Redundancy Options – WAN Availability and QoS

Depending on the cost of downtime for an organization, different levels of redundancy can be implemented for a remote site. The more critical WAN sites will use higher levels of redundancy. With any of the deployment options—MPLS WAN, hybrid WAN, or Internet WAN—you can design redundant links with redundant routers, a single router with redundant links, or a single router with a single link.

For the most critical WAN sites, you typically want to eliminate single points of failure by designing with dual routers and dual WAN links along with dual power supplies. However, this highly available option comes with a higher price tag and is more complex to manage, but it offers failover capabilities. Another option available to reduce cost is to use a single router with dual power supplies and multiple WAN links providing power and link redundancy. Non-redundant, single-homed sites are the lowest cost, but they have multiple single points of failure inherent with the design, such as the WAN carrier or WAN link.

Single-Homed Versus Multi-Homed WANs

The advantages of working with a single WAN carrier are that you have only one vendor to manage, and you can work out a common QoS model that can be used throughout your WAN. The major drawback with a single carrier is that if the carrier has an outage, it can be catastrophic to your overall WAN connectivity. This also makes it difficult to transition to a new carrier because all your WAN connectivity is with a single carrier.

On the other hand, if you have dual WAN carriers, the fault domains are segmented, and there are typically more WAN offerings to choose from because you are working with two different carriers. This design also allows for greater failover capabilities with routing and software redundancy features. The disadvantages with dual WAN carriers are that the overall design is more complex to manage, and there will be higher recurring WAN costs.

Single-Homed MPLS WANs

In a single-MPLS-carrier design, each site is connected to a single MPLS VPN from one provider. For example, you might have some sites that are single-homed and some sites that are dual-homed to the MPLS VPN. Each site will consist of CE routers peering with the provider using eBGP, and iBGP will be used for any CE-to-CE peering. Each CE will advertise any local prefixes to the provider with BGP and redistribute any learned BGP routes from the provider into the IGP or use default routing. Common IGPs are standard-based OSPF and EIGRP.

Figure 9-2 illustrates a single-MPLS-carrier design with single- and dual-homed sites.

Figure 9-2 Single-MPLS-Carrier Design Example