Nomadix Access Gateway, User Manual

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32 Introduction
ISP link Selection Criteria
In a load-balancing scenario, some criteria must be used to decide which ISP is selected for
outgoing traffic. There a number of factors that influence this decision, including:

Identity of the users: Is a random ISP section used or is it desirable to have certain
users steered toward a particular ISP?

For random ISP: Whether subscriber, destination address or session-based link
selection is used?
User-Based ISP Selection versus Random ISP Selection
User-based ISP selection is the process whereby the ISP link that is selected in a load-balanced
environment is based on the identity of the user. For example, all users from guest rooms may
be steered toward one ISP link, and all meeting room users steered toward another ISP link that
is only used for meetings and conferences.
The alternative is to use random ISP selection, whereby the load balancer or NSE selects the
ISP to be used according to the current load conditions. The Nomadix NSE uses random ISP
selection by default.
Link Availability Detection Method and Time
Load balancing and failover requires some form of monitoring of each ISP link to determine its
availability for executing load balancing and failover decisions. Generally, link monitoring is
accomplished by two different methods:
1.
Periodic probing of predefined hosts using HTTP or ICMP ping requests.
2.
Periodic DNS queries to the DNS servers provided by each ISP.
The period between successive link tests is usually configured, and is typically set to between
30 seconds and 60 seconds. This represents the maximum time for which a user will remain
connected to a failed ISP connection before being re-routed to a working ISP link in an ISP
failure scenario.
Traffic Balancing and Weighting
Load balancers have some form of weighting of traffic between links to achieve a desired
balance scenario. With the Nomadix NSE, traffic is balanced by individual subscriber numbers,
and weighted according to the speed of the ISP connected to each port. For example, if an NSE
has 2 x 10M links connected and currently has 100 active subscribers, then 50 users would be
connected to each link. If the ISP links were 10 Mbps and 40Mbps, then 20 users would be
connected to the 10M link and 80 users to the 40M link, and so on.