The effectiveness of the DRP internal metric is determined by the quality of data in the IGP routing tables. When the server availability parameter is enabled for a distributed server, the Director uses periodic, temporary TCP connections to verify that the server is available and prevents the Director from redirecting clients to a server that cannot respond. This means that an initial request to the Director from a specific local DNS server triggers DRP querying and DRP reply sorting. The Director caches the sorted DRP replies, along with the IP address of the querying local DNS server. This is efficient because the local DNS server will probably issue DNS requests for large numbers of clients, and the caching eliminates DRP querying for multiple users in the same topological proximity as their shared DNS server.
Performance is improved and network overhead is decreased.
However, the TTL attached to DNS replies is configurable. Access lists enable the network administrator to limit DRP server agent access to devices having specific source IP addresses. This technique enables DRP devices to authenticate the identities of DRP query and response sources. Figure 1-4 shows the front panel of the Cisco DistributedDirector 2501. Figure 1-5 shows the rear panel.