Computer Certification

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.

To create the virtual server:

MLS(config)# ip slb vserver VIRTUAL_SERVER

MLS(config-slb-vserver)# serverfarm ServFarm

MLS(config-slb-vserver)# virtual 210.1.1.14

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 210.1.1.14
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 210.1.1.0 /24 to connect to the virtual server.

MLS(config-slb-vserver)# client 210.1.1.0 0.0.0.255

SLB is the server end's 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 today's networks
so make sure you're comfortable with SLB before taking on the exam.

Passing the BCMSN exam and getting one step closer to the CCNP certification means learning and noticing details that you were not presented with in your CCNA studies. (Yes
I know – you had more than enough details then
right?) One protocol you’ve got to learn more details about is VTP
which seemed simple enough in your CCNA studies! Part of learning the details is mastering the fundamentals
so in this tutorial we’ll review the basics of VTP.


In show vtp status readouts
the "VTP Operating Mode" is set to "Server" by default. The more familiar term for VTP Operating Mode is simply VTP Mode
and Server is the default. It's through the usage of VTP modes that we can place limits on which switches can delete and create VLANs.

In Server mode
a VTP switch can be used to create
modify
and delete VLANs. This means that a VTP deployment has to have at least one switch in Server mode
or VLAN creation will not be possible. Again
this is the default setting for Cisco switches.

Switches running in Client mode cannot be used to create
modify
or delete VLANs. Clients do listen for VTP advertisements and act accordingly when VTP advertisements notify the Client of VLAN changes.

VTP Transparent mode actually means that the switch isn't participating in the VTP domain as Servers and Clients do. (Bear with me here.) Transparent VTP switches don't synchronize their VTP databases with other VTP speakers. They don't even advertise their own VLAN information! Therefore
any VLANs created on a Transparent VTP switch will not be advertised to other VTP speakers in the domain
making them locally significant only. (I know you remember that phrase from your CCNA studies!)

Devices running VTP Transparent mode do have a little something to do with the other switches in the VTP domain
though. When a switch running in Transparent mode receives a VTP advertisement
that switch will forward that advertisement to other switches in that VTP domain.

Configuring switches as VTP Clients is a great way to “tie down” VLAN creation capabilities to switches that are under your physical control. However
this occasionally leads to a situation where only the VTP clients will have ports that belong to a given VLAN
but the VLAN still has to be created on the VTP server. (VLANs can be created and deleted in transparent mode
but those changes aren't advertised to other switches in the VTP domain.)


In the next BCMSN tutorial
we’ll take a look at the details of VTP.