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| 1.2 |  | Key Characteristics of Scalable Internetworks |
| 1.2.2 |  | Making the network reliable and available |

A reliable and available network provides users with 24 hour a day, seven day a week access. In a highly reliable and available network, fault tolerance and redundancy make outages and failures invisible to the end user. The high-end devices and telecommunication links that ensure this kind of performance come with a high price tag. Network designers constantly have to balance the needs of users with the resources at hand.

When choosing between high performance and low cost at the core layer, the network administrator should choose the best available routers and dedicated WAN links. The core must be designed to be the most reliable and available layer. If a core router fails or if a core link becomes unstable, routing for the entire internetwork might be adversely affected.

Core routers maintain reliability and availability by rerouting traffic in the event of a failure. Robust netowrks can adapt to failures quickly and effectively. To build robust networks, the Cisco IOS offers several features that enhance reliability and availability. These features include the following:

- Support for scalable routing protocols
- Alternate paths
- Load balancing
- Protocol tunnels
- Dial backup

The following sections describe these features.

Scalable Routing Protocols
Routers in the core of a network should converge rapidly and maintain reachability to all networks and subnetworks within an Autonomous System (AS). Simple distance vector routing protocols, such as RIP, take too long to update and adapt to topology changes to be viable core solutions. Compatibility issues may require that some areas of a network run simple distance vector protocols such as RIP and Routing Table Maintenance Protocol (RTMP), an Apple proprietary routing protocol. It is best to use a scalable routing protocol in the core layer. Good choices include Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), or Enhanced Interior Gateway Routing Protocol (EIGRP).

Alternate Paths
Redundant links maximize network reliability and availability, but they are expensive to deploy throughout a large internetwork. Core links should always be redundant. Other areas of a network may also need redundant telecommunication links. If a remote site exchanges mission-critical information with the rest of the internetwork, that site would be a candidate for redundant links. To provide another dimension of reliability, an organization may even invest in redundant routers to connect to these links. A network that consists of multiple links and redundant routers will contain several paths to a given destination. If a network uses a scalable routing protocol, each router maintains a map of the entire network topology. This map helps routers select an alternate path quickly if a primary path fails. EIGRP actually maintains a database of all alternate paths if the primary route is lost.

Load Balancing
Redundant links do not necessarily remain idle until a link fails. Routers can distribute the traffic load across multiple links to the same destination. This process is called load balancing. Load balancing can be implemented using alternate paths with the same cost or metric. This is called equal-cost load balancing. They can also be implemented over alternate paths with different metrics. This is referred to as unequal-cost load balancing. When routing IP, the Cisco IOS offers two methods of load balancing. They are know as per packet and per destination load balancing. If fast switching is enabled, only one of the alternate routes will be cached for the destination address. All packets in the packet stream bound for a specific host will take the same path. Packets bound for a different host on the same network may use an alternate route. This way, traffic is load balanced on a per destination basis.

Per packet load balancing requires more CPU time than per destination load balancing. However, per packet load balancing allows load balancing that is proportional to the metrics of unequal paths, which can help utilize bandwidth efficiently. The proportional distribution makes per packet load balancing better than per destination load balancing.

Protocol Tunnels
An IP network with Novell NetWare running Internetwork Packet Exchange (IPX) at a handful of remote sites may provide IPX connectivity between the remote sites by routing IPX in the core. Even if only two or three offices sparingly use NetWare, this will create additional overhead associated with routing a second routed protocol, or IPX, in the core. It would also require that all routers in the data path have the appropriate IOS and hardware to support IPX. For this reason, many organizations have adopted ’IP only‘ policies at the network core because IP has become the dominant routed protocol.

Tunneling gives an administrator a second and more agreeable option. The administrator can configure a point-to-point link through the core between the two routers using IP. When this link is configured, IPX packets can be encapsulated inside IP packets. IPX can then traverse the core over IP links and the core can be spared the additional burden of routing IPX. Using tunnels, the administrator increases the availability of network services.

Dial Backup
Sometimes two redundant WAN links are not enough or a single link needs to be fault tolerant. However, the possibility of purchasing a full-time redundant link is too expensive. In these cases a backup link can be configured over a dialup technology, such as ISDN, or even an ordinary analog phone line. These relatively low-bandwidth links remain idle until the primary link fails.

Dial backup can be a cost-effective insurance policy, but it is not a substitute for redundant links that can effectively double throughput by using equal-cost load balancing.

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