Cisco Cisco IOS Software Release 12.2(14)ZA

In dispatched mode, the virtual server address is known to the real servers; you must configure the virtual 
server IP address as a loopback address, or secondary IP address, on each of the real servers. IOS SLB 
redirects packets to the real servers at the media access control (MAC) layer. Since the virtual server IP 
address is not modified in dispatched mode, the real servers must be Layer 2-adjacent to IOS SLB, or 
intervening routers might not be able to route to the chosen real server.

For Catalyst 6500 Family Switches, dispatched mode with hardware data packet acceleration generally 
yields better performance than directed mode.

Refer to the “Configuring Logical Interfaces” chapter of the Cisco IOS Interface Configuration Guide, 
Release 12.2 for more information about configuring the loopback address.

In directed mode, the virtual server can be assigned an IP address that is not known to any of the real 
servers. IOS SLB translates packets exchanged between a client and a real server, using NAT to translate 
the virtual server IP address to a real server IP address.

IOS SLB supports the following types of NAT:

You can use both server NAT and client NAT for the same connection.

IOS SLB does not support FTP or firewall load balancing in directed mode. Therefore, FTP and firewall 
load balancing cannot use NAT.

IOS SLB supports only client NAT for TCP and UDP virtual servers.

IOS SLB supports only server NAT (but not server port translation) for Encapsulation Security Payload 
(ESP) virtual servers or Generic Routing Encapsulation (GRE) virtual servers.

Server NAT involves replacing the virtual server IP address with the real server IP address (and vice 
versa). Server NAT provides the following benefits:

Servers can be many hops away from the load-balancing device.

Intervening routers can route to them without requiring tunnelling.

Loopback and secondary interfaces are not required on the real server.

The real server need not be Layer 2-adjacent to IOS SLB.

The real server can initiate a connection to a virtual server on the same IOS SLB device.