The internetwork
should be reliable and available at all layers, but most critically
at the core layer. Recall that core routers must be reliable because
they carry information about all the routes in an internetwork. If
one of these routers goes down, it affects routing on a large scale.
Core routers are reliable when they can accommodate failures by
rerouting traffic and respond quickly to changes in the network
topology. Some protocols that enhance network reliability and
availability that the Cisco IOS software supports are scalable
routing protocols, tunnels, and dial backup.
Scalable Protocols
Scalable protocols include Open Shortest Path First (OSPF) and
Enhanced Interior Gateway Routing Protocol (EIGRP). These protocols
provide the following features:
Reachability --- Scalable internetworks, even those using a
hierarchical design, can have a large number of reachable networks
or subnetworks. These networks can be subject to reachability
problems due to metric limitations of distance vector routing
protocols. Scalable routing protocols, such as OSPF, NetWare Link
Services Protocol (NLSP), and EIGRP, use metrics that expand the
reachability potential for routing updates because they use cost,
rather than hop count, as a metric.
Fast convergence time --- Convergence time is defined as the
amount of time required to propagate new route information from one
end of the internetwork to the other end of the internetwork.
Scalable protocols can converge quickly because the router can
detect failure rapidly and because each router maintains a network
topology map. Routers also forward network changes quickly to all
routers in the network topology.
Congestion control --- Scalable routing protocols generally add
less traffic overhead to the network for carrying routing
information by providing summarizations of network information.
Alternate Paths
Many internetwork backbones carry mission-critical information.
Organizations running such backbones are usually interested in
protecting the integrity of this information at virtually any cost.
Routers must offer sufficient reliability so that they are not the
weak link in the internetwork chain. The key is to provide
alternative paths that can come on line whenever link failures occur
along active networks.
End-to-end reliability is not ensured by simply making the
backbone fault tolerant. If communication on a local segment within
any building is disrupted for any reason, that information will not
reach the backbone. End-to-end reliability is possible only when
redundancy is employed throughout the internetwork. Because this
redundancy is usually cost-prohibitive, most companies prefer to
employ redundant paths on only those segments that carry
mission-critical information.
What does it take to make the backbone reliable? Routers hold the
key to reliable internetworking. Depending on the definition of
reliability, this can mean duplicating every major system on each
router and possibly every component. However, hardware component
duplication is not the entire solution because extra circuitry is
necessary to link the duplicate components to allow them to
communicate. This solution is usually very expensive, but more
importantly, it does not completely address the problem. Even
assuming all routers in your network are completely reliable
systems, link problems between nodes within a backbone can still
defeat a redundant hardware solution.
To really address the problem of network reliability, links must
be redundant. Further, it is not enough to simply duplicate all
links. Dual links must terminate at multiple routers unless all
backbone routers are completely fault tolerant (no single points of
failure). Otherwise, backbone routers that are not fault tolerant
become single points of failure. The inevitable conclusion is that a
completely redundant router is not the most effective solution to
the reliability problem because it is expensive and still does not
address link reliability.
Scalable protocols, such as EIGRP and OSPF, enable a router to
maintain a map of the entire network topology, so when a failure is
detected the router can reroute traffic by looking at the network
topology and finding another path. EIGRP is also a feasible solution
because it keeps a record of alternate routes in case the preferred
route goes away.
Load Balancing
Load balancing is the easiest way to add bandwidth in a network
with multiple links. Routers provide built-in load balancing for
multiple links and paths. You can use up to four paths to a
destination network. In some cases, the paths do not need to be of
equal cost.
Within IP, routers provide load balancing on both a per-packet
and a per-destination basis. For per-destination load balancing,
each router uses its route cache to determine the output interface.
If IGRP or EIGRP routing is used, unequal-cost load balancing is
possible. The router uses metrics to determine which paths the
packets will take; the user can adjust the amount of load balancing.
Because scalable protocols have a map of the entire network
topology, and because of the way in which they maintain their
routing tables, they are able to transport data simultaneously
across multiple paths to a given location, as shown in the figure to
the left.
Tunnels
Software tunnels can provide communication across WAN links into
network areas that were previously unreachable. Tunnels allow you to
configure a point-to-point link between two discontiguous networks
running a given protocol, such as IPX, without configuring the
entire cloud for IPX. Not only does this provide availability, but
it also eliminates the overhead associated with running an
additional routing protocol over the link. In addition, tunneling
provides a means for encapsulating packets inside a routable
protocol via virtual interfaces.
Dial Backup
On WAN connections, you can configure backup links when you need
to perform the following tasks:
- Make the primary WAN connection more reliable by configuring
one or more on backup connections.
- Increase availability by configuring the backup connections to
be used when a primary connection is experiencing congestion.