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Multihoming enables an interface to assume multiple IP addresses on the same network. Multihoming can be used for all IP networks bound to a router. This is irrespective of whether the networks are bound to the same interface or to different interfaces.
The most common use of multiple addresses on the same network is to enable a Web server to operate as though it were several Web servers. One application is to use each secondary IP address to point to a different Web page on the same Web server, depending on the Domain Name System (DNS) name that is used to reach the server.
Multihoming is also commonly used with network address translation (NAT), the proxy server, and the virtual private network (VPN). In all cases, the secondary IP address can be configured on the same interface that has the primary IP address. Or the secondary address can be configured on a different interface. When there are multiple interfaces, the secondary address is associated with the interface that is bound to the network that uses the same address. If the secondary address is not valid on any of the networks bound to existing interfaces, the address is rejected and an error message is produced.
The multihoming feature has been extended to help configure the stack for load balancing and fault tolerance at the NIC/Link level. The TCP/IP stack also supports grouping NICs with similar characteristics in order to facilitate load balancing and fault tolerance across them.
In NetWare 6, TCP/IP has two levels of enabling load balancing and fault tolerance: one at the system level and another at the local group level. To benefit from this, make sure that you have fully enabled TCP/IP at both levels.
Five types of multihoming configurations are provided:
Single NIC Multiple IP Addresses (Different IP Addresses)---In this type of multihoming, client-to-server traffic can be distributed across the routers. The required level of load balancing can be achieved through static routers and dynamic DNS. Any dynamic algorithm will take care of the fault tolerance. With a static route configuration, the Multiple Default Gateway with Dead Gateway Detection support will help achieve the same results.
Multiple NIC Multiple IP Address (Different IP Network)---This is a normal router configuration. All the configuration and the advantages gained by the previous type of multihoming are applicable with this type also. In addition, load balancing server-to-client traffic across the NICs and routers is also possible. This can be achieved with the help of multipath routing.
Multiple NIC Multiple IP Address (Same IP Network)---This type of multihoming allows for both server-to-client and client-to-server load balancing and fault tolerance at the Link level. This type of multihoming is especially helpful in cases where Route level load balancing and fault tolerance are not required and create overhead. Once this type of multihoming is supported on the server side, the outgoing traffic load is NIC based on various parameters such as the destination IP address and interference lead. If a NIC fails, the lead is automatically distributed among the remaining NICs to achieve fault tolerance.
Multiple NIC Single IP Address---The clients use the same IP address to communicate with the server. This is achieved through using the round-robin method of distributing the NIC addresses for the ARP requests sent by the clients. This solution avoids extra configuration and transparently achieves the load balancing for incoming packets. If a NIC fails, the server sends a message to the clients to forcefully use them as the other interface's MAC address.
Secondary IP Address (Multiple Logical Hosts)---This type of multihoming allows you to create multiple logical hosts that belong to the same network. In a multihoming setup where multiple NICs are grouped to support a single network, the secondary IP address with this type of configuration supports an option to select one of the NICs in the group. By using the non-ARPable option, these addresses can be used as virtual IP addresses for load balancing solutions. Through this option, the same IP address can be configured on all servers and the load balancer can distribute the client load across these servers.
In case of multihoming, if fault tolerance is enabled and if either the link is down, or the card is not connected or the card has a fatal error (not able to send packets out), a user can not add a secondary IP address. If the user still wants to add a secondary IP address an error message 'cannot allocate resources to add secondary IP address' will be flashed. This indicates that there is no board to which an IP address can be bound.
For more information, see Configuring Load Balancing and Configuring Fault Tolerance.
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