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Today I ran into problem of MTU in LVS/Tunneling.
Small packets were forwarded to real servers without problems, but the
bigger ones weren't and TCP retransmissions occurred. I noticed the
problem dissapeared when I switched to LVS/DR so this gave me hint to
where the problem might be. MTU 1480 had to be set on the outgoing
interface of realservers with tunl0 having standard 1500. Directors have
1500 on all interfaces. This way TCP syn ack contained correct MTU and
client didn't send big packets that were discarded on director anymore.
IP header is 20 bytes long so 1480 is the maximum value that works.
It isn't mentioned in documentation that this problem will occur if
tunneling is used and can be forgotten and discovered later. I think it
would be helpful to add this to documentation so people count with it
from the beginning.
Jaro
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