What does P router mean in MPLS ?

What does P router mean in MPLS ? It is used in MPLS VPN networks mostly but can be used with any MPLS application, use case.

This is very important node in MPLS, and crucial to understand MPLS.  

MPLS is one of the most commonly used encapsulation mechanism in Service Provider networks and before studying more advanced mechanisms, this article is must read.

Let’s take a look at below figure.


Figure -1 MPLS network PE, P and CE routers

In Figure-1 MPLS network is shown. This can be an Enterprise or Service Provider network. MPLS is not only a service provider technology. It can provide segmentation/multi tenancy for the enterprise environment as well.  

Three different types of router are shown. CE , PE and P routers.  

PE and CE devices were explained in this post.  

If it is Enterprise network, WAN routers can be considered as PE routers, and the switches can be a CE devices.

PE routers don’t have to be connected to P routers. PE routers can be directly connected to each other.  

P router’s job is to provide reachability between the PE devices. You can think PE devices in MPLS, as local access switches in campus networks. In the case of campus network, service can be provided by Layer 2 VLAN and it starts from the access switches, in the case of Service Provider network, MPLS service starts from PE device.  

PE devices need to reach each other and P device provide it. By the way, P stands for Provider , thus P router means Provider Router.  

P router doesn’t have a connection to the customer. If there is customer connection from any particular router, then we cannot mention from that router as P router, because it is PE router.  

P router, by swapping (changing incoming MPLS Label with outgoing one) MPLS label, provide a reachability between PE routers.  

Important note about P router, P router doesn’t keep the customer specific information in the memory. So layer 2 or layer 3 information of the customers are not learned by P router.  

This provides scalable network.  

PE devices, when packet arrives first lookup IP destination address and then uses MPLS label to function.  

P devices don’t do IP lookup at all but only switch MPLS labels.

In MPLS, service can be Layer 2 or Layer 3.  

In Layer 3 MPLS VPN, IP routing is enabled between PE and CE devices.  

Routing protocols in theory can be Static Routing, RIP, EIGRP, OSPF, IS-IS and BGP.  

All of them are IETF standard. But in real life, most service providers only provide Static Routing and BGP as a PE-CE routing protocol.  

PE-CE interface is only IP and not MPLS. Specific application of PE-CE as MPLS is called CSC (Carrier Supporting Carrier). But in basic MPLS Layer 3 MPLS VPN, PE-CE link is always IP.  

Conclusion :

P stands for Provider in MPLS

2. P router doesn’t have a connection to the customers and don’t keep any customer MAC address or IP address information

3. P router by swapping MPLS label between incoming and outgoing ports, provide a reachability between PE (Edge) routers.

4. If PE routers are directly connected, you may not have P router in your MPLS enabled network.

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

He created OrhanErgun.Net 10 years ago and has been serving the IT industry with his renowned and awarded training.

Wrote many books, mostly on Network Design, joined many IETF RFCs, gave Public talks at many Forums, and mentored thousands of his students.  

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