Server Load Balancing (SLB)

    May 15, 2006

When youre working on your BCMSN exam on your way to CCNP certification, youll read at length about how Cisco routers and multilayer switches can work to provide router redundancy – but theres 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. Theyre represented to the hosts as the virtual server

The hosts will seek to communicate with the server at, not knowing that theyre 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. Well first add to the server farm:

MLS(config)# ip slb serverfarm ServFarm

MLS(config-slb-sfarm)# real

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 farms workload. The real and inservice commands should be repeated for each server in the server farm.

To create the virtual server:

MLS(config)# ip slb vserver VIRTUAL_SERVER

MLS(config-slb-vserver)# serverfarm ServFarm

MLS(config-slb-vserver)# virtual

MLS(config-slb-vserver)# inservice

>From the top down, the vserver was named VIRTUAL_SERVER, which represents the server farm ServFarm. The virtual server is assigned the IP address, and connections are allowed once the inservice command is applied.

You may also want to control which of your network hosts can connect to the virtual server. If hosts or subnets are named with the client command, those will be the only clients that can connect to the virtual server. Note that this command uses wildcard masks. The following configuration would allow only the hosts on the subnet /24 to connect to the virtual server.

MLS(config-slb-vserver)# client

SLB is the server ends answer to HSRP, VRRP, and GLBP – but you still need to know it to become a CCNP! Knowing redundancy strategies and protocols is vital in todays networks, so make sure youre comfortable with SLB before taking on the exam.

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