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MPLS and L3VPN lab 2 of 11

MPLS forwarding: traceroute, TTL and MTU

Trace a label-switched path across a core that runs no BGP, by hand and then with MPLS ping and traceroute. Hide the core from customers with TTL settings, find out why big transfers fail while small pings work, and fix the MTU on both vendors.

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 02 - MPLS forwarding - traceroute TTL and MTU

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

The scenario

Wrenfield Networks carries a customer between two sites over its label core, and only its edge routers run BGP. P1 and P2 have no idea where the customer's addresses are, and that's the point: a BGP-free core. Every customer packet has to stay on a label from PE1 all the way to the router before PE2, because a P router that gets it unlabelled has nowhere to send it.

Two things are on your list today. The on-call team wants to learn the tools that prove a label path works. And the customer's backup software fails over the core with "message too long", though small pings are fine.

DeviceInterfaceAddressConnects to
CE1Loopback1192.168.1.1/24
CE1Gi0/0172.16.1.2/30PE1 Gi0/0
PE1Loopback010.255.0.1/32
PE1Gi0/110.0.12.1/30P1 Gi0/0
PE1Gi0/0172.16.1.1/30CE1 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
PE2Loopback010.255.0.4/32
PE2Gi0/110.0.34.2/30P2 ether2
PE2Gi0/0172.16.2.1/30CE2 Gi0/0
CE2Loopback1192.168.2.1/24
CE2Gi0/0172.16.2.2/30PE2 Gi0/0

What you’ll practise

  • Follow an LSP hop by hop through the forwarding tables
  • Use MPLS ping and traceroute and read their result codes
  • Explain why a BGP-free core depends on a continuous LSP
  • Control TTL propagation for customers and for your own traffic
  • Work out the MTU a label stack needs and raise it across a mixed core

The tasks

  1. Follow the LSP by hand
  2. Prove the LSP with MPLS ping and traceroute
  3. What the customer sees: TTL
  4. Find the MTU problem
  5. Fix it across the core

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 MPLS forwarding - traceroute TTL and MTU6 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 MPLS forwarding: traceroute, TTL and MTU, 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 02 - MPLS forwarding - traceroute TTL and MTU 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.