Brocade Communications Systems ServerIron ADX 12.4.00 Manual Manual page 89

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GSLB hash-based persistence
GSLB provides two methods for persistence- Sticky method and Hash-based persistence. Sticky
GSLB is suitable for single-box and HA (hot standby, symmetric, sym-active) topologies. However, if
there are two GSLB controllers across a network providing GSLB for the same domain but are not in
an HA configuration, and if persistence is desired when the same client is directed to either of
these two GSLB controllers, then hash-based GSLB persistence should be used. hash-based
Persistence provides GSLB controller persistence in multiple GSLB controller environments. When
users perform a DNS query for a domain, the users will get the same IP address for that domain
regardless of which GSLB controller is contacted. Currently hash-based persistence distributes
hash buckets in a round robin fashion.
GSLB weighted hash-based persistence
In addition to providing hash-based persistence, we will now provide weighted hash-based
persistence. Weighted hash-based persistence allocates the hash buckets in a weighted round
robin fashion. This enables the user not only to maintain persistence, but also to determine what
percentage of the traffic goes to a particular domain IP address.
Hashing scheme
Each domain maintains a separate hash table. For instance, if GSLB controller has the following
two domains www.foo.com and www.test.com configured, then it will maintain one hash table for
each domain. The number of hash buckets for each hash table is 256.
The client IP address is hashed to generate a value between 0 and 255.
After the Client IP address is hashed to an index in the hash table, the IP address associated with
the hash index in the hash table is selected as the best IP address for the client. The GSLB
controller reorders the IP address in the DNS server's response so that the best IP address is
placed in the first position. It then forwards the modified response to the client.
IP address allocation
Firstly, IP addresses are ordered with the lowest IP having rank 1. IPs will be allocated to hash
buckets in a weighted round robin fashion starting with lowest IP first. This is done so that no
synchronization is required across Controllers.
Example
Consider the example where user has configured IPs 1.1.1.44, 1.1.1.43 and 1.1.1.42 for
www.foo.com. The IP addresses are first sorted in ascending order.
1.1.1.42 (rank 1)
1.1.1.43 (rank 2)
1.1.1.44 (rank 3)
User also configures hash weights for these IP addresses. Say the weights for the IP addresses are
as follows.
1.1.1.42: weight 1
1.1.1.43: weight 1
1.1.1.44: weight 2
ServerIron ADX Global Server Load Balancing Guide
53-1002437-01
Weighted distribution of sites with hash-based persistence
"Disabling rehash on change in hash weight configuration" on page 79
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