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I've been using a combination of interface bond-, vlan- and bridge-interfaces to provide redundancy and different logical network layers to xen domU's.

This setup is working well however i'm a bit uncertain as to how different settings on these interfaces affect one-another. To illustrate, here's my setup on a typical dom0:

                 /- vlan10 -- br10
eth0 -\         /
       > bond0 <--- vlan20 -- br20
eth1 -/         \
                 \- vlan30 -- br30

Considering bond-, vlan- and bridge-interfaces are logical rather than physical, does setting MTU on these interfaces have any effect if the physical (eth0, eth1) interfaces have a different MTU set serve a purpose?

KvH
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2 Answers2

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From your example, the MTU settings on bond interface need to be as large as largest MTU settings on each VLAN. And I think you don't need to set it on slave eth0,eth1.

tonioc
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  • To add to the above, it is my understanding that if eth0 MTU is say 9000 while vlan20 mtu is 1500 you will get MTU of 1500 when traversing all these interfaces – Konrads Jan 12 '15 at 23:29
  • Do we need to set MTU on `bridge` interface too? – Satish Aug 16 '18 at 14:54
  • I reckon the general rule is probably: yes, packets going through virtual interfaces are affacted by both the physical and the logical interface’s MTU. Packets must not be larger than any of the MTU values. In theory the values could be different, though. Can somebody confirm this understanding? – Daniel Böhmer Jan 20 '21 at 14:29
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from the Red Hat site https://access.redhat.com/solutions/963753:
Issue
jumbo frames needs to be configured in the vlan tagged interface. Adding MTU=9000 in vlan tagged interface i:e ifcfg-ethX.Y or ifcfg-bondX.Y does not help and ifconfig output still shows vlan interface with MTU:1500
Resolution
When one of the VLANs belonging to the interface needs an MTU of 9000 the underlying main non-vlan interface needs to have that same MTU as well. Other VLANs may use a different MTU as per requirement. So along with adding MTU=9000 in ifcfg-ethX.Y / bondX.Y , add it in ifcfg-ethX / bondX as well.
Root Cause
Base interface of VLAN needs to have the required MTU as to propagate changes in VLAN interface.

grant
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