350-401 MPLS — Timed 30 Questions

Question 1

An engineer is troubleshooting an MPLS VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are running OSPF with the CE routers. On PE1, the 'show ip route vrf CUSTOMER' output shows 10.2.2.0/24 as an OSPF route, but the prefix is not present in the global BGP table. What is the most likely cause?

Accepted Answer

Redistribution from OSPF into BGP under the VRF is not configured on PE1.. In MPLS L3VPN, CE routes must be redistributed into BGP (VPNv4) on the PE. If OSPF routes are present in the VRF but not in BGP, redistribution is missing. Option A correctly identifies this. Option B is wrong because OSPF is running; Option C is irrelevant; Option D would affect label allocation, not route advertisement.

Answer

The OSPF adjacency between PE1 and CE1 is down.

Answer

The VRF forwarding table on PE1 is full.

Answer

MPLS LDP is not enabled on the PE1-CE1 link.

Question 2

A service provider uses MPLS L3VPN with OSPF as the PE-CE routing protocol. A customer reports that a new subnet added on CE1 is not reachable from CE2, even though the PE1 router has the route in its VRF and BGP table. The 'show ip bgp vpnv4 vrf CUSTOMER' on PE2 shows the prefix with a valid next-hop. What should the engineer check next?

Accepted Answer

Verify that LDP has allocated a label for the BGP next-hop address on PE2.. In MPLS L3VPN, even if BGP has the route, the packet forwarding requires a valid MPLS label for the next-hop. If LDP is not resolving the BGP next-hop, the route is not usable. Option A is correct. Option B is wrong because OSPF is fine; Option C is wrong because the route is in BGP; Option D is wrong because redistribution is already working.

Answer

Check if OSPF is redistributed into BGP on PE1.

Answer

Ensure the route is present in the global BGP table on PE2.

Answer

Confirm that the VRF on PE2 has the correct route-target import.

Question 3

An engineer is designing an MPLS L3VPN service for a customer that requires overlapping IP addresses between two sites. The customer uses OSPF as the PE-CE protocol. The engineer configures VRFs on the PE routers and assigns unique route distinguishers (RDs) and route targets (RTs). However, the customer reports that routes from one site are not being installed in the other site's VRF. What is the most likely cause?

Accepted Answer

The route-target export on PE1 does not match the route-target import on PE2.. In MPLS L3VPN, route targets control the import/export of routes between VRFs. If the RTs are not configured correctly, routes will not be exchanged. Option A is correct. Option B is wrong because overlapping addresses are handled by VRFs; Option C is wrong because OSPF can handle overlapping addresses with proper configuration; Option D is wrong because RDs only make prefixes unique, they do not control route exchange.

Answer

The overlapping IP addresses cause a routing loop in OSPF.

Answer

OSPF cannot carry overlapping prefixes in different VRFs.

Answer

The route distinguisher is not unique between the two sites.

Question 4

A network engineer is configuring MPLS TE (Traffic Engineering) in an MPLS core to optimize bandwidth utilization. After enabling MPLS TE on all core routers and configuring tunnels, the engineer notices that traffic is not being rerouted when a link fails. The 'show mpls traffic-eng tunnels' shows the tunnels are up but not using the backup path. What is the most likely missing configuration?

Accepted Answer

MPLS TE FRR (Fast Reroute) is not configured on the tunnels.. MPLS TE requires explicit backup path configuration, such as FRR (Fast Reroute) or backup tunnels, to provide protection. Without it, traffic will not be rerouted automatically. Option A is correct. Option B is wrong because LDP is not required for TE; Option C is wrong because RSVP is the signaling protocol, not the issue; Option D is wrong because link-state routing is fine.

Answer

LDP is not enabled on the core interfaces.

Answer

RSVP is not configured on the core routers.

Answer

OSPF is not configured with MPLS TE extensions.

Question 5

An enterprise is implementing MPLS L3VPN to connect multiple branch offices. The PE routers are using eBGP to exchange VPNv4 routes. The engineer notices that some VPN routes are not being advertised to the remote PE. The 'show bgp vpnv4 unicast all' on the local PE shows the routes as valid but not best. What is the most likely reason?

Accepted Answer

The route has a higher local preference than the best path.. In BGP, only the best path is advertised to peers. If a route is valid but not best, it may be due to a higher local preference or other BGP path selection criteria. Option A is correct. Option B is wrong because the route is valid; Option C is wrong because the route is present; Option D is wrong because MED is not typically set by default.

Answer

The route is not valid due to a missing label.

Answer

The route is not in the BGP table.

Answer

The route has a higher MED value than the best path.

Question 6

A network engineer is troubleshooting an MPLS L2VPN (VPWS) where two customer sites are connected via a pseudowire. The engineer has configured the xconnect on both PE routers, but the customer reports that the link is down. The 'show mpls l2transport vc' command on PE1 shows the VC state as 'down'. What is the most likely cause?

