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How to change the instance's Linux bridge and tap interface MTU

Hi,

I use DevStack to deploy OpenStack with Linux Bridge instead of OVS in a multi-node set up.

I’m testing jumbo frames and want to set MTU to 9000.

At the Network node, the bridges and tap interfaces are created with MTU = 9000:

localadmin@qa4:~/devstack$ brctl show bridge name bridge id STP enabled interfaces brq09047ecb-1c 8000.7c69f62c4f2f no eth1 tapedbcd5b1-a6 brq319688ab-93 8000.3234d6ee3a18 no bond0.300 tap4e230a86-cb tapfddaf12e-85 virbr0 8000.000000000000 yes

localadmin@qa4:~/devstack$ ifconfig brq09047ecb-1c brq09047ecb-1c Link encap:Ethernet HWaddr 7c:69:f6:2c:4f:2f
inet6 addr: fe80::3c79:c8ff:fe23:2fe7/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:696 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:30617 (30.6 KB) TX bytes:648 (648.0 B)

localadmin@qa4:~/devstack$ ifconfig brq319688ab-93 brq319688ab-93 Link encap:Ethernet HWaddr 32:34:d6:ee:3a:18
inet6 addr: fe80::e0ec:3bff:fe09:4318/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:30 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:4236 (4.2 KB) TX bytes:648 (648.0 B)

localadmin@qa4:~/devstack$ ifconfig tapedbcd5b1-a6 tapedbcd5b1-a6 Link encap:Ethernet HWaddr ae:fb:64:53:f7:2d
inet6 addr: fe80::acfb:64ff:fe53:f72d/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:65 errors:0 dropped:0 overruns:0 frame:0 TX packets:947 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:10223 (10.2 KB) TX bytes:80510 (80.5 KB)

localadmin@qa4:~/devstack$ ifconfig tap4e230a86-cb tap4e230a86-cb Link encap:Ethernet HWaddr 32:34:d6:ee:3a:18
inet6 addr: fe80::3034:d6ff:feee:3a18/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:2073 errors:0 dropped:0 overruns:0 frame:0 TX packets:2229 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:8878139 (8.8 MB) TX bytes:8914532 (8.9 MB)

localadmin@qa4:~/devstack$ ifconfig tapfddaf12e-85 tapfddaf12e-85 Link encap:Ethernet HWaddr d2:33:29:9b:2c:e8
inet6 addr: fe80::d033:29ff:fe9b:2ce8/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:152 errors:0 dropped:0 overruns:0 frame:0 TX packets:295 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:15237 (15.2 KB) TX bytes:51849 (51.8 KB)

The instance launched at the Compute node has interface eth0 MTU = 9000:

ubuntu@qa5-vm2:~$ ifconfig eth0 Link encap:Ethernet HWaddr fa:16:3e:05:36:58
inet addr:10.0.0.4 Bcast:10.0.0.255 Mask:255.255.255.0 inet6 addr: fe80::f816:3eff:fe05:3658/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:1169 errors:0 dropped:0 overruns:0 frame:0 TX packets:384 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:1408206 (1.4 MB) TX bytes:1336535 (1.3 MB)

However, the associated bridge and tap interface MTU is set to default 1500:

localadmin@qa5:~/devstack$ brctl show bridge name bridge id STP enabled interfaces brq319688ab-93 8000.6805ca302558 no bond0.300 tapa7acee8a-54 virbr0 8000.000000000000 yes

localadmin@qa5:~/devstack$ ifconfig brq319688ab-93 brq319688ab-93 Link encap:Ethernet HWaddr 68:05:ca:30:25:58
inet6 addr: fe80::a8ef:dfff:feb1:8eec/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:162 errors:0 dropped:0 overruns:0 frame:0 TX packets:8 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:910353 (910.3 KB) TX bytes:648 (648.0 B)

localadmin@qa5:~/devstack$ ifconfig tapa7acee8a-54 tapa7acee8a-54 Link encap:Ethernet HWaddr fe:16:3e:05:36:58
inet6 addr: fe80::fc16:3eff:fe05:3658/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:395 errors:0 dropped:0 overruns:0 frame:0 TX packets:1180 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:500 RX bytes:1338757 (1.3 MB) TX bytes:1408932 (1.4 MB)

localadmin@qa5:~/devstack$ ifconfig bond0.300 bond0.300 Link encap:Ethernet HWaddr 68:05:ca:30:25:58
inet6 addr: fe80::6a05:caff:fe30:2558/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:9000 Metric:1 RX packets:11507 errors:0 dropped:0 overruns:0 frame:0 TX packets:11590 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:14831272 (14.8 MB) TX bytes:14943870 (14.9 MB)

As a result, jumbo frames are fragmented coming out from the Compute node.

What is the appropriate local.conf configuration so that the instance’s bridge and tap interface MTU is set to 9000 when the instance is created?

Thanks, Danny