Local Clustering and Failover in Visual Studio .NET

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Local Clustering and Failover
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to change one of the first six 4-bit values in the address. MAC address prefixes are assigned in blocks to vendors, so all equipment with the 8:0:20 prefix is from Sun Microsystems.2 If you choose 8:0:21 (which is not assigned to any company) as a prefix for a private MAC address (keeping the same 24-bit suffix), and verify that you have no other equipment using that address, you should be safe. The second way is to follow some of the guidelines for locally assigned numbers in Internet RFC 1597, which is already rather dated. You ll need to be sure that the primary recognizes that a takeover has occurred and goes back to its default or builtin MAC address; you also want to be sure that if the primary reboots and comes back as the secondary node, it also uses its built-in default MAC address rather than the locally created, public one. The advantage of moving the MAC address is that clients don t have to do a thing. The IP-to-MAC-address mapping stays the same, since you move both the logical IP address and MAC address to the secondary host. However, some network media do not support migrating MAC addresses, and more and more clustering software does not support it either.
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Wait for the clients to realize that the host formerly listening on that MAC address has gone away, and have clients send new ARP requests for the public IP address. If ARP entries are only cached for 30 seconds, this means that there s a 30-second MTTR before a new ARP request is sent, and the clients see a short pause (probably no longer than is required to get the secondary machine up and running anyway). You ll need to be sure that the client application can tolerate this delay, and that your ARP cache entries timeout quickly. Many system administrators tune the ARP cache expiration period up to over two hours, reducing the volume of ARP traffic on the network, but making it impossible for clients to recover from an IP address migration quickly. In short, you want to keep ARP cache timeouts at their minimum levels on all clients of HA systems, because you want to be sure they ll find the new server after a takeover, even if the client misses (or ignores) a gratuitous ARP sent by the secondary server.
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There are other side effects of moving MAC addresses between hosts. Switches and hubs that track MAC addresses for selective forwarding need to handle the migration. Not all equipment does this well. You want to be sure that moving a MAC address doesn t trigger a spanning tree algorithm that splits the network at a switch that believes a loop has been created through improper connections or cabling. We took a closer look at redundant networks, moving IP addresses, and the use of virtual addresses for ease of management in great detail in 9.
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IEEE maintains an online database of MAC prefixes and the companies who own them. If you are interested, visit http://standards.ieee.org/regauth/oui/index.shtml.
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IP Addresses and Names
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There are three different kinds of hostnames that a system might have in a failover configuration; each type maps to a type of network, as discussed previously. One is its private name. This is the name found in system configuration files, and the one that shows up if you ask the host for its name. Depending on how the network is configured, that name may or may not be resolved on the network. If your machine s private name is shirley, that does not necessarily mean that other hosts on the same network know the machine as shirley. The second kind of name is the public name. This is the name that maps to IP addresses that other systems on the network know about. In a failover configuration, this name and IP address should not map to a particular machine, but rather to the application that is requested through that name. The third kind of name is the administrative interface name. That name may or may not match the system s private name. It s probably best if it does, but it does not have to. For example, in a configuration with an administrative network, one public network, two heartbeat networks, and an NFS server, a pair of servers might use the following hostnames:
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laverne and shirley are the real hostnames. They are the names assigned to the administrative network interfaces, which means that only the system administrators will access the systems using these names. They refer to physical machines and not logical services. milwaukee-nfs is the logical hostname. It doesn t correspond precisely to one interface, but will instead be bound to the primary server s (shirley s) public network interface by default and migrate to the takeover server s (laverne s) public network link during a failover. shirley-net1 is the administrative hostname assigned to the public network on the primary server. Similarly, laverne-net1 is the administrative hostname on the secondary server. Further discussion of administrative networks can be found in the next section. shirley-hb1, shirley-hb2, laverne-hb1, and laverne-hb2 are the hostnames for the interfaces connected to the redundant heartbeat networks. (Note that some FMSs do not require IP addresses or hostnames on their heartbeat ports.)
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Moral of the story: You re going to use an awful lot of hostnames and IP addresses. Make sure that you have a sufficient number of subnets available and IP address space to allocate for each server pair. Also, keep the names obvious, and use suffixes indicating what net, what kind of network, and what number the interface is on a net if required. Refer back to Key Design Principle #1 about simplicity: It counts when you re sorting through 14 network interfaces, trying to figure out why a secondary server can t send NFS replies back to clients connected through a router.
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