Design for High Availability
Most businesses need a high level of availability, especially for their critical applications. The goal of high availability is to remove the single points of failure in the network design by using software features or hardware-based resiliency. Redundancy is critical in providing high levels of availability for the enterprise. Some technologies have built-in techniques that enable them to be highly available. For technologies that do not have high availability, other techniques can be used, such as additional WAN circuits or backup power supplies.
Defining Availability
System availability is a ratio of the expected uptime to the amount of downtime over the same period of time. Let’s take an example of 4 hours of downtime per year. There are 365 days in a year, which equals 8760 hours (365 × 24 = 8760). Now, if we subtract 4 hours from the annual total of 8760 hours, we get 8756. Then, if we figure 8756 / 8760 × 100, we get the amount of availability percentage, which in this case is 99.95%.
Table 9-5 shows the availability percentages from 99% to 99.999999%, along with amounts of downtime per year.
Table 9-5 Availability Percentages
Availability |
Downtime per Year |
The Nines of Availability |
Targets |
99.000000% |
3.65 days |
Two nines |
|
99.900000% |
8.76 hours |
Three nines |
|
99.990000% |
52.56 minutes |
Four nines |
Branch WAN high availability |
99.999000% |
5.256 minutes |
Five nines |
Branch WAN high availability |
99.999900% |
31.536 seconds |
Six nines |
Ultra high availability |
99.999990% |
3.1536 seconds |
Seven nines |
Ultra high availability |
99.999999% |
.31536 seconds |
Eight nines |
Ultra high availability |
Figure 9-1 illustrates WAN router paths and the impacts to availability depending on the level of redundancy used.
)
Figure 9-1 Router Paths and Availability Examples
Deployment Models
There are three common deployment models for WAN connectivity, each with pros and cons:
MPLS WAN: Single- or dual-router MPLS VPN
Hybrid WAN: MPLS VPN and Internet VPN
Internet WAN: Single- or dual-router Internet VPN
An MPLS WAN involves single or dual routers for the MPLS VPN connections. It provides for the highest in SLA guarantees for both QoS capabilities and network availability. However, this option is the most expensive, and it ties the organization to the service provider.
A hybrid WAN combines an MPLS VPN and an Internet VPN on a single router or on a pair of routers. This deployment model offers a balanced cost option between the higher-cost MPLS VPN connection and the lower-cost Internet VPN for backup. With a hybrid WAN, traffic can be split between the MPLS VPN for higher-priority-based traffic and Internet VPN for lower-priority-based traffic. Newer WAN designs are also using SDWAN with both MPLS and Internet-based transports.
An Internet WAN includes a single router or dual routers using Internet-based VPN only. This deployment model is the lowest-cost option but lacks the SLAs and QoS capabilities offered by carriers. The enterprise would be responsible for providing SLAs to the end users.
Redundancy Options
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; however, 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 only have 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 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
Multi-Homed MPLS WANs
In a dual-MPLS-carrier design, each site is connected to both provider A and provider B. Some sites might have two routers for high availability, and others might have only a single router but with two links for link and provider redundancy. For example, each CE router would redistribute local routes from EIGRP into BGP. Routes from other sites would be redistributed from BGP into EIGRP as external routes. For sites that have two routers, filtering or tagging of the routes in and out of BGP would be needed to prevent routing loops.
Figure 9-3 illustrates a dual-MPLS-carrier design with single and dual routers.
)
Figure 9-3 Dual-MPLS-Carrier Design Example
Hybrid WANs: Layer 3 VPN with Internet Tunnels
Hybrid WAN designs involve using an MPLS VPN for the primary connection and an Internet tunnel for the backup connection. In this design, eBGP would be used to peer with the MPLS VPN provider, and EIGRP would be used for routing for the IGP internally. At each site, the CE router would learn routes from the MPLS VPN via BGP and redistribute the routes from BGP into EIGRP. Then each site would redistribute EIGRP routes into BGP and use EIGRP to peer with other local routers at each site. The Internet tunnel routers would use EIGRP to exchange routes inside the VPN tunnels, and they would not need to redistribute routing information because they would run only EIGRP. On the MPLS VPN router, BGP-learned routes would be preferred because the BGP routes that would be redistributed into EIGRP routes would have a lower administrative distance. In this case, if you want the MPLS VPN router to be the primary path, you need to run an FHRP between the dual-homed routers, with the active router being the MPLS VPN-connected router. That way, it would choose the MPLS VPN path as the primary path and use the Internet tunnel path as the backup path for failover. Another option would be to modify the routing protocol metrics so that the MPLS VPN path is preferred.
Figure 9-4 illustrates a hybrid WAN design with an MPLS VPN and an Internet VPN.
)
Figure 9-4 Hybrid WAN Design Example