350-501 Practice Test 26 — 15 Questions

Question 1

A customer reports that CE routers attached to PE1 and PE2 in the same VRF cannot ping each other. Based on the exhibit, what is the most likely cause?

Accepted Answer

LDP is not enabled on the core interfaces between PE1 and PE2. B is correct because LDP must be enabled on the core interfaces between PE1 and PE2 to establish LDP sessions, which are required to exchange MPLS labels for the transport LSP. Without LDP, the MPLS forwarding path between the PEs is broken, preventing CE-to-CE ping even if BGP VPNv4 routes are correctly advertised.

Answer

Missing address-family ipv4 for BGP neighbor

Answer

Mismatched route distinguisher (RD) values on PE1 and PE2

Answer

Route target (RT) import/export mismatch

Question 2

A service provider is deploying an MPLS L3VPN solution for a customer with two sites. The PE routers are running OSPF as the IGP and LDP for label distribution. The provider wants to ensure that customer traffic is load-balanced across two equal-cost paths between the PEs. The network team notices that all traffic is taking only one path despite the IGP showing equal-cost routes. The team has verified that the MPLS forwarding table on the P router shows only one label entry for the BGP next-hop. The team suspects a load-balancing issue. Which action best resolves the problem?

Accepted Answer

Enable per-flow load balancing on all routers with 'load-balance per-flow'. Option B is correct because the issue is that MPLS L3VPN traffic is not being load-balanced despite equal-cost IGP paths. This typically occurs when the MPLS forwarding table on the P router has only one label entry for the BGP next-hop, indicating that per-flow load balancing is not enabled. Enabling 'load-balance per-flow' on all routers ensures that the CEF (Cisco Express Forwarding) uses per-flow hashing to distribute traffic across multiple equal-cost LSPs (Label Switched Paths), rather than per-packet or default behavior that may favor a single path.

Answer

Increase the OSPF maximum-paths value to 4

Answer

Enable BGP additional-paths on the PE routers

Answer

Configure LDP to use the 'mpls ldp igp sync' command

Question 3

A service provider is deploying EVPN-MPLS for L2VPN services. The customer requires that MAC addresses learned from one PE are not advertised to other PEs unless they are active. Which EVPN route type is used for MAC address withdrawal?

Accepted Answer

Route Type 2: MAC/IP Advertisement. Option C is correct because EVPN Route Type 2 (MAC/IP Advertisement) carries both MAC addresses and their associated IP addresses, and it supports a 'sticky' or 'withdraw' mechanism via the BGP Withdraw message. When a MAC address becomes inactive on a PE, the PE sends a BGP Withdraw for the specific Route Type 2 route, effectively removing that MAC from the control plane of other PEs. This ensures that only active MAC addresses are advertised, meeting the customer requirement.

Answer

Route Type 5: IP Prefix

Answer

Route Type 1: Ethernet Auto-Discovery (A-D)

Answer

Route Type 3: Inclusive Multicast Ethernet Tag

Question 4

An ISP is designing an MPLS core network and needs to choose an IGP that supports fast convergence. Which IGP meets this requirement and is most commonly used in MPLS core networks?

Accepted Answer

IS-IS. IS-IS is the correct choice because it is a link-state IGP that inherently supports fast convergence through mechanisms like incremental SPF (iSPF) and prefix-independent convergence (PIC). It is widely deployed in MPLS core networks due to its scalability, extensibility via TLVs, and native support for MPLS Traffic Engineering (MPLS-TE) without requiring additional protocol extensions like OSPF's opaque LSA.

Answer

OSPFv3

Answer

EIGRP

Answer

RIPng

Question 5

A service provider is deploying a new MPLS core network. The network has four routers: P1, P2, PE1, and PE2. OSPF is used as the IGP. The engineer configures MPLS LDP on all interfaces. After enabling LDP, the engineer notices that the LDP session between P1 and P2 is established, but no labels are exchanged for the loopback0 interfaces of PE1 and PE2. The loopback0 addresses are advertised in OSPF. The engineer verifies that the OSPF routes are present in the routing table of all routers. What is the most likely reason for the missing labels?

Accepted Answer

An access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes.. The most likely reason is that an access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes. LDP by default advertises labels for all prefixes in the routing table, but the 'advertise-labels' command can filter which prefixes receive labels. If the loopback0 prefixes of PE1 and PE2 are denied by such an access-list, no labels will be advertised for them, even though OSPF routes are present.

Answer

LDP is using UDP for label exchange.

Answer

The OSPF cost to the loopbacks is too high.

Answer

The loopback interfaces are not enabled with 'mpls ip'.

Question 6

Which THREE are valid methods for label distribution in an MPLS network?

Accepted Answer

Segment Routing via IGP. Segment Routing via IGP (Option A) is a valid label distribution method because it uses the IGP (OSPF or IS-IS) to advertise prefix-SIDs, which are MPLS labels. The IGP distributes these labels as part of its link-state database, eliminating the need for a separate label distribution protocol like LDP. This is defined in RFC 8660 and RFC 8665 for OSPF and IS-IS respectively.

Answer

BGP

Answer

TDP

Question 7

A service provider is deploying segment routing (SR) with MPLS data plane. The network uses OSPF as the IGP. Which configuration is required to enable SR-MPLS and ensure that routers advertise prefix-SIDs for their loopback interfaces?

