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MPLS and L3VPN lab 9 of 11

Troubleshooting the label path

BGP is up, every loopback pings, and the customer's VPN is dead. Walk the label path link by link across IOS and RouterOS, and find four faults left by a night of maintenance: LDP authentication, a missing LDP interface, an unreachable LSR ID and a shrunken MPLS MTU.

Run this lab

It needs Lite: its 6 nodes are set to 6 vCPU and 2,816 MB in total. That fits the free demo too.

In your workspace: Shared, then MPLS and L3VPN guided labs, then 09 - Troubleshooting the label path

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Level
Advanced
Time
About 75 minutes
Nodes need
6 vCPU, 2,816 MB
Smallest plan
Lite

The scenario

You're on call for Wrenfield Networks. A night of maintenance on the core has finished, and the morning's first ticket has arrived.

Ticket 4471, Alder Foods. Nothing works between our two sites. Your NOC says "all BGP sessions are up, and all routers ping each other". We don't care. Fix it.

The NOC is right on both counts, and that's your first clue. The VPN's configuration wasn't touched: RDs, RTs and BGP are as they should be. All the faults are in the label path, on the routers between CE1 and CE2. There are four, each from a different engineer's work last night, on different links.

Your ticket: the two sites must exchange traffic, and that must include full-size packets: 1492 bytes with DF set is the largest this 1500-byte core can carry through a two-label stack. Don't change the MTU of any link to 1600 today, because that's a planned job for another night.

DeviceInterfaceAddressConnects to
CE1Loopback1192.168.1.1/24
CE1Gi0/0172.16.1.2/30PE1 Gi0/0
PE1Loopback010.255.0.1/32
PE1Gi0/0172.16.1.1/30CE1 Gi0/0
PE1Gi0/110.0.12.1/30P1 Gi0/0
P1Loopback010.255.0.2/32
P1Gi0/010.0.12.2/30PE1 Gi0/1
P1Gi0/110.0.23.1/30P2 ether1
P2loopback10.255.0.3/32
P2ether110.0.23.2/30P1 Gi0/1
P2ether210.0.34.1/30PE2 Gi0/1
PE2Loopback110.255.1.4/32
PE2Loopback010.255.0.4/32
PE2Gi0/0172.16.2.1/30CE2 Gi0/0
PE2Gi0/110.0.34.2/30P2 ether2
CE2Loopback1192.168.2.1/24
CE2Gi0/0172.16.2.2/30PE2 Gi0/0

What you’ll practise

  • Explain why an L3VPN can fail while BGP and IP routing look healthy
  • Walk an LSP link by link from interface to forwarding table on both vendors
  • Tell a hello problem from a TCP session problem
  • Find a fault that only big packets show

The tasks

  • A method for the label path

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of Troubleshooting the label path6 nodes, 5 links. Every link is listed as text below the diagram.CE1PE1P1P2PE2CE2Gi0/0Gi0/0Gi0/1Gi0/0Gi0/1ether1ether2Gi0/1Gi0/0Gi0/0
6 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • CE1 Gi0/0 to PE1 Gi0/0
  • PE1 Gi0/1 to P1 Gi0/0
  • P1 Gi0/1 to P2 ether1
  • P2 ether2 to PE2 Gi0/1
  • PE2 Gi0/0 to CE2 Gi0/0
Nodes in Troubleshooting the label path, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
CE1Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
PE1Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
P1Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
P2MikroTik RouterOSmikrotik-7.14.21 vCPU, 256 MB
PE2Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB
CE2Cisco IOSvvios-adventerprisek9-m.spa.159-3.m91 vCPU, 512 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder MPLS and L3VPN guided labs.

  3. Open 09 - Troubleshooting the label path and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Cisco, Juniper Networks and MikroTik. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.