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Load Balancing Servers, Firewalls and Caches - Kopparapu C.

Kopparapu C. Load Balancing Servers, Firewalls and Caches - Wiley Computer Publishing, 2002. - 123 p.
ISBN 0-471-41550-2
Download (direct link): networkadministration2002.pdf
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Figure 4.15: High availability #8.
Another issue in this design is that an active NIC or the link to the active NIC may fail on one or more servers, causing the standby NIC to take over. The active load balancer continues to function, but will have no access to the servers if the standby load balancer does not forward normal traffic. In general, this design is prone to bugs and should be avoided unless one takes adequate care to work around these issues.
In the design shown in Figure 4.15, the active interfaces from servers are divided between the two load balancers. We need to configure the load balancers in active-active mode and bind VIP1 to RS1 and RS2, and VIP2 to RS3 and RS4. Set the default gateway for RS1 and RS2 to gateway IP1, and for RS3 and RS4 to gateway IP2. When both load balancers are working, we will be able to utilize all the load balancers and the servers. If load balancer 1 fails, VIP1 and gateway IP1 fail over to load balancer 2. But the key, again, is to ensure that the active NIC interface connected to the load balancer also fails over at the same time to provide connectivity to load balancer 2, as discussed in the previous design shown in Figure 4.14. But one improvement in this design is that if one of the active NIC interfaces fails, the standby interface takes over to provide connectivity through the other load balancer. If the active NIC interface on RS1 fails, load balancer 1 will still be able to access RS1 through load balancer 2 because, since we are using active-active mode, load balancer 2 is forwarding traffic. Since we set the default gateway for RS1 to gateway IP1, the server reply traffic will still flow through load balancer 1. If the default gateway were not properly matched with the VIP that’s bound to the server, or if the default gateway were set to the router instead of the load balancer’s source IP, we would have an asymmetrical reply flow.
61
Multiple VIPs
We can take advantage of shared VIP in this design in which both load balancers can process traffic for a given VIP. With shared VIP, we don’t have to worry about how the reply traffic comes back and whether default gateway is set right. We can bind each VIP to all servers and whichever load balancer gets the reply packet first will process it.
Using active-active configuration allows us to access each server from any load balancer. Using shared VIP frees us from having to bother with how the reply traffic flows back and allows us to bind each VIP to all servers.
In the design shown in Figure 4.16, we now introduce the active-active NIC interfaces, in which both interfaces are active at the same time. It’s important to keep in mind that each real server IP address looks like one real server to the load balancer. So, a real server with two active NICs, each with its own IP address, will look like two independent real servers to the load balancer. Depending on the operating system on the server, we need to configure the IP addresses for the NIC interfaces. For example, Linux allows IP addresses for both the NIC interfaces to be in the same subnet. Some operating systems may require that the IP addresses for the two NIC interfaces be on different subnets. We also need to check whether we can set the default gateway for each NIC interface or not. If the operating system only allows one default gateway to be set, all the replies will go back through the same default gateway no matter which interface gets the requests, causing asymmetric traffic flows. Therefore, it’s good to use DSR or source NAT when connecting servers to multiple load balancers using two or more NIC interfaces in the servers, unless we exactly understand the operating system and NIC behavior.
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Figure 4.16: High availability #9.
When a NIC fails, the load balancer will consider the real server identified by the IP address on that NIC to be down, although that real server continues to be available through the second NIC. In general, active-active NIC interfaces can pose problems because of the issues just discussed.
One of the reasons to use multiple NIC interfaces in a server is not only to get high availability, but also to get more throughput. As processing power has grown in servers, the servers have become capable of performing increasing amounts of network I/O throughput. Any decent server can easily fill a 100-Mbps NIC interface today. A medium- to high-end server may be able to fill a 1-Gbps interface. Driving more than 1 Gbps of throughput will probably require a lot of optimizations and very high-end hardware. So, the easiest way to get more than 100 Mbps of throughput from a server is to use gigabit NIC interfaces rather than multiple 100-Mbps links. This will avoid all the issues we have seen with using multiple NICs. But many users are wary of losing a server costing $100,000, just because a NIC costing $1,000 failed. Therefore, dual NIC interfaces help protect the server from NIC-interface or link failures. But dual NIC interfaces bring a host of issues along with them, as we discussed in the aforementioned designs. If the user can work with DSR and active-active configuration, that’s a great way to go when using dual NICs because the reply traffic can flow in any path with DSR. But not everyone may want to use DSR because one may not want to configure the
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