Evaluating Backbone Services
Load balancing bridged traffic over serial lines is also supported. Serial lines can be assigned to
circuit groups. If one of the serial links in the circuit group is in the spanning tree for a network, any
of the serial links in the circuit group can be used for load balancing. Data ordering problems are
avoided by assigning each destination to a serial link. Reassignment is done dynamically if interfaces
go down or come up.
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 simply by 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 only possible when redundancy is employed
throughout the internetwork. Because this is usually cost prohibitive, most companies prefer to
employ redundant paths only on 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.
