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When you’re working on your BCMSN exam on your way to CCNP certification, you’ll read at length about how Cisco routers and multilayer switches can work to provide router redundancy - but there’s another helpful service, Server Load Balancing, that does the same for servers. While HSRP, VRRP, and CLBP all represent multiple physical routers to hosts as a single virtual router, SLB represents multiple physical servers to hosts as a single virtual server.

In the following example, three physical servers have been placed into the SRB group ServFarm. They’re represented to the hosts as the virtual server 210.1.1.14.

The hosts will seek to communicate with the server at 210.1.1.14, not knowing that they’re actually communicating with the routers in ServFarm. This allows quick cutover if one of the physical servers goes down, and also serves to hide the actual IP addresses of the servers in ServFarm.

The basic operations of SLB involves creating the server farm, followed by creating the virtual server. We’ll first add 210.1.1.11 to the server farm:

MLS(config)# ip slb serverfarm ServFarm

MLS(config-slb-sfarm)# real 210.1.1.11

MLS(config-slb-real)# inservice

The first command creates the server farm, with the real command specifying the IP address of the real server. The inservice command is required by SLB to consider the server as ready to handle the server farm’s workload. The real and inservice commands should be repeated for each server in the server farm.
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