300-410 MPLS Operations — 15 Questions

Question 1

A network engineer is troubleshooting an MPLS L3VPN where CE1 cannot reach CE2. The PE routers are running OSPF as the IGP and LDP for label distribution. On PE1, the engineer sees that the VRF route for CE2's subnet is present, but the corresponding MPLS label is missing in the LFIB. The show mpls ldp neighbor command shows LDP neighbors are up. What is the most likely cause of the missing label?

Accepted Answer

LDP is not enabled on the interface facing the next-hop router.. The missing label in the LFIB despite the VRF route being present indicates that LDP is not binding a label to the BGP next-hop of the VPN route. Since the IGP route to the BGP next-hop is present but LDP has not assigned a label for it, the issue is likely that LDP is not enabled on the interface facing the next-hop router.

Answer

The VRF route is not redistributed into BGP on the remote PE.

Answer

MTU mismatch on the link between PE1 and P causes label imposition failure.

Answer

The mpls label protocol ldp command is missing under the VRF.

Question 2

A network engineer is troubleshooting MPLS traffic engineering (TE) tunnels. A TE tunnel from Router A to Router B is configured but remains down. The engineer runs show mpls traffic-eng tunnels and sees that the tunnel is in 'down' state with the error 'Path computation failed'. The IGP is OSPF with MPLS TE enabled, and the network is fully meshed. What is the most likely root cause?

Accepted Answer

MPLS TE is not enabled on all interfaces along the path.. The 'Path computation failed' error indicates that the headend router cannot find a path meeting the constraints. Since OSPF TE is enabled, the issue is likely that the traffic engineering database (TED) is incomplete because MPLS TE is not enabled on all intermediate interfaces, preventing the headend from learning link attributes.

Answer

The tunnel destination is not reachable via the IGP.

Answer

RSVP is not configured on the tunnel interface.

Answer

The tunnel bandwidth is set too high, exceeding available bandwidth.

Question 3

A network engineer is troubleshooting an MPLS L2VPN (VPWS) where the pseudowire between two PE routers is down. The show mpls l2transport vc command displays state 'down' and the VC ID is correct on both ends. The engineer checks the MPLS LDP session and sees it is up, but the targeted LDP session for the pseudowire is not established. What is the most likely cause?

Accepted Answer

The mpls ldp neighbor command is missing on one or both PEs.. For a pseudowire to be established, a targeted LDP session is required between the two PE routers. If the targeted LDP session is not established, the pseudowire cannot exchange labels. The most common cause is that the mpls ldp neighbor command is missing or misconfigured on one or both PEs.

Answer

The VC ID does not match on both ends.

Answer

The IGP is not converging, causing reachability issues.

Answer

The mpls label protocol ldp command is missing globally.

Question 4

A network engineer is troubleshooting MPLS traffic where packets are being dropped at a P router. The engineer runs show mpls forwarding-table and sees that the outgoing label for a specific FEC is 'Untagged' instead of a valid label. The IGP is running correctly, and LDP neighbors are established. What is the most likely cause?

Accepted Answer

LDP is not enabled on the outgoing interface.. The 'Untagged' label in the LFIB indicates that LDP has not assigned a label for that FEC on the outgoing interface. This typically happens when LDP is not enabled on the outgoing interface, either because the interface is not configured with mpls ip or because the interface is passive under LDP.

Answer

The IGP metric is too high, causing LDP to prefer a different path.

Answer

The mpls label range is exhausted.

Answer

The router is configured with mpls ldp advertise-labels for host routes only.

Question 5

A network engineer is troubleshooting an MPLS L3VPN where CE1 can ping the PE1 interface but cannot ping CE2. On PE1, show ip route vrf CUSTOMER shows the route to CE2's subnet, but show bgp vpnv4 unicast all neighbors 10.0.0.2 advertised-routes does not show the route. The BGP session between PE1 and PE2 is established. What is the most likely cause?

Accepted Answer

A route-map applied to the VRF export is filtering the route.. The route is present in the VRF but not advertised to the remote PE, indicating a BGP policy issue. Since the BGP session is up, the most likely cause is that the route is not being redistributed into BGP or is being filtered by a route-map or prefix-list on the VRF export.

Answer

The VRF export route-target does not match the import route-target on the remote PE.

Answer

The BGP session is not using the correct update-source.

Answer

The next-hop-self command is missing under the VRF address-family.

