Technical data
ServerIron ADX Advanced Server Load Balancing Guide 49
53-1002435-03
Other transparent cache switching options
2
You can reduce the burden on the BAR by enabling cache route optimization. This feature
configures the ServerIron ADX to use the information in its Layer 4 session table to recognize that
the return traffic actually should go to the RAS instead of the BAR, and send the return traffic
directly to the RAS. Thus, the return traffic does not needlessly add overhead to the BAR.
FIGURE 10 Cache route optimization
Why ICMP redirects do not solve the problem
The ServerIron ADX redirects HTTP traffic destined for the Internet to the cache server. When the
cache server responds to the client, it does so by sending its packets to its default gateway
because the users are not in the same subnet as the cache server. However, at Layer 3, the packet
is addressed to a client that is actually accessible through the remote access server (RAS). The
border access router (BAR) knows the proper next hop router is the RAS, through a routing protocol,
and retransmits the packet to the RAS, at Layer 2. The RAS forwards the packet to the client. Thus
every packet to every client must go to the BAR and then be retransmitted. The BAR port is already
carrying all the fetch and refresh traffic from that cache and this additional traffic can overload it.
• The BAR does not send an ICMP redirect in this case, as you might expect. A router sends ICMP
redirects only if the following conditions are met:
• The interface on which the packet comes into the router is the same as the interface on which
the packet gets routed out.
Internet
www.stockquotes.com
1.0.0.2
www.oldnews.com
1.0.0.1
www.livenews.com
1.0.0.3
Web servers
Border Access
Router (BAR)
e18 (output port)
e17
Remote Access
Server (RAS)
Web Queries
208.95.6.3
208.95.7.3
208.95.8.3
Cache server3
209.157.22.205
Cache server4
209.157.22.215
Cache server5
209.157.22.225
Each cache server has a
default route to the BAR,
which must send the traffic
back through the ServerIron
to the RAS.
The Cache Router Optimization
feature configures the ServerIron
to send the return traffic directly
to the RAS.
SI