Accepted Answer

Configure 'segment-routing mpls' globally and assign a prefix-SID to the loopback interface under OSPF.. Option D is correct because to enable SR-MPLS with OSPF, you must globally enable segment routing with the 'segment-routing mpls' command, and then assign a prefix-SID to the loopback interface under the OSPF process using the 'prefix-sid' command. This ensures that routers advertise the prefix-SID for their loopback via OSPF extensions, which is the fundamental requirement for SR-MPLS operation without LDP.

Answer

Configure 'mpls ip' on all interfaces and 'router ospf 1 mpls ldp auto-config'.

Answer

Configure 'label mode per-prefix' under the OSPF process and assign a label to the loopback.

Answer

Configure 'segment-routing global-block 16000 23999' globally and enable 'mpls ip' on all interfaces.

Question 8

An engineer is configuring an MPLS L3VPN and needs to ensure that the PE router installs VPNv4 routes from a remote PE into the VRF of a customer. The remote PE sends a VPNv4 route with route-target 100:1. Which configuration on the local PE causes the route to be imported into the VRF?

Accepted Answer

vrf definition CUSTOMER
 rd 100:1
 route-target import 100:1. Option C is correct because the `route-target import 100:1` command under the VRF definition configures the local PE to accept VPNv4 routes that carry the specified route-target (100:1) from the remote PE. This import RT must match the export RT of the remote PE for the route to be installed into the VRF's routing table. The `rd 100:1` defines the route distinguisher, which is separate from the RT and ensures uniqueness of the VPNv4 prefix.

Answer

router bgp 100
 address-family ipv4 vrf CUSTOMER
 route-target import 100:1

Answer

vrf definition CUSTOMER
 rd 100:1
 route-target both 100:1 route-map IMPORT

Answer

vrf definition CUSTOMER
 rd 100:1
 route-target export 100:1

Question 9

Refer to the exhibit. A PE router in an MPLS L3VPN network shows the above output. The VRF CUSTOMER contains two routes. Which statement about forwarding for these routes is true?

Accepted Answer

Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.. The output shows that for the VRF CUSTOMER, the route to 10.1.1.0/24 has a next hop of 192.168.1.2 and is associated with label 3 (implicit null). In MPLS L3VPN, label 3 instructs the upstream router to pop the MPLS label and forward the packet using IP routing to the directly connected next hop. Therefore, traffic to 10.1.1.0/24 will be label-switched with an implicit null label, meaning the label is removed before forwarding to the CE router.

Answer

Both routes are reachable via MPLS.

Answer

Traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label.

Answer

Traffic to 10.2.2.0/24 will be forwarded using IP routing.

Question 10

Based on the exhibit, which prefix is NOT reachable via MPLS forwarding?

Accepted Answer

10.5.5.5/32. Option D (10.5.5.5/32) is correct because the exhibit shows that the LDP label bindings are only present for prefixes 10.1.1.1/32, 10.2.2.2/32, and 10.3.3.3/32. The prefix 10.5.5.5/32 is not in the LDP label information base (LIB), meaning no MPLS label has been assigned to it, so it cannot be forwarded via MPLS and must be forwarded using standard IP routing instead.

Answer

10.1.1.1/32

Answer

10.3.3.3/32

Answer

10.2.2.2/32

Question 11

Based on the exhibit, what is the purpose of the 'mpls ldp neighbor ... password cisco' commands?

Accepted Answer

To authenticate the LDP session using MD5.. The 'mpls ldp neighbor ... password cisco' command configures a Message Digest 5 (MD5) authentication password for the LDP session with a specific neighbor. This ensures that the TCP connection used for LDP exchanges is authenticated, preventing spoofed or unauthorized LDP messages from being accepted, as defined in RFC 5036.

Answer

To synchronize LDP and IGP convergence.

Answer

To control the label allocation for prefixes from that neighbor.

Answer

To enable LDP session protection for the neighbor.

Question 12

A service provider is deploying L3VPN with inter-AS option B (ASBR-to-ASBR). Which TWO statements are true about this design?

Accepted Answer

ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes.. In inter-AS Option B, ASBRs peer using eBGP and exchange labeled VPN-IPv4 prefixes (AFI 1, SAFI 128). This allows the VPNv4 routes to be carried across AS boundaries without requiring a full mesh of MP-IBGP between PEs, as the ASBRs re-advertise the routes with a new next-hop and perform label allocation for the VPN labels.

Answer

The VPN label is removed by the ASBR before forwarding to the neighbor AS.

Answer

Route reflectors are required to propagate VPN routes between ASes.

Answer

ASBRs exchange unlabeled IPv4 routes and use MP-BGP to carry VPNv4 routes.

Question 13

A network engineer is troubleshooting MPLS LSP connectivity. The MPLS LDP session is up on both endpoints, but some MPLS labels are missing in the LFIB. Which configuration change would most likely resolve the issue?

Accepted Answer

Enable 'mpls ldp advertise-labels'. Option B is correct because the 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.

Answer

Configure 'label protocol ldp' on all routers

Answer

Configure 'mpls ip' on all interfaces

Answer

Enable 'mpls ldp explicit-null' on the ingress

Question 14

Drag and drop the steps to configure a GRE tunnel on a Cisco router into the correct order.

Question 15

Drag and drop the steps to configure a VLAN on a Cisco switch into the correct order.

350-501 Practice Test 26 — 15 Questions

Exhibit

350-501 Practice Test 26 — 15 Questions

Exhibit