Accepted Answer

LDP is not enabled on the core interfaces between the PEs.. In MPLS L2VPN, the VC state depends on the MPLS label path. If LDP is not exchanging labels for the pseudowire, the VC will remain down. Option A is correct. Option B is wrong because the VC ID must match; Option C is wrong because the VC type must match; Option D is wrong because the encapsulation must match.

Answer

The VC ID is different on the two PEs.

Answer

The VC type is not set to Ethernet.

Answer

The encapsulation is set to VLAN instead of Ethernet.

Question 7

An engineer is configuring MPLS L3VPN with OSPF as the PE-CE protocol. The customer wants to ensure that routes from the MPLS backbone are not redistributed back into the customer's OSPF domain. What configuration should the engineer apply on the PE router?

Accepted Answer

Configure the 'domain-id' under the OSPF process in the VRF.. In MPLS L3VPN, the 'domain-id' OSPF attribute prevents redistribution of routes from the backbone into the customer's OSPF domain. Option A is correct. Option B is wrong because 'sham-link' is for backdoor links; Option C is wrong because 'capability vrf-lite' is for VRF-lite; Option D is wrong because 'redistribute connected' is not related.

Answer

Configure a sham-link between the PE routers.

Answer

Use the 'capability vrf-lite' command under OSPF.

Answer

Apply a route-map to filter routes redistributed into OSPF.

Question 8

A service provider is migrating from a traditional IP core to an MPLS core. The engineer has configured LDP on all core routers and verified that LDP sessions are established. However, some prefixes learned via OSPF are not being assigned labels. The 'show mpls ldp bindings' command shows missing bindings for certain routes. What is the most likely cause?

Accepted Answer

The routes are not present in the global routing table on the router.. LDP by default only assigns labels to routes in the routing table that are not BGP routes. If the routes are not in the routing table (e.g., due to summarization or filtering), LDP will not assign labels. Option A is correct. Option B is wrong because LDP does not require IGP; Option C is wrong because LDP can assign labels to any route; Option D is wrong because LDP does not require BGP.

Answer

The OSPF process is not redistributed into LDP.

Answer

LDP is configured to only assign labels to BGP routes.

Answer

The 'mpls ldp autoconfig' command is missing on OSPF.

Question 9

An engineer is configuring MPLS L3VPN with BGP as the PE-CE protocol. The customer uses eBGP between CE and PE. The engineer notices that the CE router is not receiving any VPN routes from the PE. The 'show bgp vpnv4 unicast all' on the PE shows the routes as valid and best. What is the most likely missing configuration?

Accepted Answer

The 'redistribute bgp' command is missing under the VRF BGP address-family on the PE.. In MPLS L3VPN, when using eBGP between PE and CE, the PE must redistribute BGP routes into the VRF BGP process. This is done using the 'redistribute bgp' command under the VRF address-family. Option A is correct. Option B is wrong because the routes are already in BGP; Option C is wrong because the session is up; Option D is wrong because the VRF is configured.

Answer

The BGP session between PE and CE is not established.

Answer

The route-target import is not configured on the PE.

Answer

The VRF is not configured on the PE.

Question 10

A network engineer runs the following command on Router R1:

R1# show mpls ldp neighbor
    Peer LDP Ident: 10.1.1.2:0; Local LDP Ident 10.1.1.1:0
        TCP connection: 10.1.1.2.646 - 10.1.1.1.179
        State: Oper; Msgs sent/rcvd: 120/115; Downstream
        Up time: 02:30:15
        LDP discovery sources:
          GigabitEthernet0/0, Src IP addr: 192.168.1.2
        Addresses bound to peer LDP Ident:
          10.1.1.2         192.168.1.2      10.2.2.2

Based on this output, what can be concluded?

Accepted Answer

The LDP session is using TCP port 646 for LDP and port 179 for BGP, and the session is operational.. The output shows an established LDP session (State: Oper) between LSRs 10.1.1.1 and 10.1.1.2, with TCP port 646 (LDP) and port 179 (BGP) shown. The 'Downstream' label distribution mode is indicated. The peer's addresses include loopback and interface IPs.

Answer

The LDP session is down because the state is 'Oper' but the TCP connection shows port 179.

Answer

The LDP neighbor is using label distribution mode 'Upstream' based on the output.

Answer

The LDP session has been up for 2 hours and 30 minutes, and the peer has sent 115 messages.

Question 11

A network engineer runs the following command on Router R2:

R2# show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
16         Pop Label  10.1.1.1/32      0             Gi0/0      192.168.1.1
17         18         10.2.2.0/24      1500          Gi0/1      192.168.2.3
18         Untagged   10.3.3.0/24      0             Gi0/2      192.168.3.4

Based on this output, what is the correct interpretation?

