Technical data

56 Brocade ServerIron ADX Advanced Server Load Balancing Guide
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Other transparent cache switching options
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ServerIron ADX examines its Layer 4 header and checks to see whether it matches an entry in the
state table. The ServerIron ADX also examines the source MAC address to verify that the cache sent
the packet. If the MAC address field in the state table is null, and it will be for the first packet, the
ServerIron ADX simply forwards the packet at Layer 2 to the cache’s default gateway, the BAR.
When the packet is re-transmitted by the BAR, the ServerIron ADX examines the Layer 4 header
again, and sees that it matches an existing connection. The ServerIron ADX also examines the
source MAC address to be sure the cache server sent the packet. In this case, the source MAC
address is the BAR’s MAC, not the cache server’s. The ServerIron ADX concludes that this packet
has been retransmitted and places the destination MAC address of the packet, the RAS’s MAC, into
the state table’s MAC address field for this connection. Then the packet is forwarded to the RAS at
Layer 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 RAS. 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:
ServerIron(config)#server cache-name server1
ServerIron(config-rs-server1)#dest-nat
Syntax: dest-nat
Destination NAT is disabled by default.
NOTE
FTP is not supported when destination NAT is enabled.