Question 6

A network engineer is troubleshooting MPLS LDP where the LDP session between two directly connected routers is not forming. The engineer runs show mpls ldp discovery and sees that LDP hellos are being sent and received on the link. However, show mpls ldp neighbor shows no neighbors. What is the most likely cause?

Accepted Answer

The LDP router-id is not reachable via the IGP.. LDP hellos are exchanged, but the session does not form, indicating a problem with the transport connection. The most common cause is that the routers cannot establish a TCP connection to the LDP transport address, often due to a missing route to the LDP router-id or an ACL blocking TCP port 646.

Answer

The mpls label protocol ldp command is missing globally.

Answer

The interface is configured with mpls ldp igp sync.

Answer

The LDP session is using a non-default transport address.

Question 7

A network engineer is troubleshooting MPLS traffic where packets are being forwarded without a label (IP forwarding) instead of being label-switched. The engineer runs show mpls forwarding-table and sees that the FEC for the destination prefix has a valid label. However, show ip cef shows that the outgoing interface is not MPLS-enabled. What is the most likely cause?

Accepted Answer

The outgoing interface is missing the mpls ip command.. The LFIB has a label, but CEF is not imposing it, indicating that the outgoing interface is not configured for MPLS forwarding. Even if LDP assigns a label, the interface must have mpls ip enabled for label imposition to occur.

Answer

The CEF is disabled globally.

Answer

The mpls label protocol ldp command is missing on the interface.

Answer

The interface is a loopback interface.

Question 8

A network engineer is troubleshooting MPLS TE where a tunnel is configured with explicit path but the tunnel remains down. The show mpls traffic-eng tunnels command shows 'Path computation failed' and the show mpls traffic-eng topology command shows that the explicit path includes a router that is not in the TED. What is the most likely cause?

Accepted Answer

The router in the explicit path does not have MPLS TE enabled.. The explicit path includes a router that is not in the TED, meaning the headend cannot find that router in the traffic engineering database. This is because MPLS TE is not enabled on that router or its interfaces, so its link attributes are not advertised.

Answer

The explicit path is configured with a loose next-hop.

Answer

The tunnel destination is not reachable via the IGP.

Answer

The mpls traffic-eng tunnels command is missing on the headend.

Question 9

A network engineer is troubleshooting MPLS L3VPN where traffic from CE1 to CE2 is being dropped. On PE1, show mpls forwarding-table shows that the label for the BGP next-hop is 'Pop tag' (implicit null). The engineer checks the remote PE and sees that it is advertising implicit null for the BGP next-hop. What is the most likely reason for the traffic drop?

Accepted Answer

The remote PE does not have the CE route in its VRF.. Implicit null (pop tag) means the penultimate hop pops the label and forwards the IP packet. If the traffic is dropped, the issue is likely that the remote PE does not have a route to the CE in its VRF, so it drops the IP packet after label pop.

Answer

The penultimate hop is not popping the label correctly.

Answer

The MTU on the link between P and PE2 is too small.

Answer

The remote PE is configured with explicit null for the BGP next-hop.

Question 10

A network engineer runs the following command on Router R1:

R1# show mpls ldp neighbor
    Peer LDP Ident: 10.0.0.2:0; Local LDP Ident 10.0.0.1:0
        TCP connection: 10.0.0.2.646 - 10.0.0.1.52868
        State: Oper; Msgs sent/rcvd: 123/120; Downstream
        Up time: 02:15:30
        LDP discovery sources:
          GigabitEthernet0/0, Src IP addr: 192.168.1.2
        Addresses bound to peer LDP Ident:
          10.0.0.2        192.168.1.2

Based on this output, which statement is correct?

Accepted Answer

The LDP session is established and operational.. The output shows a single LDP neighbor with state 'Oper' (operational), indicating the LDP session is up. The 'Downstream' mode is default. The peer LDP Ident is 10.0.0.2:0, and the local LDP Ident is 10.0.0.1:0. The addresses bound include the peer's router-id and the interface IP. No problems are indicated.

Answer

The LDP session is down because the state is 'Oper'.

Answer

The LDP neighbor is using upstream label distribution.

Answer

The LDP session is using TCP port 179.

Question 11

A network engineer runs the following command on Router R1:

R1# show mpls forwarding-table
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop
Label      Label      or Tunnel Id     Switched      interface
16         Pop Label  10.1.1.0/24      0             Gi0/0      192.168.1.2
17         21         10.2.2.0/24      1500          Gi0/0      192.168.1.2
18         Untagged   10.3.3.0/24      0             Gi0/1      192.168.2.2

Based on this output, what is the problem with the label entry for 10.3.3.0/24?

