Enterprise Internet (WAN) Link Connectivity – Redundancy and Load Balancing

While taking a single 2 Mbps Internet Leased Line Link might be more cost effective than taking two 1 Mbps ILL Links from multiple service providers, the various enterprise WAN Link termination/ connectivity devices come with good redundancy and load balancing options  with multiple ILL Links, which can avoid single point failures at the most important point in the network – WAN/ Internet Gateway.

Terminating Internet (WAN) Links on a UTM Device (Unified Threat Management Device):

WAN Link redundancy and UTM device redundancy (high availability) and load balancing architecture diagramYou can terminate a WAN Link (Including Internet Leased Lines) on to a UTM (Unified Threat Management) device directly, without having to terminate it on a router first, and having the UTM device behind it. UTM’s have grown beyond just allowing termination of links – they provide device level redundancy as well as link level redundancy.

As shown in the left hand side diagram, with some UTM vendors, if two UTM devices are connected in that fashion, full device level redundancy can be achieved. That is, if one UTM device fails, the other device takes over. Mostly in such configurations, one of the devices is in passive mode./ both of them can be in active mode, as well. Its important to have such device level redundancy at the WAN gateway to avoid WAN disconnections as having just one device leads to single point of failure.

On the right hand side of the diagram, the link level redundancy and the load balancing features of the UTM devices are represented. If you have a couple of Internet links (recommended), you could terminate both of them on a UTM device (either in active-active or in active-passive mode) so that when one link fails, the Internet traffic continues to flow through the other link. If both the links are up, some vendors even allow load balancing of the Internet traffic, between them. The maximum number of links supported in this configuration can be more than just two, and depends on the UTM device vendor.

The Load balancing can be done on a per destination basis, round robin basis, percentage (50%-50%) basis or maximum threshold basis, and some vendors allow customers to choose from any of the options.

Terminating Internet (WAN) Links on a Router:

WAN device redundancy and link redundancy and link failover in Routers

Routers have been the most popular way of terminating Internet Leased Lines (and other WAN connections). So, routers have built in device level redundancy (the configuration shown in the diagram is just an example, and the connectivity may change depending on the vendor) so that even if one router fails, the Internet (WAN) traffic is forwarded through the other router. These routers can be in active-active or active-passive mode. The diagram on the right shows the link level redundancy where if one of the Internet links fail, the traffic continues to go through the other link. Load balancing can be enabled within all the links that are connected to the router.

Routers can terminate multiple types of WAN Interfaces including E1/T1/T3, DS3, FE, ADSLx (broadband), Serial, ISDN, 3G, G.SHDSL, Dial-up, etc. Many enterprise routers are modular in nature, and can accommodate specific modules at any point of time to include any of the supported WAN interfaces for that model.

Terminating Multiple Internet WAN Links on a Link Load Balancer:

Link Load Balancer architecture diagram

A Link Load Balancer is a hardware appliance which accepts multiple Internet (WAN) links from multiple service providers and allows the users to use the full capacity of all the links connected to it and fail over to other links, when a link from a particular service provider is suddenly down. This provides link level redundancy to the Internet Links terminating to an organization (assuming that each link is from a different service provider). Link Load Balancers also do load balancing of Internet (WAN) traffic across the various links connected to it, so that all the lines are optimally utilized.

Some Link Load Balancers provide even device level redundancy when two such devices are used. But this feature is available only with certain vendors. Some of them support QoS policies and bandwidth shaping policies to be applied to critical traffic like voice, video, etc to ensure that certain minimum bandwidth/ priority is allocated to real time traffic traversing over the Internet.

So, its possible to achieve both link level and device level redundancy for enterprise Internet (WAN) link connectivity to try and provide uninterrupted Internet access to all the employees, as far as possible. It is also recommended that companies use these redundancy and load balancing features for connecting WAN/ Internet links.

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