1 Site persistence in GSLB using hashing
Example
1.1.1.42yields hash index 45 {(1+1+1+42 %256) = 45} 172.168.10.1 yields hash index 95 {(172+168+10+1 %256) = 95}
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 ServerIron reorders the IP address in the DNS server’s response so that the best IP address is first. Then it forwards the modified response to the client.
IP address allocation
IP addresses are first ordered with the lowest IP having rank 1. IPs are allocated to hash buckets in a round robin fashion starting with lowest IP first.
Example
Assume a user has configured IPs 1.1.1.44 and 1.1.1.42 for www.foo.com. The IP addresses are sorted in ascending order.
1.1.1.42(rank 1)
1.1.1.44(rank 2)
The hash allocation for www.foo.com looks like the following:
0 1 2
.42 | .44 | .42 | ................................. |
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255
.44
If the IP address of a client querying for www.foo.com hashes to hash index 2, then 1.1.1.42 will be selected as the best IP address for this client.
IP address failure or removal from domain
In the previous example, assume a user removes 1.1.1.44 for domain www.foo.com. The IP address for www.foo.com is 1.1.1.42 (rank 1)
In this scenario, all the hash indexes allocated to 1.1.1.44 will be cleaned up. All the empty hash indexes will be reassigned to existing IP addresses in round robin fashion as described in the section "IP Address Allocation".
0 | 1 | 2 |
|
.42 | .42 | .42 | ................................. |
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255
.42
Rehash: new IP address for a domain or change of state
This section describes how the ServerIron ADX handles the introduction of a new IP address for a domain or change of state of an IP address from down to healthy (rehash mechanism).
Assume the
1.1.1.42(rank 1)
1.1.1.44(rank 2)
72 | ServerIron ADX Global Server Load Balancing Guide |
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