System information

98 Deploying Samba on IBM Eserver BladeCenter
In the example in Figure 6-1 on page 97, virtual server load balancing is used. The switch has
two different V_IPs configured which correspond to each of the services (Sales and
Marketing). DNS responds with these V_IPs when these services are requested. The GbESM
is also configured for the real IP addresses (R_IPs) that are owned by blades that support
requests coming into each V_IP. In this case, all server blades can respond to requests for all
services, so each V_IP is assigned the entire set of R_IPs.
A TCP connection consists of a series of packets sent back and forth from two destinations. If
packets are misrouted and sent to the wrong destination, the session will slow down due to
retransmissions and possibly fail if misrouting continues. Therefore, a Layer 4-7 switch needs
to be TCP-aware in the sense that it needs to be able to:
򐂰 Identify the beginning of a new TCP connection
򐂰 Assign that connection to a real server
򐂰 Make sure that all ensuing packets related to that TCP connection continue to be sent to
the same real server
This requirement is what puts the “Layer 4” in L4-7. The GbESM provides this capability. All
packets arriving that are destined to a V_IP are inspected to determine whether they are
associated with an existing TCP connection (in which case they are re-directed to the real
server already assigned) or whether they are a request to set up a new TCP connection. If the
received packet is a request to establish a new TCP connection (a TCP SYN packet), the
GbESM will determine the best available server to re-direct that request to and then do so.
The “best available” consideration can be made based upon a number of factors that are
beyond the scope of this chapter. It is enough to say that “best available” is based on a
consideration of the present health and load of each of the candidate real servers.
This chapter introduces the concepts of Layer 4-7 Switching and how it benefits customers
deploying applications on the IBM Eserver BladeCenter. For a more in-depth look at Layer
4-7 Switching, refer to IBM Eserver BladeCenter Layer 2-7 Network Switching, REDP3755.
In addition to expanding on the concepts discussed here, this Redpaper is an excellent guide
to deploying the GbESM in both Nortel and Cisco networking environments.