<h1>High Availability - Networks (I)</h1>

Redundant devices

 
Today’s networks are high-tech and most times high speed. Common to most Wide Area Network (WAN) designs is the need for a backup to take over in case of any type of failure to your main link. A simple scenario would be if you had a single T1 connection from your core site to each remote office or branch office you connect with. What if that link went down? How would you continue your operations if it did?
 
Adding redundancy is the most common way to increase your uptime. First, make sure there's redundancy within your core router; redundant CPU cards, power supplies and fans usually can be added to chassis-based routers and switches, and some router and switch vendors have equipment with dual backplanes. With redundant CPU cards, you can force a failover to one card while you upgrade the second one, instead of having to bring the whole router down for the upgrade.
 
The goal of redundant topologies is to eliminate network downtime caused by a single point of failure. All networks need redundancy for enhanced reliability and this is achieved through reliable equipment and network designs that are tolerant to failures and faults and networks should be designed to reconverge rapidly so that the fault is bypassed.
 
Network redundancy is a simple concept to understand. If you have a single point of failure and it fails you, then you have nothing to rely on. If you put in a secondary (or tertiary) method of access, then when the main connection goes down, you will have a way to connect to resources and keep the business operational.
 
The critical point is that highly reliable network equipment is expensive because it is designed not to break and this typically includes things like dual power supplies, watchdog processors and redundant disk systems.
 
A highly available system may be built out of less expensive network products but these components may lack the redundant power supplies or other features of high-reliability equipment, and therefore, they may fail more often than the more expensive equipment. However, if the overall network design takes into account the fact that equipment may fail, then end users will still be able to access the network even if something goes wrong.
 

Redundant switching

 

 
One of the basic high availability techniques is the use of redundant switches. Two switches can be configured as a virtual router and used as a gateway. One of the core switches is automatically selected as the active router. Should that switch fail, the second switch automatically responds to requests from clients for the virtual gateway address.
 
You should also always consider the LAN connections of your routers and the LAN’s default gateway assignment. For better results create a redundant LAN by installing two switches and making them the default gateway. This way, if the router fails completely (such as looses power), the default gateway address is still intact, and the switch can make the decision, not the router. In the next figure there is a redundant LAN connection where routers are connected and the switches make the routing decisions in the case where the router with the default gateway assignment is at risk.
 

 
It is recommended to purchase hardware with multiple hot-swappable redundant power supplies, and make sure that the power source is on a generator, has a backup, and/or is phase correctly so you can survive a power lose entirely. Enterprise UPS systems, backup generators and so on can be used to provide alternate and redundant power solutions when needed.
 

Multiple Ethernet ports

 
In the next figure each server has two Ethernet interfaces. In this example, one interface is connected to one of the switches and the other interface is connected to the other. If one interface fails, the computer is still connected to the network.

 

 

Redundant Topologies

 
In a redundant mesh topology network devices are connected with many redundant interconnections between network nodes. In a true mesh topology every node has a connection to every other node in the network and the level of redundancy can be measured by network connectivity. Redundant network topologies are designed to ensure that networks continue to function in the presence of single points of failure. Reliability is increased by redundancy.
 

 
On a redundantly connected network if a router were to fail connectivity would be preserved by routing traffic through a redundant connection. Furthermore, each router should have two or more points, or 'legs', with which to keep redundancy. The next figure shows the layout of an extremely common design; branch offices need to connect to a headquarters site where centralized resources are located such as financial applications, enterprise resource planning (ERP) software, databases, file server data and so on.

 

 
Commonly, the router is the networks default gateway, where all packets are sent that are not found locally. The router needs to make a routing decision and since the main link is up, decides to send via that connection. When the main link drops, commonly the alternate link is used if set up correctly. This provides your remote site with a new path to reach the resources needed to continue operations.

The core or backbone of a network usually handles the most traffic so, if it goes down, it will likely affect the most users. If your redundant core router or switch equipment is connected and ready to kick in automatically when a problem occurs, you can reduce an outage from hours of manual labor to an automated process that takes just a few seconds. This is called High Availability, where identical core routers must be ready to take over should the primary fail.

 

 
This means that the next layer, the aggregator switches, has to have a connection to each router, which also provides some redundancy for the links themselves and this lets you put each core router in different geographic locations.

Next figure shows a more common solution for a large company.

 

 

Here you have multiple branch offices connecting to a central location. Redundant links provide an alternate solution to main link failure on your WAN and provide remote sites access to core resources.