Installation guide

Chapter 1. Red Hat Cluster Suite Overview 31
Figure 1-23. LVS Implemented with Direct Routing
In a typical direct-routing LVS configuration, an LVS router receives incoming server re-
quests 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 net-
work, 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 (usu-
ally every 15 minutes) and refilled with IP/MAC associations.