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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