Infrastructure Software

I suggest to search the web for RAC and Jumbo frames.

There is also Oracle documentation about Jumbo Frames for the Cluster Interconnect:

https://docs.oracle.com/database/121/HABPT/config_cw.htm#HABPT5363

You need to check if your hardware including network switches support it. If your equipment isn't 15 years old it most likely will.

Why do you think that loopback MTU size is an issue?

Loopback traffic does not cross h/w boundaries - thus is not use for cluster Interconnect. Thus RAC or no RAC does not seem relevant. Loopback is typically used for IPC.

Thus are there IPC issues?

Did you change your initial question? If not then I completely misunderstood. I was referring to regular TCP/IP devices for the cluster interconnect that use a MTU of 1500. Anyway, you should not change the MTU size of the loopback adapter. It is set to 64k by default.

What information have you been reviewing that suggests otherwise?

What does the loopback adapter have to do with RAC does not apply to any other installation?

One of my Rac clusters got following error below.   The dbas insisted I follow this Doc ID and change lo MTU size to 16436.   Not sure if Oracle Rac is using lo and if this adjustment has anything to do with actual communication or if its just a configuration parameter which can be adjusted to free up available memory which can be used for network buffers.  I do have current cisco hardware but switches don't support Jumbo Frames.  This is actual lowering of the MTU from default of 65536

Oracle Linux: ORA-27301:OS Failure Message: No Buffer Space Available (Doc ID 2041723.1)

Think its as simple as RHEL default for lo 65536 is too large and wastes buffers/memory.   cluverify doesn't pick up on this and don't see setting in any install manuals.

In addition I believe the 16436 is:

16 * 1024 would be 16384 then if you add the 52 byte TCP/IP header information, it comes out to the 16436 number.

(52 bytes for the header is.. 20 bytes for the IP header, 20 bytes for the TCP header, then it pads it to the next 32 bit boundary. )

The lo adapter is part of the TCP/IP stack. There is no hardware and no swtiches involved with the lo adapter. The loopback adapter uses 127.0.0.1 and must not be changed.

Under MS Windws the MTU of the loopback adpater is 4294967295 and under XP it was 32767.

Have you seen https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0cf833aefaa85bbfce3ff70485e5534e09254773

https://www.ora-solutions.net/web/2018/05/12/beware-of-loopback-mtu-size-with-rac-on-oracle-linux-7/

Why not give it a try and see what happens? What can you loose?