Hi All,

I am having an interesting issue with configuring the MTU on my KVM guest VMs. The VMs have two nics. One for management (eth0) which connects to vibro and eth1 which connects to a host device macvtap (which is configured for MTU 9000) and will be used for NFS/iSCSI traffic. As such I need to configure the MTU on the interface to 9000.

I am able to ping from the host through macvtap device at 9000. Unfortunately, the machine is ignoring the configuration and reports the MTU as 1500. I believe I need to change something in the virtual network configuration, but I do not know where to look. I found someone who had the same issue, however there was no concrete response.

https://www.centos.org/forums/viewtopic.php?t=2226

Thank you for your help

Regards,
WS


kvm-host # ping -I vlan100 11.11.11.12 -s 9000
PING 11.11.11.12 (11.11.11.12) from 11.11.11.22 vlan100: 9000(9028) bytes of data.
9008 bytes from 11.11.11.12: icmp_seq=1 ttl=255 time=0.186 ms
9008 bytes from 11.11.11.12: icmp_seq=2 ttl=255 time=0.183 ms
9008 bytes from 11.11.11.12: icmp_seq=3 ttl=255 time=0.184 ms
9008 bytes from 11.11.11.12: icmp_seq=4 ttl=255 time=0.162 ms
9008 bytes from 11.11.11.12: icmp_seq=5 ttl=255 time=0.139 ms


kvm-guest
# ping -I eth1 11.11.11.12 -s 1490
PING 11.11.11.12 (11.11.11.12) from 11.11.11.33 eth1: 1490(1518) bytes of data.
1498 bytes from 11.11.11.12: icmp_seq=1 ttl=255 time=0.413 ms
1498 bytes from 11.11.11.12: icmp_seq=2 ttl=255 time=0.464 ms
1498 bytes from 11.11.11.12: icmp_seq=3 ttl=255 time=0.290 ms
1498 bytes from 11.11.11.12: icmp_seq=4 ttl=255 time=0.279 ms

ping -I eth1 11.11.11.12 -s 9000
PING 11.11.11.12 (11.11.11.12) from 11.11.11.33 eth1: 9000(9028) bytes of data.
^C
--- 11.11.11.12 ping statistics ---
88 packets transmitted, 0 received, 100% packet loss, time 87422ms


kvm-guest# more /etc/sysconfig/network-scripts/ifcfg-eth1
TYPE=Ethernet
BOOTPROTO=none
IPADDR=11.11.11.33
PREFIX=24
DEFROUTE=yes
IPV4_FAILURE_FATAL=yes
IPV6INIT=no
NAME=eth1
UUID=ad1e7c9e-29ab-4f67-bb2f-32cdd2d84031
ONBOOT=yes
HWADDR=52:54:005:56:6B
MTU=9000
LAST_CONNECT=1455222597


mtu3.png