300-410 MPLS L3VPN — Timed 15 Questions

Question 1

A network engineer is troubleshooting an MPLS L3VPN where CE1 (192.168.1.0/24) cannot reach CE2 (192.168.2.0/24). The PE routers are running OSPF with the CEs. On PE1, the VRF configuration includes route-target import and export 100:100. The show ip vrf detail command on PE1 shows the VRF is active, but the CE1 loopback is not present in the VRF routing table. The show ip route vrf CUSTOMER command on PE1 shows only directly connected interfaces. What is the most likely cause?

Accepted Answer

The OSPF process on PE1 is not configured under the VRF.. The CE routes are not being learned via OSPF into the VRF because OSPF process is not associated with the VRF. Without the 'router ospf vrf ' command, OSPF runs in the global routing table and does not populate the VRF.

Answer

The route-target import on PE1 is misconfigured.

Answer

The CE1 interface is not in the VRF.

Answer

The MP-BGP session between PE1 and PE2 is down.

Question 2

An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2 (the PE2 loopback), but the show ip route vrf CUSTOMER command does not have this route. The show mpls forwarding-table on PE1 does not show a label for 192.168.1.2. What is the most likely cause?

Accepted Answer

LDP is not enabled on the core-facing interfaces of PE1 or the P routers.. The VPNv4 route is received but not installed in the VRF routing table because the recursive lookup fails: the next-hop (PE2 loopback) is not reachable via LDP. Without an LDP label for the BGP next-hop, the route cannot be installed.

Answer

The VRF route-target import is missing on PE2.

Answer

The MP-BGP session is not using the loopback interface.

Answer

The VRF on PE1 has the wrong route-target export.

Question 3

A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using OSPF with the CEs and MP-BGP between them. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route as well. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 but the output interface is 'no route'. What is the most likely cause?

Accepted Answer

The PE2 loopback is not advertised into the IGP (OSPF/IS-IS) of the service provider core.. CEF has a next-hop but no output interface because the recursive routing table lookup for the BGP next-hop (192.168.1.2) fails. The IGP (OSPF or IS-IS) does not have a route to the PE2 loopback, so CEF cannot resolve the adjacency.

Answer

The OSPF process on PE1 is not redistributing connected routes.

Answer

The VRF route-target import is misconfigured.

Answer

MPLS is not enabled on the core-facing interfaces.

Question 4

An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, but the show ip route vrf CUSTOMER command does not have this route. The show ip bgp vpnv4 all 10.2.2.0/24 command on PE1 shows the route is received but not best. What is the most likely cause?

Accepted Answer

The BGP next-hop (PE2 loopback) is not reachable in the global routing table.. The route is received but not marked as best, so it is not installed in the routing table. Common reasons include the route being suppressed due to a higher AD from another source or the next-hop being unreachable. In this scenario, the most likely cause is that the BGP next-hop is not reachable in the global routing table.

Answer

The route-target import on PE1 is missing.

Answer

The VRF on PE1 has a different route-target export.

Answer

The MP-BGP session is using an incorrect address family.

Question 5

A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using eBGP with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label but the outgoing interface is 'aggregate'. What is the most likely cause?

Accepted Answer

The PE2 loopback address is accidentally configured on PE1.. The label for the BGP next-hop is pointing to 'aggregate', which means the router is the egress LSR for that prefix. This occurs when the PE2 loopback is also configured on PE1, causing the router to think it is the destination. The traffic is then dropped or looped because the router tries to process the packet locally instead of forwarding it.

Answer

LDP is not enabled on the core-facing interfaces.

Answer

The VRF route-target import is misconfigured.

Answer

The MP-BGP session is using the wrong update-source.

Question 6

An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using OSPF with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show ip route 192.168.1.2 command on PE1 shows the route with a next-hop of 10.0.0.2 and output interface GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label with outgoing interface GigabitEthernet0/0. What is the most likely cause?

Accepted Answer

The CE1 router does not have a default route or specific route to 10.2.2.0/24.. All forwarding components appear correct: the route is in the VRF, CEF has a valid next-hop and interface, and MPLS has a label. The issue is likely on the CE side, such as a missing route on CE1 or a firewall blocking traffic. The engineer should check CE1's routing table.

