Red Hat Enterprise Linux 5 Cluster Suite Overview page 35

Routing Methods
outgoing packets through the LVS router. Direct routing reduces the possibility of network performance
issues by relegating the job of the LVS router to processing incoming packets only.
Figure 1.20. LVS Implemented with Direct Routing
In a typical direct-routing LVS configuration, an LVS router receives incoming server requests through
a virtual IP (VIP) and uses a scheduling algorithm to route the request to real servers. Each real server
processes requests and sends responses directly to clients, bypassing the LVS routers. Direct routing
allows for scalability in that real servers can be added without the added burden on the LVS router to
route outgoing packets from the real server to the client, which can become a bottleneck under heavy
network load.
While there are many advantages to using direct routing in LVS, there are limitations. The most
common issue with direct routing and LVS is with Address Resolution Protocol (ARP).
In typical situations, a client on the Internet sends a request to an IP address. Network routers typically
send requests to their destination by relating IP addresses to a machine's MAC address with ARP.
ARP requests are broadcast to all connected machines on a network, and the machine with the
correct IP/MAC address combination receives the packet. The IP/MAC associations are stored in
an ARP cache, which is cleared periodically (usually every 15 minutes) and refilled with IP/MAC
associations.
The issue with ARP requests in a direct-routing LVS configuration is that because a client request to
an IP address must be associated with a MAC address for the request to be handled, the virtual IP
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