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Peter_125719's avatar
Peter_125719
Icon for Nimbostratus rankNimbostratus
Dec 14, 2012

MTU is reduced to 576 when VS is attached

Hi,   I am a newcomer to BIG-IP. I have a LTM deployed in L2 mode by putting two untagged interface to one VLAN group.   Without attaching any VS, everything is normal and the MTU is 1500, from...
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Peter_125719's avatar
Peter_125719
Icon for Nimbostratus rankNimbostratus
Dec 15, 2012
Yes. I checked the MTU of the VLANs in the VLAN group. They are set to 1500. There is nothing special from the virtual server and the pool. The virtual server is actually a web proxy. So if I fail open with no active members available, the client can still visit the original web server. Standard HTTP profile is used for the virtual server.

 

 

I did run traffic capture on BIG-IP on both internal and external interfaces for the fail open case. In the SYN-ACK from server side to LTM external, MSS is 1430. But in the same packet forwarded from LTM to client, MSS is set to be 1460 (1460 + 20 + 20 = 1500), which is perfectly consistent with the MTU in the internal VLAN interface. But the actual maximum packet size on the internal interface is 612, which includes the 22 extra bytes LTM padding (590 + 22 = 612). While on the external interface size, the server side maximum packet size is 1506:

 

1430 (MSS) + 20 (TCP header) + 20 (IP header) + 14 (Ethernet header) + 22 (LTM padding with VS name)= 1506

 

 

Thanks,

 

 

Peter