CARP VLAN MTU question

[Webster, Andrew]

Hi,

 

When setting CARP over VLAN configured interfaces, the MTU value of the
CARP interface reflects that of the physical interface, not the VLAN
interface.  Should it not be indicative of the MTU available on the VLAN
interface (4 less than the physical interface) since it is to the CARP
interface that the data is ultimately directed?

 

 

System: FreeBSD 5.4-p8 with CARP device compiled in.  

 

Interface vxn1 is the physical interface on which two vlans (vlan0 and
vlan1) are configured.  Two CARP interfaces (carp0 and carp1) are added,
one for each vlan.

As seen below, the MTU for the physical interface is 1500, and the VLAN
interfaces 1496, but the CARP interfaces are showing 1500.

 

vxn0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500

        inet6 fe80::20c:29ff:fef1:9f42%vxn0 prefixlen 64 scopeid 0x1 

        inet 10.1.1.1 netmask 0xffffff00 broadcast 10.1.1.255

        ether 00:0c:29:f1:9f:42

vxn1: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu
1500

        inet6 fe80::20c:29ff:fef1:9f4c%vxn1 prefixlen 64 scopeid 0x2 

        ether 00:0c:29:f1:9f:4c

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384

        inet 127.0.0.1 netmask 0xff000000 

        inet6 ::1 prefixlen 128 

        inet6 fe80::1%lo0 prefixlen 64 scopeid 0x5 

vlan0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu
1496

        inet 10.10.10.11 netmask 0xffffff00 broadcast 10.10.10.255

        inet6 fe80::20c:29ff:fef1:9f42%vlan0 prefixlen 64 scopeid 0x6 

        ether 00:0c:29:f1:9f:4c

        vlan: 10 parent interface: vxn1

vlan1: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu
1496

        inet 10.10.20.11 netmask 0xffffff00 broadcast 10.10.20.255

        inet6 fe80::20c:29ff:fef1:9f42%vlan1 prefixlen 64 scopeid 0x7 

        ether 00:0c:29:f1:9f:4c

        vlan: 20 parent interface: vxn1

carp0: flags=41<UP,RUNNING> mtu 1500

        inet 10.10.10.10 netmask 0xffffff00 

        carp: MASTER vhid 1 advbase 1 advskew 0

carp1: flags=41<UP,RUNNING> mtu 1500

        inet 10.10.20.10 netmask 0xffffff00 

        carp: MASTER vhid 1 advbase 1 advskew 0

 

 

Andrew 

 

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