Brocade Communications Systems ServerIron ADX 12.4.00 Service Manual Page 62

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50 ServerIron ADX Advanced Server Load Balancing Guide
53-1002435-01
Other TCS options
2
When the cache server transmits the next packet, the ServerIron ADX compares its Layer 4 header
to the state table and gets a match and now the entry has a MAC address in the MAC address field.
The ServerIron ADX replaces the destination address with the stored MAC address and transmits
the packet at Layer 2 using the new “optimum” MAC address. Thus all packets except the first
packet are sent directly to the optimum router.
Because this scheme works at the MAC layer, it is compatible with all routing protocols. Moreover,
because the scheme is session specific, it can handle any number or RASs. When a session is
terminated, the table entry is deleted and so is the “optimization”. Thus changes in the network at
Layer 3 are immediately implemented.
Enabling Destination NAT
By default, the ServerIron ADX translates the destination MAC address of a client request into the
MAC address of the cache server. However, the ServerIron ADX does not translate the IP address of
the request to the cache server’s IP address. Instead, the ServerIron ADX leaves the destination IP
address untranslated.
This behavior assumes that the cache server is operating in promiscuous mode, which allows the
cache server to receive requests for any IP address so long as the MAC address in the request is
the cache server’s. This behavior works well in most caching environments. However, if your cache
server requires that the client traffic arrive in directed IP unicast packets, you can enable
destination NAT.
NOTE
This option is rarely used. If your cache server operates in promiscuous mode, you probably do not
need to enable destination NAT. Otherwise, enable destination NAT. Consult your cache server
documentation if you are unsure whether you need to enable destination NAT.
To enable destination NAT, enter commands such as the following:
ServerIronADX(config)# server cache-name server1
ServerIronADX(config-rs-server1)# dest-nat
Syntax: dest-nat
Destination NAT is disabled by default.
NOTE
FTP is not supported when destination NATis enabled.
Configuring Source NAT
Normally, when the ServerIron ADX redirects a client’s web request to a cache server, the
ServerIron ADX translates the destination MAC address of a client request into the MAC address of
the cache server. However, the ServerIron ADX does not translate the source or destination IP
addresses in the client’s request.
Generally, in network topologies where the ServerIron ADX and cache server are directly connected
or connected through a Layer 2 switch or bridge, the cache’s response to a client query always
passes back through the ServerIron ADX. The ServerIron ADX uses the cache response to assess
the health of the cache server. When the ServerIron ADX passes a cache response to the client,
the ServerIron ADX assumes the cache server is healthy.
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