Accepted Answer

For prefix 10.1.1.1/32, the router will pop the MPLS label before forwarding because the outgoing label is 'Pop Label'.. The forwarding table shows label operations. For prefix 10.1.1.1/32, the outgoing label is 'Pop Label' meaning PHP (Penultimate Hop Popping). For 10.2.2.0/24, label 17 is swapped to 18. For 10.3.3.0/24, 'Untagged' means the packet is forwarded without an MPLS label (e.g., to a non-MPLS interface).

Answer

For prefix 10.2.2.0/24, the router will impose label 18 onto the packet.

Answer

For prefix 10.3.3.0/24, the router will forward the packet with an MPLS label of 18.

Answer

The router has received 1500 bytes for prefix 10.2.2.0/24, and the outgoing interface is Gi0/1.

Question 12

A network engineer runs the following command on Router R3:

R3# show mpls ldp bindings
  lib entry: 10.1.1.1/32, rev 2
        local binding:  label: 16
        remote binding: lsr: 10.1.1.2:0, label: 17
  lib entry: 10.2.2.0/24, rev 4
        local binding:  label: 18
        remote binding: lsr: 10.1.1.2:0, label: 19
  lib entry: 10.3.3.0/24, rev 6
        local binding:  label: 20
        remote binding: lsr: 10.1.1.2:0, label: 21

Based on this output, what is true?

Accepted Answer

For prefix 10.1.1.1/32, the local label is 16 and the remote label from LSR 10.1.1.2 is 17.. The output shows the LIB (Label Information Base). For each prefix, there is a local label assigned by this router and a remote label learned from LSR 10.1.1.2. The rev number indicates the revision of the entry.

Answer

The router has learned label bindings from two different LDP peers.

Answer

The label for prefix 10.2.2.0/24 is 18 locally and 19 remotely, indicating that the remote LSR will use label 18.

Answer

The LIB entry for 10.3.3.0/24 has a revision number of 6, meaning it was the sixth entry added.

Question 13

A network engineer runs the following command on Router R4:

R4# show mpls interfaces
Interface              IP            Tunnel   BGP Static Operational
GigabitEthernet0/0     Yes           No       No  No     Yes
GigabitEthernet0/1     Yes           No       No  No     Yes
GigabitEthernet0/2     No            No       No  No     No

Based on this output, what can be concluded?

Accepted Answer

MPLS is enabled and operational on GigabitEthernet0/0 and GigabitEthernet0/1.. The command shows which interfaces are enabled for MPLS. The 'IP' column indicates if MPLS is enabled for IP forwarding. 'Operational' shows if MPLS is actually up. Gi0/2 has MPLS disabled (IP: No) and is not operational.

Answer

MPLS is enabled on all three interfaces, but only Gi0/0 and Gi0/1 are operational.

Answer

BGP and Static label switching are enabled on Gi0/0 and Gi0/1.

Answer

Tunnel interfaces are configured on Gi0/0 and Gi0/1.

Question 14

A network engineer runs the following command on Router R5:

R5# show mpls ldp discovery
 Local LDP Identifier: 10.5.5.5:0
 Discovery Sources:
   Interfaces:
     GigabitEthernet0/0: xmit/recv
       LDP Id: 10.5.5.4:0, no hello (expired)
     GigabitEthernet0/1: xmit/recv
       LDP Id: 10.5.5.6:0

Based on this output, what is the state of the LDP session with neighbor 10.5.5.4?

Accepted Answer

The LDP session with 10.5.5.4 is not established because no hello messages have been received from that neighbor.. The output shows LDP discovery sources. For Gi0/0, the neighbor 10.5.5.4:0 has 'no hello (expired)', meaning the hello messages have not been received recently, so the neighbor is considered down or the session is not established.

Answer

The LDP session with 10.5.5.4 is operational because the interface is in xmit/recv mode.

Answer

The LDP session with 10.5.5.4 is down because the interface is not operational.

Answer

The LDP session with 10.5.5.4 is using targeted discovery.

Question 15

A network engineer runs the following command on Router R6:

R6# show mpls ldp neighbor 10.6.6.7 detail
    Peer LDP Ident: 10.6.6.7:0; Local LDP Ident 10.6.6.6:0
        TCP connection: 10.6.6.7.646 - 10.6.6.6.179
        State: Oper; Msgs sent/rcvd: 200/195; Downstream
        Up time: 1d04h
        LDP discovery sources:
          Targeted Hello 10.6.6.6 -> 10.6.6.7, active, passive
        Addresses bound to peer LDP Ident:
          10.6.6.7         192.168.6.7
        Hold time: 15 seconds; keepalive interval: 5 seconds
        Peer hold time: 15 seconds; keepalive interval: 5 seconds

Based on this output, what type of LDP session is this?

