| A reliable and available network
provides users with 24-hours-a-day, 7-days-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 steep 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, you should opt for the
best available routers and dedicated WAN links. You must design the
core to be the most reliable and available layer. If a core router
went down, or if a core link became 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. Networks that can deal with
failures quickly and effectively are said to be robust. To build
robust networks, the Cisco IOS offers several features that enhance
reliability and availability. These include support for scalable
routing protocols, alternative paths, load balancing, protocol
tunnels, and 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 sometimes require that some areas of a network
run simple distance-vector protocols such as RIP and Routing Table
Maintenance Protocol (RTMP, an Apple Computer proprietary routing
protocol). Whenever possible, a scalable protocol
such as Open Shortest Path First (OSPF) or Enhanced Interior Gateway
Routing Protocol (EIGRP) should be implemented, especially in the
core layer.
Alternate Paths
Redundant links maximize network reliability and availability, but
they are expensive to deploy throughout a large internetwork. Links
in the core layer should always be made redundant, but other areas of a
network may also need redundant telecommunication lines. 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, such as OSPF or EIGRP, its
routers will maintain a map of the entire network topology. This
will allow the routers to reroute traffic quickly by selecting an
alternate path. In fact, EIGRP maintains a database of all alternate
paths just in case the preferred 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. It can be
implemented using alternate paths with the same cost or metric
(equal-cost load balancing), or over alternate paths with different
metrics (unequal-cost load balancing). When routing IP, the Cisco
IOS offers two methods of load balancing: per-packet and
per-destination load balancing. If process switching is enabled, the
router will alternate paths on a per-packet basis. If fast switching
is enabled, only one of the alternate routes will be cached for the
destination address and 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. On the plus side,
per-packet load balancing allows load balancing that is proportional
to the metrics of unequal paths, rather than round-robin path
selection, which can help utilize bandwidth efficiently.
Tunnels
Consider an IP network with Novell NetWare running IPX at a handful of
remote sites. One way to provide IPX connectivity between
the remote sites is to route 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 (IPX) in
the core. It would also require that all routers in the data path have
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 world's dominant routed
protocol.
Tunneling allows an administrator a
second and more palatable option: 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, or packaged, 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, administrators increase the availability of network
service.
Dial Backup
Sometimes two redundant WAN links are not enough, or a single link
needs to be fault-tolerant, but 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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