Answer

The VRF route-target import on PE1 is misconfigured.

Answer

The OSPF process on PE1 is not redistributing BGP routes into OSPF.

Answer

The MP-BGP session is not using the loopback interface.

Question 7

A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using eBGP with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 command on PE1 shows the route is received and best, but the show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 neighbors 192.168.1.2 advertised-routes command on PE2 shows the route is not advertised. What is the most likely cause?

Accepted Answer

The VRF on PE2 does not have the CE2 route in its routing table.. The route is not being advertised from PE2 to PE1. This could be due to a missing network statement or redistribute command under the BGP VRF address family on PE2, or the route is not in the VRF routing table on PE2. The engineer should check PE2's VRF routing table for the CE2 route.

Answer

The MP-BGP session between PE1 and PE2 is down.

Answer

The route-target export on PE2 is misconfigured.

Answer

The VRF on PE1 has the wrong route-target import.

Question 8

An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using OSPF with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label with outgoing interface GigabitEthernet0/0. The show ip route 192.168.1.2 command on PE1 shows the route with a next-hop of 10.0.0.2 and output interface GigabitEthernet0/0. The show ip cef 192.168.1.2 command on PE1 shows the next-hop as 10.0.0.2 and output interface GigabitEthernet0/0. What is the most likely cause?

Accepted Answer

The PE2 router does not have a label for the CE1 prefix in its LFIB.. All forwarding components on PE1 are correct. The issue is likely on the remote side, such as PE2 not having a label for the return traffic or CE2 not having a route back. The engineer should check PE2's forwarding table for the CE1 prefix.

Answer

The VRF route-target import on PE2 is misconfigured.

Answer

The OSPF process on PE1 is not redistributing BGP routes into OSPF.

Answer

The MP-BGP session is using an incorrect update-source.

Question 9

A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using eBGP with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 command on PE1 shows the route is received and best, but the show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 command on PE1 also shows the route has the 'r' flag (RIB-failure). What is the most likely cause?

Accepted Answer

There is a static route in the VRF for 10.2.2.0/24 with a lower administrative distance.. A RIB-failure indicates that the route is received and best in BGP but is not installed in the routing table because another route with a lower administrative distance exists. This could be due to a static route or an IGP route for the same prefix in the VRF.

Answer

The route-target import on PE1 is misconfigured.

Answer

The MP-BGP session is not using the loopback interface.

Answer

The VRF on PE1 has a different route-target export.

Question 10

A network engineer runs the following command on Router PE1:

PE1# show mpls forwarding-table 192.168.1.0 24
Local  Outgoing    Prefix            Bytes Label   Outgoing   Next Hop    
Label  Label       or Tunnel Id      Switched      interface              
16     Pop Label   192.168.1.0/24    0             Gi0/0      10.1.1.2   
17     22          192.168.1.0/24    0             Gi0/1      10.2.2.2

Based on this output, which statement is correct?

Accepted Answer

The router is performing load balancing across two LSPs for prefix 192.168.1.0/24.. The show mpls forwarding-table output shows two entries for the same prefix. The first entry has 'Pop Label' as outgoing label, meaning PHP (Penultimate Hop Popping) is in use for that path. The second entry has label 22, indicating a different LSP. The presence of two entries for the same prefix suggests load balancing across two equal-cost paths.

Answer

The router has a routing loop for prefix 192.168.1.0/24.

Answer

Penultimate Hop Popping is disabled on this router.

Answer

The prefix 192.168.1.0/24 is learned via BGP from two different neighbors.

Question 11

A network engineer runs the following command on Router PE2:

PE2# show ip bgp vpnv4 vrf CUSTOMER_A 10.10.10.0 24
BGP routing table entry for 10.10.10.0/24, version 15
Paths: (1 available, best #1, table CUSTOMER_A)
  Advertised to update-groups:
     1          
  Refresh Epoch 1
  Local, imported path from 10.10.10.0/24
    10.1.1.1 (metric 20) from 10.1.1.1 (10.1.1.1)
      Origin incomplete, metric 0, localpref 100, valid, internal, best
      Extended Community: RT:100:100
      mpls labels in/out 18/19

Based on this output, what is the problem?

