Understanding How The Csm Determines Vip Availability - Cisco catalyst 6500 series Configuration Note

Chapter 8 Configuring Additional Features and Options
RHI is normally restricted to intranets; for security reasons, most routers do not propagate host-route
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information to the Internet.
Routing to VIP Addresses Without RHI
Without RHI, traffic reaches the VIP address by following a route to the client VLAN to which the VIP
address belongs. When the CSM powers on, the MSFC creates routes to client VLANs in its routing table
and shares this route information with other routers. To reach the VIP, the client systems rely on the
router to send the requests to the network subnet address where the individual VIP address lives.
If the subnet or segment is reachable but the virtual servers on the CSM at this location are not operating,
the requests fail. Other CSM devices can be at different locations. However, the routers only send the
requests based on the logical distance to the subnet.
Without RHI, traffic is sent to the VIP address without any verification that the VIP address is available.
The real servers attached to the VIP might not be active.
By default, the CSM will not advertise the configured VIP addresses.
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Routing to VIP Addresses with RHI
With RHI, the CSM sends advertisements to the MSFC when VIP addresses become available and
withdraws advertisements for VIP addresses that are no longer available. The router looks in the routing
table to find the path information it needs to send the request from the client to the VIP address. When
the RHI feature is turned on, the advertised VIP address information is the most specific match. The
request for the client is sent through the path where it reaches the CSM with active VIP services.
When multiple instances of a VIP address exist, a client router receives the information it needs
(availability and hop count) for each instance of a VIP address, allowing it to determine the best available
route to that VIP address. The router chooses the path where the CSM is logically closer to the client
system.
With RHI, you must also configure probes because the CSM determines if it can reach a given VIP
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address by probing all the real servers that serve its content. After determining if it can reach a VIP
address, the CSM shares this availability information with the MSFC. The MSFC, in turn, propagates
this VIP availability information to the rest of the intranet.

Understanding How the CSM Determines VIP Availability

For the CSM to determine if a VIP is available, you must configure a probe (HTTP, ICMP, Telnet, TCP,
FTP, SMTP, or DNS) and associate it with a server farm. When probes are configured, the CSM performs
these checks:
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OL-4612-01
Probes all real servers on all server farms configured for probing
Identifies server farms that are reachable (have at least one reachable real server)
Identifies virtual servers that are reachable (have at least one reachable server farm)
Identifies VIPs that are reachable (have at least one reachable virtual server)
Catalyst 6500 Series Content Switching Module Configuration Note
Configuring Route Health Injection
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