Accepted Answer

This is a targeted LDP session established between two non-directly connected routers.. The output shows 'Targeted Hello' in the discovery sources, indicating this is a targeted LDP session (not link-local). The session is operational and uses targeted hellos.

Answer

This is a link-local LDP session because the discovery source is an interface.

Answer

This is a multicast LDP session because the hold time is 15 seconds.

Answer

This is a BGP session because the TCP connection shows port 179.

Question 16

A network engineer runs the following command on Router R7:

R7# show mpls ldp capabilities
    LDP Capabilities:
      Dynamic Capability: advertised
      Typed Wildcard FEC: advertised
      MTU Signaling: advertised
      P2MP: not advertised
      MPLS OAM: advertised
      LDP Graceful Restart: advertised
        Helper mode: enabled
        Restart mode: enabled
        Reconnect time: 120 sec
        Recovery time: 180 sec

Based on this output, which capability is NOT supported by this router?

Accepted Answer

P2MP (Point-to-Multipoint). The output lists capabilities that are advertised. P2MP (Point-to-Multipoint) is explicitly listed as 'not advertised', meaning it is not supported.

Answer

LDP Graceful Restart

Answer

MTU Signaling

Answer

Dynamic Capability

Question 17

A network engineer runs the following command on Router R8:

R8# show mpls ldp traffic
    LDP Traffic Statistics:
      Hellos sent: 5000, Hellos received: 4998
      Initialization messages sent: 2, received: 2
      Keepalive messages sent: 15000, received: 14995
      Label mapping messages sent: 100, received: 95
      Label withdraw messages sent: 5, received: 3
      Label release messages sent: 2, received: 1
      Label abort messages sent: 0, received: 0
      Notification messages sent: 1, received: 2
      Address messages sent: 10, received: 9
      Address withdraw messages sent: 1, received: 0

Based on this output, what is a likely issue?

Accepted Answer

There is a problem with label distribution because label withdraw messages are significantly higher than label release messages.. The statistics show a mismatch between sent and received messages for several types, especially label withdraw (5 sent, 3 received) and label release (2 sent, 1 received). This may indicate packet loss or a problem with the LDP session. However, the most notable discrepancy is in label mapping messages (100 sent, 95 received), suggesting some mapping messages were lost. But the question asks for a likely issue; the high number of label withdraws relative to releases could indicate instability.

Answer

The LDP session is experiencing packet loss because the number of hellos sent and received are not equal.

Answer

The router has sent 100 label mapping messages but received 95, indicating that 5 mapping messages were lost.

Answer

The router has sent 1 notification message and received 2, indicating errors in the LDP session.

Question 18

A network engineer runs the following command on Router R9:

R9# show mpls ldp bindings 10.9.9.0 255.255.255.0
  lib entry: 10.9.9.0/24, rev 10
        local binding:  label: 22
        remote binding: lsr: 10.9.9.1:0, label: 23
        remote binding: lsr: 10.9.9.2:0, label: 24
        remote binding: lsr: 10.9.9.3:0, label: 25

Based on this output, how many remote LDP peers have advertised a label for the prefix 10.9.9.0/24?

Accepted Answer

3. The output shows three remote bindings from different LSRs (10.9.9.1, 10.9.9.2, 10.9.9.3), each with a different label. So three remote peers have advertised labels.

Answer

1

Answer

2

Answer

4

Question 19

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.0
 mpls ip
 mpls label protocol ldp
!
router ospf 1
 network 10.1.1.0 0.0.0.255 area 0
!
router ldp
 interface GigabitEthernet0/1
!
What is the effect of this configuration?

Accepted Answer

MPLS forwarding is enabled on GigabitEthernet0/1, and LDP will distribute labels for all routes in the routing table.. The configuration enables MPLS forwarding and LDP on the interface, with OSPF providing IGP reachability. LDP will distribute labels for prefixes learned via OSPF.

Answer

Only OSPF routes are labeled; static routes are excluded from LDP label distribution.

Answer

The 'mpls label protocol ldp' command is redundant because LDP is the default.

Answer

The configuration will fail because 'router ldp' requires an LDP router-id to be set.

Question 20

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.0
 mpls ip
 mpls label protocol tdp
!
router ospf 1
 network 10.1.1.0 0.0.0.255 area 0
!
router ldp
 interface GigabitEthernet0/1
!
Which statement about this configuration is true?

Accepted Answer

The interface will use TDP for label distribution, ignoring the LDP configuration under router ldp.. The interface is configured with TDP (Cisco proprietary) while the router is configured for LDP (standard). This mismatch will prevent label exchange.

Answer

The router will use LDP because the global configuration overrides the interface command.

Answer

Both TDP and LDP will be used simultaneously on the interface.

Answer

The configuration will fail because TDP is not supported on this platform.