Welcome to this Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and ask questions about configuring and troubleshooting the Global Site Selector (GSS) with expert Swati Chopra.
GSS devices represent the next generation of application switches and global server load balancing (GSLB) appliances. Working together with the Cisco ACE Module and Cisco ACE 4710 appliance, these devices form an application-fluent networking solution that improves availability, acceleration, and security for data center applications.
The primary role of Cisco GSS is to implement the business continuance and disaster recovery policies of a business by optimizing and securing the Domain Name System (DNS) infrastructure of the data center. It does this by integrating with the DNS infrastructure and responding to the client DNS requests, thereby directing the client to the site that is best able to serve its needs.
Swati Chopra is a CCNA, CCNP, and VCP certified customer support engineer for content switching, covering technologies such as Cisco Application Control Engine, Cisco Wide Area Application Services, Global Site Selector, Cisco Content Services Switches, and Digital Media Suite. She has been with Cisco for more than three years and has worked with most of the high-end customers on content-related complex cases. She completed her master’s degree in finance, was heading an online education project in collaboration with e-Sylvan, and later moved to technical services because of her love for technology. She is actively involved with diverse Cisco initiatives such as Connected Women, WISE, and Cisco Career Connections and recently hosted a “Birds of Feather” table at Cisco’s Women of Impact conference.
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Because of the volume expected during this event, Swati might not be able to answer every question. Remember that you can continue the conversation in the Data Center community under subcommunity Application Networking shortly after the event. This event lasts through April 25, 2014. Visit this forum often to view responses to your questions and the questions of other Cisco Support Community members.
Hi Sarah,
The load balancing mechanism for GSS requests is done via different methods. We can use these methods to define how the load is shared for different balance clauses within the same rule. The 6 methods we use are:
–
round-robin—The GSS cycles through the list of answers that are available as requests are received. Each resource within an answer group is tried in turn. The GSS cycles through the list of answers, selecting the next answer in line for each request. This is the default.
eg: if we have 2 answers in answer group then GSS will provide them alternatively.
–least-loaded—The GSS selects an answer based on the load reported by each VIP in the answer group. The answer reporting the lightest load is chosen to respond to the request.The least-loaded option is available only for VIP-type answer groups that use a KAL-AP or Scripted keepalive, as they provide the GSS with detailed information on the SLB load and availability.
eg: if one answer has higher load than the other, than first answer will not be provided until its load goes down the other answers
–ordered—The GSS selects an answer from the list based on precedence; answers with a lower order number are tried first, while answers further down the list are tried only if preceding answers are unavailable to respond to the request.
for eg: answer with ordered number 1 will be provided first till it becomes unavailable. Once it is unavailable then answer with ordered list number 2 will be provided
–weighted-round-robin—The GSS cycles through the list of answers that are available as the requests are received, but sends requests to favored answers in a ratio determined by the weight value assigned to that resource.
eg: if one answer has more weight(80%) than the other answer(20%), then it will be used 8 times out of 10.
–hashed— When the GSS uses the hashed balance method, elements of the client's DNS proxy IP address and the requesting client's domain are extracted to create a unique value, referred to as a hash value. The unique hash value is attached to and used to identify a VIP that is chosen to serve the DNS query.
The use of hash values makes it possible to "stick" traffic from a particular requesting client to a specific VIP, ensuring that future requests from that client are routed to the same VIP. This type of continuity can be used to facilitate features, such as online shopping baskets, in which client-specific data is expected to persist even when client connectivity to a site is terminated or interrupted.
The GSS supports the following two hashed balance methods. You can apply one or both hashed balance methods to the specified answer group.
• By Source Address—The GSS selects the answer based on a hash value created from the source address of the request.
• By Domain Name—The GSS selects the answer based on a hash value created from the requested domain name.
for eg: a user using same source ip will get the same answer from GSS if we do source address hashing.
-DNS Race (Boomerang) Method-The GSS supports the DNS race (boomerang) method of proximity routing, which is a type of DNS resolution initiated by the GSS to load balance 2 to 20 sites.
The boomerang method is based on the concept that instantaneous proximity can be determined if a CRA within each data center sends an A-record (IP address) at the exact same time to the client's D-proxy. The DNS race method of DNS resolution gives all CRAs (Cisco content engines or content services switches) a chance at resolving a client request and allows for proximity to be determined without probing the client's D-proxy. The first A-record received by the D-proxy is, by default, considered to be the most proximate.
Use case is mainly with CRA's.
Hope this helps. Please feel free to revert if you have follow-up questions.
Thanks,
Swati
