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Multiprotocol Label Switching (MPLS)

A mechanism that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table.

Shared Layer 2 Backbone

This document uses this term to refer to the concept of tunneling Layer 2 and Layer 3 connections over a shared Layer 2 network.

Virtual Routing and Forwarding (VRF)

This technology allows multiple routing domains to co-exist within the same device at the same time. Routing domains are independent, so overlapping IP addresses can be used without causing conflict. This enables multiple virtual IP networks to exist independently on the same physical network.

VLAN Double Tagging (Q-in-Q)

A process in which an ethernet switch appends an extra VLAN tag onto packets that it receives on certain ports. This enables multiple VLANs to be tunneled across a Layer 2 network, encapsulated within another VLAN.

How It Works

This section examines in more detail how each of the 3 key technologies contributes to the solution.

1. VRF-Lite provides Layer 3 virtualisation

VRF-Lite enables completely separate IP networks to be overlaid on the same physical infrastructure.

The network’s routers maintain separate routing tables for individual IP networks, and separate instances of routing protocols (OSPF, RIP or BGP) to populate the separate routing tables.

The IP networks, referred to as VRF instances, operate as completely functioning routing domains, with routes that are dynamically exchanged within each domain. However, the VRF-Lite functionality on the Allied Telesis Layer 3 switches ensures total separation between the different routing domains.

The following example shows Allied Telesis x930 Series switches performing VRF-lite, and thus acting as the Provider Edge (PE) switches in the Shared Layer 2 Backbone:

Virtual Network

The diagram below shows x530 switches connecting into an EPSR ring consisting of a SwitchBlade x908 GEN2 stack, a SwitchBlade x8100 chassis, and an x950 Series switch, with separate distribution and backbone layers:

Separate Distribution and Backbone Layers

Comparison with MPLS

A number of enterprises have employed MPLS to implement their shared Layer 2 backbone. This section compares the Allied Telesis IP-over-Ethernet solution with an MPLS-based solution. It shows the claimed benefits of an MPLS-based solution, and considers whether or not it really does have an advantage over the IP-over-ethernet solution.

Table 1: MPLS claims and reponses

Switching in the backbone is the most efficient, as it is based on just the MPLS label.In a pure Ethernet backbone, switching is based only on VLAN tagging, which is equally as efficient.
MPLS hides IP addressing, so separate networks that use overlapping IP address ranges can share the same backbone switches.The key enabling technology for the separation of IP domains is VRF. It is just as effective to use VLAN tagging to confine traffic into VRF instances as it is to use MPLS labels.
MPLS is a multi-protocol solution, so protocols other than IP can be transported across the backbone.VLAN double-tagging is equally as multi-protocol as MPLS. The process of using a VLAN to encapsulate another VLAN for transportation across the backbone places no restriction on any higher-layer protocols being carried within the encapsulated VLAN.
MPLS sets up label paths automatically, without any need to statically configure the path.

Although the VLAN membership must be statically configured on backbone network ports, two points should be considered:

1. Configuring VLAN membership on ports is very simple, and can even be scripted.

2. In an Enterprise, the addition of a new entity in need of network separation is not a frequent event. So, in an Enterprise network, the total operational cost saving provided by MPLS’s automatic path creation is very little.

MPLS provides QoS functionality.

Ethernet CoS marking enables QoS to be applied to different traffic types. In the end, the QoS marking scheme is not important. The important factors in QoS are:

1. the richness of the policy engines in the distribution switches, and

2. the power of the prioritization, policing and shaping features in the backbone switches. These factors are agnostic to which protocol is carrying the QoS marking in the packets.

MPLS can quickly re-route around broken links.EPSR is an extremely effective mechanism for link recovery.


Advantages of an IP-over-Ethernet solution

There are many advantages of an IP-over-Ethernet solution. The Allied Telesis solution is simple, reliable and effective.

The major advantage is simplicity. Using MPLS in the backbone network adds an extra layer of complexity for no value. VLAN tagging is a simple and familiar technology. Given that mapping tunnels onto VLAN tags provides and effective solution, there is no need to employ another separate protocol for applying labels to tunneled traffic.

Static configuration of VLAN membership on backbone ports avoids the need to understand and troubleshoot any path establishment protocol. Backbone switch configuration can be kept very simple.

Additionally, using a pure IP-over-Ethernet solution lets you avoid being locked into having to use MPLS-capable equipment. Removing unnecessary elements from the solution, and keeping with more universal technologies broadens the range of options for equipment to use in the shared network.