We sometimes hear that:
- An MPLS Network without P is not a real MPLS Network.
- A collapsed PE Network is incomplete or not a genuine MPLS design
What exactly is a P and PE router :
In MPLS, a P router acts as a vital link within the core network, connecting different parts together efficiently. Unlike PE routers, P routers don’t deal directly with individual customers. This streamlined approach makes the network highly scalable because the core doesn’t get bogged down by the number of customers. While P routers handle the core backbone of the network, PE routers, equipped with their distinctive “Edge” functionality, act as the initial points of entry for customer traffic.
Typically, a true P router doesn’t run BGP but only IGP. However, sometimes the terms P and PE routers are used interchangeably, depending on whether routing lookup occurs via IP or Label. However, it’s important to note that a PE router is fundamentally distinct from a P router, as it incorporates the “Edge” functionality.
What looks like a Backbone with only PE routers (Collapsed P/PE)
In the diagram above, there are 4 Points of Presence (POP), each with 2 routers, resulting in a total of 6 physical interconnections to other POPs.
Now, imagine if we have 50, 100, or 200 POPs. We would then have an exponential increase in the number of physical interconnections between the POPs. For instance, with 50 POPs, the number of interconnections would be much higher than with just 4 POPs. Specifically, with 50 POPs, there would be approximately 2450 interconnections. With 100 POPs, this number would grow even further to around 9900 interconnections. This exponential growth in interconnections presents significant challenges in terms of network management, scalability, and maintenance.
What looks like a Backbone without P routers, but with Agreagation switches :
In this design, we can rely on central aggregation switches, which are less expensive than MPLS routers. However, despite the use of switches, the number of IGP adjacencies will remain the same as in the design without switches.
What looks like a Backbone with P routers :
Now, consider the design where we deploy ‘P only routers,’ simplifying the architecture and enhancing the scalability, modularity, and maintainability of the backbone. This is because both the physical interconnections and the adjacencies that each PE router needs to maintain will be reduced.
We can also integrate multiple layers of P routers, forming aggregation and core layers, alongside other. The possibilities are virtually endless and vary depending on each constraint and situation (variations that may include aggregation switches…)
You can extrapolate that this principle applies to many other designs, such as having distinct “Access, Aggregation, Core layers” compared to having a collapsed Core and aggregation layer in a campus or Datacenter design.
In summary :
- The presence of P routers in a Backbone MPLS Network is not mandatory, and this design remains entirely valid. Many companies and service providers operate their networks using a collapsed design, which is especially suitable for small to medium entities.
- P routers are simply tasked with interconnecting PE routers. This approach helps us avoid the complexity of full-mesh physical and logical interconnections, resulting in a more scalable network design.
- Considering future growth projections and port density, from a scalability standpoint, you may have multiple options, for example :
- Using a switch aggregation, which is less expensive, if the need is limited to physical interconnections.
- Deploying P routers if there is a requirement for both physical interconnections and logical IGP domain scalability.
- We can also begin with a collapsed PE Network Backbone and gradually transition to implementing P routers as the company grows.
- Of course, there are numerous designs when it comes to MPLS Backbone, including traditional P/PE architecture, P only at primary sites, collapsed P/PE, etc., depending on factors such as the number of routers, Points of Presence (POPs), and various constraints like scalability, cost, and performance. This post serves as an initial exploration of why P routers can be collapsed with PE routers or not, depending on the specific needs.
- The same principle applies to a wide range of designs, such as having “Access, Aggregation, Core layers” versus a collapsed Core and aggregation layer.
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