We apply some of the existing web server redundancy techniques for high service availability and scalability to the relatively new IP telephony context. The paper compares various failover and load sharing methods for registration and call routing servers based on the Session Initiation Protocol (SIP). In particular, we consider the SIP server failover techniques based on the clients, DNS (Domain Name Service), database replication and IP address takeover, and the load sharing techniques using

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... NS, SIP identifiers, network address translators and servers with same IP addresses. Additionally, we present an overview of the failover mechanism we implemented in our test-bed using our SIP proxy and registration server and the open source MySQL database.