The Three Types Of MPLS
MPLS is available in three types:
1. Layer 2 point to point
2. Layer 3 IP VPN
3. Layer 2 VPLS
1. MPLS Layer 2 Point to Point
Suitable for companies that require high bandwidth between a small number of sites.
Layer 2 point to point is a cost effective and flexible alternative to high bandwidth leased lines. Many wholesale network operators and providers have based their core network infrastructure on Ethernet and use Layer 2. This type of transport is protocol agnostic and allows anything running over the LAN to be sent over the WAN without having to use routers to convert packets up to Layer 3, the network layer.
Advantages of MPLS Layer 2 point to point:
– It is no longer necessary to manage complex routing tables.
– Customers save money by terminating their WAN connections straight in to a Layer 2 switch as opposed to buying expensive routers to convert Layer 3 back down to Layer 2.
Disadvantages of MPLS Layer 2 point to point: –
– Providers tend to provide only high bandwidth circuits, i.e. 10 Mbps and above
– Point to multipoint is not supported.
2. MPLS Layer 3 IP/VPN
Particularly suitable for large multi-site enterprise, i.e. retail chains, which deploy a large number of low bandwidth sites or large corporates with global offices, this type of service is a natural progression away from legacy Frame Relay and ATM services. It is a perfect fit for companies that are: –
– In the process of merging: IP/VPNs are extremely scalable for fast deployment
– require ‘any to any’ connectivity: a shorter hop count between two local sites is more efficient than – ‘tromboning’ back into a central point. This is especially relevant for global networks where latency is increased as packets travel over long geographical distances.
– preparing for voice and data convergence: to implement a blanket ‘class of service’ prioritization based on traffic type is made simple across multi-site networks.
– be migrating from traditional ATM to IP: ATM is extremely expensive to maintain
– be migrating from frame relay to IP due to its inflexible nature and management restrictions.
– have low bandwidth requirements at small branch offices: in the UK sub 2 Mbps circuit delivery is available over Kilostream circuits which a cheaper alternative to Megastream & Ethernet lines.
– Need only a secure dial-up capability in smaller locations
Advantages of MPLS Layer 3 IP/VPNs
– Offers the same service guarantees services as Frame Relay or ATM without requiring the use of any permanent virtual circuits (PVC).
– Supports Class of Service (COS) for traffic type differentiation
Disadvantages of MPLS Layer 3 IP/VPNs
– Layer 3 IP/VPN’s are proprietary; one provider, one network.
– Changes to the network configuration. For example adjusting COS has to be requested and typically takes up to 5 days. Change control is also chargeable.
– Not suitable for small networks; instead of buying one circuit between two points, customers have to buy two circuits to be brought into the private IP/VPN cloud. IP VPN are only cost effective for 5 circuits and above
– If using BT, Internet access circuits have to be purchased separately.
– IP only; other standards cannot be used over the network without conversion to Layer 3.
– MPLS can be expensive as larger carriers charge for prioritizing traffic in their annual cost.
– Change control: carriers usually reserve the right to charge for manual changes to network configuration.
How do MPLS Layer 3 IP/VPNs work?
Instead of having routers at every node or routing point of a network, traffic goes to the edge of a network and each packet has a label. Incoming packets or ingress as they are known, are assigned a label by a Label Edge Router (LER). Packets are forwarded along a Label Switch Path (LSP) where each Label Switch Router (LSR) makes forwarding decisions based solely on the contents of the label. At each hop, the LSR strip off the existing label and applies a new label, which tells the next hop how to forward the packet. In this way, a label determines how it will be transported over a network, simplifying the routing problem and giving label switching routers (LSR) high performance.
The labels, which are underlying protocol-specific identifiers, are distributed using Label Distribution Protocol (LDP) or piggybacked on routing protocols like border gateway protocol (BGP) and OSPF. Each data packet encapsulates and carries the labels during their journey from source to destination. High-speed switching of data is possible because the fixed-length labels are inserted at the very beginning of the packet or cell and can be used by hardware to switch packets quickly between links.
3. MPLS Layer 2 Virtual Private LAN Services (VPLS)
Virtual Private LAN services are growing in popularity for delivering Ethernet services. They combine MPLS and Ethernet allowing both customers and carriers to benefit. For over two decades, Ethernet switching has dominated the local area network while IP routing has dominated the carrier network. IP backbones have been used to provide Internet access and more recently to provide IP VPN access. Corporate VPN services have been typically provided with existing TDM, ATM and Frame Relay Networks. During this period, Ethernet has continued to scale the bandwidth spectrum and with its simplicity and cost effectiveness, it has established itself as the infrastructure of choice in the metro and the WAN. (Resource: vpls.org executive overview).
Otherwise known as transparent Ethernet services, VPLS is a newer protocol that works over MPLS and gives customers a combination of the benefits of the two above network types, i.e. the ability to operate a multipoint network AND pass all traffic at Layer 2 over the WAN. VPLS is popular in cities, such as Hong Kong & London & New York, where metro Ethernet networks have already been deployed. Due to its simplicity, robustness and ability to scale traffic to 10 Gbps, VPLS is popular with TV broadcasters, the financial sector and media houses.
Advantages of MPLS Layer 2 VPLS
– Transparent interface – no router hardware investment required for bandwidth upgrades
– Layer 2 means that traffic is tagged by MAC address as opposed to IP address
– Low latency – switched as opposed to routed
– Plug & Play for ease of deployment: no configuration required for new sites which appear like new devices on a LAN.
– Closed user groups can be established between customers for collaborative working
– QOS/COS costs – Layer 3 IP/VPN carriers charge an on-going fee for higher priority traffic, whereas VPLS providers only charge a one-off set-up fee to configure all levels of prioritisation.
Disadvantages of Layer 2 VPLS
VPLS has not yet been rolled out globally therefore the “any to any” feature of Layer 3 IP/VPNs is not yet fully functional with a single provider.
Comparing MPLS with IPSec and SSL VPN’s
There is no single ‘best choice’ when selecting a VPN so companies may use a combination of IP VPN, IPSec and SSL. The main difference being: –
IP Sec tunnels are encrypted over the public internet and stem from a firewall device or software on a router, whereas an MPLS network is inherently private and part of a carrier’s global network.
The public internet does not fall under the control any one provider’s network therefore service level guarantees and prioritisation can only achieved on a best effort basis.
QOS (Quality of Service) or COS (Class of Service)
Service providers will offer an SLA for packet delivery over an MPLS network whereas no-one can control the public internet. VPNs are usually offered by the service provider as a managed service, and originate and terminate in the service provider’s MPLS-enabled IP network. IPSec and SSL VPNs are typically managed by the company, and originate and terminate at the CPE. IPSec and SSL-based VPNs are also available as a managed service from certain service providers.