Within the Cloud: Technical Concepts of Cloud Computing - Deploying and Managing a Cloud Infrastructure - page 60

There are two ways to plan for redundancy, symmetrical and asymmetrical:
Symmetrical Symmetrical network redundancy can be achieved by leasing, for example,
two T3 lines from two separate providers.
Asymmetrical Asymmetrical redundancy is by leasing a T3 line from one provider and a
T1 line from another provider.
Network redundancy can be either active-active or active-passive.
Active-Active Active-active means that both lines are working together simultaneously;
this type of redundancy provides the greatest bandwidth because both lines are working
at the same time.
Active-Passive Active-passive redundancy, however, is what we consider backup redun-
dancy. Only one line is really active at a time, and that is the main line, which has a bigger
capacity. The other, lower-capacity line is used as backup in case the main line becomes
unavailable. This scheme provides redundancy at a lower cost, at the expense of bandwidth.
Having an active-active network is beneficial for organizations that deal
with high volumes of traffic from their customers. It does not simply offer
high bandwidth, it's also a good way to maintain high availability. However,
a cost-benefit ratio must be thoroughly analyzed to determine if this costly
method is suitable.
Another redundancy measure aside from a redundant wide area network (WAN) is to
be redundant at the edge of your network. It is always better to have two Internet routers
meshed to your local area network (LAN) so that when one fails, there is still network
connectivity between your data center and the outside.
Support and Maintenance Strategy
A high-tech installation such as a data center should be well maintained to keep it running in
top condition even after many years of service. Our worst enemy would be software degrada-
tion. This is eventual failure of the various applications and OSs running in your servers. Due
to processing, constant file transfers, data traffic, and other normal day-to-day processes,
settings and file systems can get corrupted and cluttered. This leads to eventual slowdowns
and even failures such as crashes and hangs. This can happen even when your hardware is
kept in the best condition possible, so it is essential to have regular server maintenance (either
automated or manual). This involves backing up critical data and cleaning or resetting various
parameters and settings to make sure everything works as expected.
Eventually something will break down, so there has to be proper procedures to follow
when it does. Backup systems have to be put in place to take over for whatever system broke
down. A proper upgrade path also needs to be laid out. Which parts of the system need to
be eventually upgraded or replaced with newer versions or standards has to be figured out
beforehand. Upgrading has to be made easy; the center has to be laid out in a way that does
not require a lot of teardown.