Junos and MPLS: L3 VPN Fundamentals

In this blog post, we will delve into the fundamentals of Layer 3 (L3) Virtual Private Networks (VPNs) in the context of Junos and Multiprotocol Label Switching (MPLS). This topic is crucial for students preparing for the Juniper Networks Certified Internet Specialist Enterprise Routing and Switching (JNCIS-ENT) certification.

What is MPLS?

MPLS is a protocol for speeding up and shaping network traffic flows. It allows most packets to be forwarded at Layer 2 (the switching level) rather than having to be passed up to Layer 3 (the routing level). Each packet gets labeled on entry into the service provider's network by the ingress router. All the subsequent routing switches perform packet forwarding based only on those labels�they never look as far as the IP header. Finally, the egress router removes the label(s) and forwards the original IP packet toward its final destination.


An MPLS L3 VPN consists of a set of sites that are interconnected by means of an MPLS provider core network. At each customer site, one or more customer edge (CE) devices attach to one or more provider edge (PE) devices.


Let's look at a basic configuration example on a Junos device:

set interfaces ge-0/0/0 unit 0 family inet address
set interfaces lo0 unit 0 family inet address
set protocols ospf area interface ge-0/0/0.0
set protocols ospf area interface lo0.0 passive
set protocols mpls interface ge-0/0/0.0
set protocols ldp interface ge-0/0/0.0
set policy-options policy-statement PS-Export term T1 from protocol ospf
set policy-options policy-statement PS-Export term T1 then accept
set policy-options policy-statement PS-Export term T2 then reject
set protocols bgp group ISP type external
set protocols bgp group ISP export PS-Export
set protocols bgp group ISP neighbor peer-as 64512
set routing-options autonomous-system 64512

In this configuration, we first set the IP addresses for the interfaces and enable OSPF on them. We then enable MPLS and LDP on the ge-0/0/0.0 interface. We create a policy statement to control the routes that are advertised through BGP. Finally, we configure BGP with the ISP and set our autonomous system number.


You can verify the MPLS L3 VPN operation using the following commands:

show mpls interface
show mpls ldp neighbor
show route table inet.3
show route table bgp.l3vpn.0

These commands will show you the MPLS-enabled interfaces, the LDP neighbors, the MPLS forwarding table, and the BGP VPNv4 routing table, respectively.


Understanding MPLS and L3 VPNs is essential for network engineers studying for the JNCIS-ENT certification. This post has provided a brief overview of these topics, along with a basic configuration example. Remember, practice is key when it comes to mastering these concepts. So, get hands-on experience whenever possible and keep exploring more advanced topics.

In the next blog post, we will delve deeper into advanced MPLS configurations and troubleshooting. Stay tuned!

© Ben Jacobson.RSS