Establishing a Resiliency Strategy - Highly Available Networks

Building a resilient IP network is not just about having redundant links and turning on the SSO features in all routers. In fact, the most difficult aspects of achieving IP resiliency is establishing an overall resiliency strategy.

With an overall resiliency strategy, you can predict what needs to be done to the network in the next few phases. You should be able to map out the logical design, knowing how it will eventually grow. From the logical design, the corresponding physical design is mapped out. Finally, both the logical and physical design translate to what sort of hardware is required, and which capacity features have to be placed in the hardware.

The strategy has to remain consistent. Many times, the resiliency of a network is compromised because of inconsistency in strategy. Shortcuts are made or different hardware is selected to do a certain task, perhaps because of shortages of funds or maybe because of changes in decision-making personnel. Problems such as these ultimately create outages later on.

Redundancy Strategy

Part of the overall resiliency strategy is how to achieve redundancy in both the logical and physical networks. Many network managers find this task challenging. For example, you might achieve physical redundancy, but because of a lack of logical redundancy, the network still experiences failure.
 
Logical Resiliency
When we talk about logical redundancy, we are mainly protecting important parts of the network, such as the following, from failing:
As mentioned previously, a network path is the route that traffic traverses between a source and a destination. It is a logical entity because network paths usually arise from some route calculations (for example, a shortest-path algorithm). The determination of the path is always done by the routing protocol and the results stored in the various routers within the network. As events occur within a network (for example, when a physical link fails), the network path for a source and destination pair may potentially change. This change might result in an alternative path, or it might result in a broken connection between the same source and destination. The task then is to make sure there is always a redundant network path to an important resource within the network (to a server, for example).

Functional entity refers to the logical functions that are performed by the routers (for example, a default gateway function or a multicast routing function). A host such as a personal computer usually needs a default gateway to help it send traffic to the rest of the world. If there is only one default gateway and it fails, the personal computer will never be able to contact any other hosts except those on the same subnet. Another example is the Area Border Router (ABR) function in the OSPF network. The multiple ABRs prevent the OSPF subarea from being disconnected from area 0.
Ensuring redundancy for these logical functions is critical, because it ensures a backup in the event of a failure. Information on logical resiliency in routing is usually found in the design guides for routing protocols such as OSPF and BGP:
 
Physical Resiliency
You might find the task of ensuring physical redundancy easier because it is a more visual exercise. You should look at several areas with regard to physical redundancy:

 

Scaling Strategy

Some people might find it strange how a scaling strategy affects resiliency of a network. For one, you might not be able to tear down everything in the network just to do improvement work on a congested link. With such high expectations on the uptime on a network (remember the five-nines challenge), it is almost impossible to do maintenance work without affecting network services. Therefore, many things have to be "preprovisioned" so as to avoid downtime.

As with a redundancy strategy, a scaling strategy also involves logical and physical aspects. To scale a network logically, consider aspects such as the IP addressing scheme, subnet size, and the number of subnets available within a network. You also need to look at how the routing design scales. For example, consider how many routers should be within an OSPF area, how many subnets should belong to a specific area, how many areas the network should be, and how many ABRs your network needs.

In the physical aspect, look into areas such as scaling a link speed. For example, you must decide whether to scale a 1-Gb backbone link by adding another 1-Gb link or by upgrading to an OC-48 link. The first option is called scaling horizontally; the latter is called scaling vertically. The correct choice depends on resource availability.

You might also look at things such as interface capacity, or so-called real estate, and router performance. In a chassis-based router, the number of slots, and thus the number of ports that it can support, dictates how large a network it can connect to in terms of number of links. In addition, the performance of the same router, in both switching capacity and forwarding capability, affects how much traffic it can carry at any one time.

By relying on features such as OIR, you can keep adding interfaces to a router and grow the network without affecting the rest. However, you can do so only if the router has enough slots in the first place. Therefore, having a capacity-planning exercise is important, and right sizing has to be done on the hardware. You do not want too large a router that costs a lot of money, nor do you want to run out of slots on a router.

Failure in this area usually results in network congestion and costly downtime.

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