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Overview of GSS Functionality

Oct. 13, 2011
Mahmoud El Safty

Mahmoud El Safty is a Global Knowledge Cisco instructor who teaches and blogs from Global Knowledge Egypt.

The Global Site Selector (GSS) leverages the Domain Name System (DNS) to provide clients with reliable and efficient content services. Domain to IP address mapping is performed with consideration for availability, location, and load of content servers. Using the GSS in combination with Cisco’s Content Services Switch (CSS), Cisco’s Catalyst 6000 Content Switching Module (CSM), or Cisco’s Application Control Engine (ACE) allows users to create Global Server Load Balancing (GSLB) networks.

The GSS may be deployed in a variety of locations in a customer’s network to serve DNS requests, providing answers based on availability and preference. The GSS combines basic concepts of DNS with monitoring of answer status and providing users with IP addresses that are most appropriate for their content requests.

The GSS provides configuration and monitoring services through a central configuration manager, the Global Site Selector Manager (GSSM), and through a CLI that is available on each GSS. Configuration for a GSS network is mostly identical on all devices (global config model) and is entered by the user on a single GSS (central configuration model).  For standard features, the customer may choose to create a network of up to 8 GSSs with global/central configuration. The customer may instead choose to configure and monitor individual devices (local configuration model), in which case the GUI runs independently on each GSS and configuration is not shared.

The GSS receives DNS queries from client DNS proxies (D-Proxy) and matches these requests with a user-defined set of DNS Rules. A match on a DNS rule provides the list of 1st, 2nd, and 3rd choice sets of answers that should be considered for the request.

Within a GSS network an answer is a host address which identifies a resource within a network that the GSS can direct a user to in order to respond to a content request. GSS answers are either a Virtual IP (VIP) Address associated with a server load balancer (SLB), a Name Server which can answer queries that the GSS cannot, or a Content Routing Agent (CRA) that use a resolution process called DNS race to send identical and simultaneous responses back to a user’s D-proxy.

The DNS rule also defines the balancing methods that should be applied for choosing from each set of possible answers and can be combined with advanced features including checking for answers with the closest network proximity to the client’s requesting D-proxy and use of a sticky database.

In addition to answering queries directly, the GSS offers the feature of forwarding requests to NS Forwarders, which will return a DNS response packet to the GSS, which in turn returns the exact same response packet to the originally requesting D-Proxy. This can be used for any query type on any domain and is not limited to the record types supported by the GSS.

All of these options are user-configurable, and their suitability depends upon the customers requirements. Please refer to the GSS Configuration Guide for more information on the latest features available.