When a router receives an IP packet, it looks at the destination IP address, checks its routing table to figure out how to forward it, and sends the packet to the next router which does the exact same thing until we reach the destination.
With the introduction of Classless Inter-Domain Routing (CIDR), routers support longest prefix matching. This means the router will use the most specific prefix in the routing table. Finding the longest prefix in the routing table is compute-intensive, putting a burden on router resources.
To reduce the burden of looking up destinations in the routing table, routers typically looked up the destination the first time they received an IP packet in software and used hardware to process other packets that belong to the same flow.
Each router makes independent decisions. On the Internet, it’s possible that there are many different routers between the source and destination, possibly owned by different ISPs.
When you transmit an IP packet from source to destination using the Internet, there are no guarantees when it comes to availability, bandwidth, and delay. It’s all “best-effort”.
What is MPLS?
Multiprotocol Label Switching (MPLS) is typically offered by service providers for private WAN links. Because service providers own these links, they can offer MPLS with IP SLAs that provide guarantees for uptime, bandwidth, and delay.
The main reason MPLS was developed in the 1990s was to reduce the amount of IP routing lookups. Back then, hardware wasn’t as powerful as it is today.
Exact lookups are much easier to implement in hardware. The idea behind MPLS is to “switch” based on labels with lookups that use exact matching instead of the compute-intensive longest prefix IP routing lookups.
Each prefix in the routing table gets a label and routers exchange labels with each other, establishing a label-switched path (LSP). Instead of lookups in the routing table, routers now “switch” packets based on the labels.
On which layer does MPLS operate?
MPLS doesn’t use one particular underlying technology. You can run it over different technologies including Ethernet, ATM, and frame-relay. MPLS doesn’t fit in nicely in the OSI model, it sits in between layers 2 and 3. Here’s an example where we use MPLS with Ethernet as the underlying technology:
Because MPLS sits in between layers 2 and 3, it’s often called a layer 2.5 protocol.
What is MPLS VPN?
MPLS VPN uses different methods where we use an MPLS backbone to create Virtual Private Networks (VPN). For example:
- Point-to-point: A layer 2 point-to-point link for connectivity between two sites.
- Virtual Private LAN Service (VPLS): A layer 2 network with VLAN support between multiple sites.
- L3 VPN: The customer peers with the service provider and the two exchange routes which are stored in a separate routing table.
MPLS does not provide encryption but it is a virtual private network which is why it is considered secure. Of course, it is possible to encrypt your traffic. For example, with IPSec.
Why is MPLS expensive?
The main reason MPLS is so expensive compared to regular broadband Internet is that you pay for a private WAN connection with an IP SLA that guarantees a certain availability, bandwidth, delay, and packet delivery rate. When there is an outage, the service provider resolves the issue within a certain time or pays a penalty.
What are the advantages of MPLS?
Originally, MPLS was created because label switching is more efficient than IP routing table lookups. Nowadays, this isn’t much of an issue because we have powerful hardware with ASICs.
Advantages
The main advantages of MPLS are that it’s scalable, offers high performance, is reliable, and supports QoS by defining LSPs that meet specific IP SLA agreements. For example, the service provider could offer three service levels for different types of traffic. One for voice over IP (VoIP), one for delay-sensitive traffic, and one for best-effort traffic.
Routers from different vendors support MPLS. You might be stuck with the service provider, but there is no “vendor lock-in” when it comes to hardware.
Disadvantages
There are also some disadvantages to MPLS. First of all, it’s expensive. MPLS is a service you pay for, which is way more expensive than regular broadband Internet access. This is one of the reasons why SD-WAN is becoming so popular.
You also rely on the service provider. If you have a global network, it could be difficult to find a service provider who offers MPLS on a global level. Many service providers work together with other service providers to offer global coverage. Deployment time can also take much longer than regular broadband Internet access.
Nowadays, businesses also require connectivity to cloud providers like Amazon AWS or Microsoft Azure. This can be difficult to set up as MPLS is a closed network.
Is MPLS still being used?
MPLS is still being used and plays an important role when connecting different sites. However, throughout the years, Internet access has become cheaper, more reliable, and offers high bandwidth. This is why SD-WAN is becoming so popular nowadays.
Businesses have to calculate the risks and rewards of using expensive but reliable MPLS connections vs regular broadband Internet connections.