Accepted Answer

The next-hop router 192.168.2.2 is not an LDP neighbor for this prefix.. The entry for 10.3.3.0/24 shows 'Untagged' in the outgoing label column, meaning the router is forwarding IP packets without an MPLS label. This indicates that the next-hop router (192.168.2.2) did not assign a label for this prefix, possibly because LDP is not enabled on that interface or the neighbor is not an LDP peer.

Answer

The label 18 is being swapped correctly.

Answer

The prefix 10.3.3.0/24 is being forwarded with MPLS label 18.

Answer

The label 18 is being popped by the next-hop router.

Question 12

A network engineer runs the following command on Router R1:

R1# show mpls l2transport vc 100 detail
Local interface: Gi0/0/0 up, line protocol up
  Destination: 10.0.0.2, VC ID: 100, VC status: down
  Last error: No remote LDP session

Based on this output, what is the most likely cause of the VC being down?

Accepted Answer

The LDP session between the routers is not established.. The output explicitly states 'No remote LDP session' as the last error. This indicates that the LDP session between the two routers (10.0.0.1 and 10.0.0.2) is not established, which is required for the pseudowire to come up.

Answer

The local interface Gi0/0/0 is administratively down.

Answer

The remote router 10.0.0.2 is not reachable via IP.

Answer

The VC ID 100 is mismatched on the remote router.

Question 13

A network engineer runs the following command on Router R1:

R1# show bgp ipv4 unicast summary
BGP router identifier 10.0.0.1, local AS number 65001
BGP table version is 10, main routing table version 10
Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.0.2        4        65002     123      120       10    0    0 02:15:30        5
10.0.0.3        4        65003      50       45       10    0    0 00:00:12 Idle (PfxRcd: 0)

Based on this output, what is the problem with neighbor 10.0.0.3?

Accepted Answer

The BGP session to 10.0.0.3 is flapping.. The neighbor 10.0.0.3 shows 'Idle' state with no prefixes received (PfxRcd: 0). The 'Idle' state in BGP indicates that the session is not established, often due to a misconfiguration (e.g., wrong AS number, no route to neighbor, or BGP not activated). The brief uptime (00:00:12) suggests it keeps trying but fails.

Answer

The neighbor 10.0.0.3 is operational and exchanging prefixes.

Answer

The neighbor 10.0.0.3 has sent 5 prefixes.

Answer

The BGP table version is mismatched.

Question 14

A network engineer runs the following command on Router R1:

R1# show ip route ospf
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
O        10.1.1.0/24 [110/20] via 192.168.1.2, 00:15:30, GigabitEthernet0/0
O        10.2.2.0/24 [110/30] via 192.168.1.2, 00:15:30, GigabitEthernet0/0

Based on this output, which statement is correct?

Accepted Answer

The router has two OSPF routes to different subnets.. The output shows two OSPF routes via the same next hop. The administrative distance is 110 (default for OSPF), and the metrics are 20 and 30. No problems are indicated; the routes are present and valid.

Answer

The router has a default route via OSPF.

Answer

The OSPF neighbor is down.

Answer

The metric for 10.2.2.0/24 is 20.

Question 15

A network engineer runs the following command on Router R1:

R1# show dmvpn
Legend: Attrb -> S: Static, D: Dynamic, I: Incomplete
        N: NATed, L: Local, X: No Socket
        # Ent -> Number of NHRP entries with same NBMA peer
        NHS Status: E => Expecting Replies, R => Responding, W => Waiting
        UpDn Time -> Up or Down Time for a Tunnel
==========================================================================
Interface: Tunnel0, IPv4 NHRP Details
Type:Hub, NHRP Peers:2,

# Ent  Peer NBMA Addr Peer Tunnel Add State  UpDn Tm Attrb
 ----- --------------- --------------- ----- -------- -----
     1 192.168.1.2     10.0.0.2        UP    00:15:30 D
     1 192.168.1.3     10.0.0.3        UP    00:10:20 D

Based on this output, what is the role of Router R1?

Accepted Answer

Router R1 is a DMVPN hub.. The output shows 'Type:Hub', indicating that Router R1 is configured as a DMVPN hub. It has two dynamic peers (spokes) connected.

Answer

Router R1 is a DMVPN spoke.

Answer

Router R1 is a DMVPN server.

Answer

Router R1 is a DMVPN client.