192.168.2.0/24 is not the best path because it has a lower local preference.

Answer

The route to 192.168.3.0/24 via 10.4.4.4 is the best path because it has a shorter AS path.

Question 4

Refer to the exhibit. Which statement is correct about the LDP bindings for prefix 10.0.0.0/24?

Accepted Answer

There are two remote label bindings for this prefix from different LSRs.. The exhibit shows two remote label bindings for prefix 10.0.0.0/24, one with an incoming label of 18 from LSR 192.168.1.2 and another with an incoming label of 19 from LSR 192.168.1.3. This indicates that two different LSRs have advertised label bindings for the same prefix, which is typical in an MPLS network with multiple paths. Therefore, option A is correct.

Answer

Both remote bindings are from the same LSR.

Answer

The local label for this prefix is 18.

Answer

The router is performing PHP for this prefix.

Question 5

A service provider operates a large MPLS VPN network using OSPF as the IGP and LDP for label distribution. The PE routers (PE1, PE2, PE3) are connected to a core of P routers. Recently, a new link was added between P2 and P3. After the link came up, the engineering team noticed that several VPN routes that were previously reachable via PE2 are now being blackholed when traffic is sent from PE1 to those prefixes. The teams verify that the VPNv4 routes are present in the BGP table on PE1 with valid next-hops, but traffic fails. The traceroute from PE1 to the CE behind PE2 stops at P2. The show mpls forwarding-table on P2 shows the correct label for the VPN prefix, but the outgoing interface is null. Which action should the engineer take to resolve the issue without causing additional disruption?

Accepted Answer

Clear LDP neighbor sessions on P2 to re-initiate label exchange.. The issue is that P2 has a label for the VPN prefix but a null outgoing interface, indicating an LDP label mapping problem. Clearing LDP neighbor sessions on P2 forces re-establishment of LDP sessions and re-exchange of label bindings, which should resolve the missing or incorrect label mapping for the BGP next-hop without disrupting other services.

Answer

Add a static route on P2 for the BGP next-hop pointing to Null0.

Answer

Shut down the new link between P2 and P3.

Answer

Clear BGP sessions on PE1 to force re-advertisement of VPN routes.

Question 6

A service provider wants to ensure that customer traffic is not impacted during a planned maintenance on a core LSR in an MPLS network. Which MPLS feature should be used?

Accepted Answer

MPLS TE Fast Reroute. MPLS TE Fast Reroute (FRR) is the correct feature because it provides local protection against link or node failures by pre-computing backup paths (bypass tunnels) that are activated within 50 milliseconds of a failure. This ensures that customer traffic is not impacted during planned maintenance on a core LSR, as the backup path is already in place and can be triggered by a manual administrative action (e.g., shutting down the interface) to seamlessly redirect traffic before the maintenance begins.

Answer

MPLS TTL propagation

Answer

MPLS LDP synchronization

Answer

MPLS OAM

Question 7

A service provider's network is experiencing suboptimal routing for BGP prefixes received from a customer. The customer is multihomed to two different PE routers in the same AS. Which BGP feature can ensure that traffic ingresses via the correct PE?

Accepted Answer

MED. MED (Multi-Exit Discriminator) is the correct BGP attribute for influencing inbound traffic from a multihomed customer. When the customer advertises the same prefix to two PE routers in the same AS, setting a lower MED value on one PE will cause the customer's upstream AS to prefer that path for returning traffic. This allows the service provider to control which PE receives the ingress traffic without altering any outbound routing policies.

Answer

Local Preference

Answer

AS-path prepend

Answer

Weight

Question 8

A network engineer is troubleshooting OSPFv3 on a service provider's IPv6 network. The router shows that OSPFv3 adjacency never reaches FULL, says 'Init'. The neighbor is directly connected over a point-to-point link. What is the most likely cause?

Accepted Answer

Missing 'ipv6 ospf' interface command on one side. The 'Init' state in OSPFv3 indicates that the router has received a Hello packet from the neighbor but the neighbor has not received a Hello packet back. On a point-to-point link, the most common cause is that the 'ipv6 ospf area ' interface command is missing on one side, which prevents OSPFv3 from sending Hellos on that interface. Without this command, the interface is not enabled for OSPFv3, so the neighbor never sees a Hello and the adjacency cannot progress to FULL.

Answer

LSA throttling prevents exchanges

Answer

Mismatched OSPF router-id

Answer

Mismatched area IDs

Question 9

A service provider is implementing MPLS L3VPN and needs to ensure that BGP route advertisement uses a specific next-hop. Which technique ensures BGP advertises the PE-CE next-hop instead of the PE-PE loopback?

