Scenario: A Default Only, One Primary, and One Backup Link
In the scenario illustrated in Figure
, the customer wants only default routes toward the provider, and does not
want to accept partial or full Internet routes. The customer wants to use
one link as the primary traffic conduit and the other as a backup in case
the primary link goes down.
(If there were more than two connections to the provider, the customer
could set up multiple defaults with varying preference levels.)
Customer's Outbound Traffic
Where a single router is used to connect to the provider in
multiple locations, multiple static default routes with different
administrative distances can be used. The default with the lower
distance will be the primary.
If the customer's network administrator would rather learn the
default route (or few aggregate routes) dynamically from the provider,
they can ask the ISP to filter the routes advertised to the minimum
possible. Local preference can be used to prefer one default to the
other.
Assume in Figure
that the default to New York is more preferred than the default to San
Francisco. In normal operations, the customer will use the New York
link as the primary link and the San Francisco link as a backup.
For outbound traffic, load balancing is not an option because all
traffic is sent over the primary line, and the secondary is kept as
backup. Absence of load balancing is offset by the fact that the
customer's router requires less memory and processing power.
Customer's Inbound Traffic