Accepted Answer

The route is functioning correctly with no issues.. The output shows a VPNv4 route for VRF CUSTOMER_A. The route is marked as 'imported path from 10.10.10.0/24', which indicates it was imported from the global table or another VRF. The route is valid and best, with labels assigned. No problem is indicated; the route is functioning correctly.

Answer

The route is not being advertised to any BGP peer.

Answer

The route is missing the required Route Target community.

Answer

The route has an incorrect label binding.

Question 12

A network engineer runs the following command on Router P1:

P1# 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 - 10.0.0.1.48632
        State: Oper, Msgs sent/rcvd: 120/118, Downstream
        Up time: 00:12:34
        LDP discovery sources:
          GigabitEthernet0/0, Src IP addr: 10.1.1.2
        Addresses bound to peer LDP Ident:
          10.0.0.2         192.168.1.1

Based on this output, which statement is correct?

Accepted Answer

The LDP session is up and functioning correctly.. The show mpls ldp neighbor output shows a single LDP neighbor with IP 10.0.0.2. The state is 'Oper' (operational), and the neighbor has been up for 12 minutes. The output indicates a healthy LDP session.

Answer

The LDP session is down.

Answer

The router is not receiving label bindings from the neighbor.

Answer

The LDP router ID is misconfigured.

Question 13

A network engineer runs the following command on Router CE1:

CE1# show ip route vrf CUSTOMER_B 10.20.20.0 24
Routing Table: CUSTOMER_B
Codes: 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

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets
B       10.20.20.0 [20/0] via 10.1.1.2, 00:02:34

Based on this output, what is the problem?

Accepted Answer

The route is functioning correctly.. The output shows a BGP route for 10.20.20.0/24 in VRF CUSTOMER_B. The route is learned via BGP with an administrative distance of 20, which is typical for external BGP. The route is valid. No problem is evident.

Answer

The route is not being installed in the routing table.

Answer

The VRF is not configured correctly.

Answer

The next hop is unreachable.

Question 14

A network engineer runs the following command on Router PE3:

PE3# show ip vrf interfaces
Interface      IP-Address      VRF                              Protocol
Gi0/0          10.1.1.1        CUSTOMER_C                       up      
Gi0/1          10.2.2.1        CUSTOMER_D                       up

Based on this output, which statement is correct?

Accepted Answer

Both interfaces are correctly assigned to VRFs and are operational.. The show ip vrf interfaces command displays VRF-to-interface mappings. Both interfaces are up and assigned to VRFs CUSTOMER_C and CUSTOMER_D respectively. This indicates proper VRF configuration.

Answer

Interface Gi0/0 is down.

Answer

VRF CUSTOMER_C has no routes.

Answer

The router has no VRF configuration.

Question 15

A network engineer runs the following command on Router PE4:

PE4# show bgp vpnv4 unicast all summary
BGP router identifier 10.0.0.4, local AS number 65001
BGP table version is 25, main routing table version 25
5 network prefixes using 640 bytes of memory
5 path entries using 400 bytes of memory
3/3 BGP path/bestpath attribute entries using 360 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1424 total bytes of memory
BGP activity 15/10 prefixes, 20/15 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.0.5        4        65001    1020    1015       25    0    0 00:12:34        5
10.0.0.6        4        65002     500     495       25    0    0 00:06:20        0

Based on this output, what is the problem?

Accepted Answer

Neighbor 10.0.0.6 is not sending any prefixes.. The BGP summary shows two neighbors: 10.0.0.5 (AS 65001) with 5 prefixes received, and 10.0.0.6 (AS 65002) with 0 prefixes received. The neighbor 10.0.0.6 has been up for 6 minutes but has not sent any prefixes. This indicates a problem with prefix advertisement from that neighbor.

Answer

Neighbor 10.0.0.5 is not exchanging prefixes.

Answer

Both neighbors are in the Idle state.

Answer

The BGP table is empty.