在Oracle RAC的官方文档上有这样一句话：
Crossover cables are not supported for use with Oracle Clusterware interconnects.

也就是说Oracle不支持使用交叉线作为ClusterWare的interconnects。
这里的Not Support不是说交叉线不能用，而是说Oracle不建议这样做，同时不承担这样做而产生的一系列问题。

在Metalink上有这样一段描述说的非常清楚，摘录一下：

CROSS OVER CABLES ARE NOT SUPPORTED.
The requirement is to use a switch:

 

Detailed Reasons:
1) cross-cabling limits the expansion of RAC to two nodes
2) cross-cabling is unstable:
a) Some NIC cards do not work properly with it. They are not able to negotiate the DTE/DCE clocking, and will thus not function.
These NICS were made cheaper by assuming that the switch was going to have the clock. Unfortunately there is no way to know which NICs do not have that clock.
b) Media sense behaviour on various OS's (most notably Windows) will bring a NIC down when a cable is disconnected.
Either of these issues can lead to cluster instability and lead to ORA-29740 errors (node evictions).

Due to the benefits and stability provided by a switch, and their afforability ($200 for a simple 16 port GigE switch), and the expense and time related to dealing with issues when one does not exist, this is the only supported configuration.

实际上，交换机的引入是为了更高的扩展性和稳定性，这是合理的投入，在搭建RAC环境时值得投入。