Information Technology Reference
In-Depth Information
Redundancy is a critical component of WAN design for the remote site because of the un-
reliable nature of WAN links, when compared to LANs that they connect. Most enterprise
edge solutions require high availability between the primary and remote site. Because
WA N l i n k s h a v e l o w e r r e l i a b i l i t y a n d l a c k b a n d w i d t h , t h e y a r e g o o d c a n d i d a t e s f o r m o s t
WA N b a c k u p d e s i g n s .
Branch offices should have some type of backup strategy in the event of a primary link
failure. Backup links can be either dialup, permanent WA N , or Internet-based connections.
WA N b a c k u p o p t i o n s a r e a s f o l l o w s :
Dial backup:
ISDN provides backup dialup services in the event of a primary failure
of a WA N circuit. The backup link is initiated if a failure occurs with the primary link.
The ISDN backup link provides network continuity until the primary link is restored,
and then the backup link is terminated such as with floating static route techniques.
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Key
To p i c
Secondary WAN link:
Adding a secondary WAN link makes the network more fault
tolerant. This solution offers two key advantages:
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Backup link:
Provides for network connectivity if the primary link fails.
Dynamic or static routing techniques can be used to provide routing consistency
during backup events. Application availability can also be increased because of
the additional backup link.
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Additional bandwidth:
Load sharing allows both links to be used at the same
time, increasing the available bandwidth. Load balancing can be achieved over the
parallel links using automatic routing protocol techniques.
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Shadow PVC:
SPs can offer shadow Frame Relay PVCs, which provide additional
PVCs for use if needed. The customer is not charged for the PVC if it does not ex-
ceed limits set by the provider while the primary PVC is available. If the limit is ex-
ceeded, the SP charges the customer accordingly.
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IPsec tunnel across the Internet:
An IPsec VPN backup link can direct redirect
traffic to the corporate headquarters when a network failure has been detected.
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Load-Balancing Guidelines
Load balancing can be implemented per packet or per destination using fast switching. If
WA N l i n k s a r e l e s s t h a n 5 6 k b p s , p e r - p a c k e t l o a d b a l a n c i n g i s p r e f e r r e d . F a s t s w i t c h i n g i s
enabled on WAN links that are faster than 56 kbps, and per-destination load balancing is
preferred.
A major disadvantage of using duplicate WAN links is cost. Duplicate WAN links require
additional WAN circuits for each location, and more network interfaces are required to
terminate the connections. However, the loss of productivity if a site loses network con-
nectivity and becomes isolated can be greater than the cost of the duplicate WAN link.