Accepted Answer

next-hop-self. In MPLS L3VPN, the PE-CE next-hop (the PE interface facing the CE) must be advertised to the remote PE so that the remote PE knows to forward traffic directly to the local PE's CE-facing interface. The `next-hop-self` command on the PE forces BGP to set the next-hop to the PE's own IP address (typically the loopback or the interface used for BGP peering), which overrides the default behavior of preserving the original next-hop. This ensures that the remote PE uses the correct next-hop for VPN traffic.

Answer

Route-map with set next-hop

Answer

next-hop-unchanged

Answer

Disable next-hop-check

Question 10

When implementing MPLS TE tunnels in a service provider core, what is the purpose of the 'affinity' attribute?

Accepted Answer

To define administrative groups for link inclusion/exclusion. The 'affinity' attribute in MPLS TE is used to define administrative groups (also known as link colors) that allow you to include or exclude specific links from a TE tunnel path based on user-defined properties. This enables traffic engineering policies such as forcing traffic to avoid certain links or preferring links with specific characteristics, without modifying the underlying IGP metric.

Answer

To set the color of the tunnel

Answer

To adjust the cost of TE tunnels

Answer

To bind tunnels to specific interfaces

Question 11

Which TWO of the following are characteristics of MPLS LDP?

Accepted Answer

Label distribution based on IP routing table. B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.

Answer

Distributes labels for BGP routes

Answer

Requires an IGP like OSPF for session reachability

Answer

TCP for both discovery and session

Question 12

Which TWO BGP path attributes are considered before AS-path length in the default best-path selection process?

Accepted Answer

Weight. Weight (D) is a Cisco-proprietary attribute that is checked first in the BGP best-path selection process, before AS-path length. Local Preference (E) is the second attribute evaluated, also preceding AS-path length. Both are considered before the AS-path length comparison in the default BGP decision process.

Answer

Origin code

Answer

Community

Answer

MED

Question 13

Which THREE of the following are required for successful operation of OSPFv3 in an IPv6 service provider network?

Accepted Answer

Interface activation with 'ipv6 ospf area '. OSPFv3 requires interface-level activation using the 'ipv6 ospf area ' command to enable the OSPFv3 process on a specific interface and associate it with an area. Without this, the interface will not participate in OSPFv3 neighbor discovery or routing updates, even if the process is configured globally.

Answer

OSPFv3 authentication configured

Answer

OSPFv3 LSA type 9 for link-local addresses

Question 14

A service provider has recently deployed MPLS L3VPN to provide IP connectivity to multiple enterprise customers. One customer reports that they cannot reach a remote site that is connected to a different PE router. The engineer checks the BGP VPNv4 table on the customer's PE and sees the route for the remote site, but the next-hop is unreachable. The interface between the PE and P routers is up/up, and IGP reachability to the PE's loopback is fine. What is the most likely cause? Consider that the network uses LDP for label distribution and OSPF as the IGP.

Accepted Answer

MPLS LDP is not enabled on the interface connecting to the P router. Since the interface between the PE and P routers is up/up and IGP reachability to the PE's loopback is fine, the issue is that MPLS LDP is not enabled on that interface. Without LDP, the PE cannot advertise a label for its loopback to the P router, so the P router cannot push the correct label for packets destined to the remote PE. This makes the BGP VPNv4 next-hop unreachable even though the route itself is present in the table.

Answer

BGP next-hop-self is not configured on the PE

Answer

The VRF route-target import/export is misconfigured

Answer

The PE router does not have an LSP to the remote PE

Question 15

A network engineer is troubleshooting a BGP convergence issue in a large service provider network. After a link failure in the core, BGP sessions between route reflectors take a long time to reconverge. The RRs are receiving updates from many clients. Which technology can be implemented to improve convergence time? The network currently uses standard BGP with default timers.

Accepted Answer

BGP Add-Path. BGP Add-Path allows a route reflector to advertise multiple paths for the same prefix to its clients, which reduces the need for clients to re-advertise updates after a failure. This speeds up convergence by enabling the route reflector to immediately select and propagate an alternate path without waiting for BGP reconvergence from other clients. In large service provider networks with many clients, this minimizes the delay caused by the route reflector having only a single best path per prefix.

Answer

BGP TCP MD5 authentication

Answer

BGP route-refresh

Answer

BGP next-hop-self