[j-nsp] MTU: old style vs new style trunk configuration on MX80

[Dawid Gajownik]

Hi!

I've been troubleshooting today MTU issues on my new MX80 router
(actually MX5) with Junos 11.4R1.14. I have noticed that MTU of
interfaces differs if I set trunks with old-style or new-style
configuration.

Documentation http://www.juniper.net/techpubs/en_US/junos11.4/topics/example/layer-2-bridge-domain-environment-example-interfaces-and-vlans-mx-solutions.html
does not mention it, but with a interface-mode you have to manually
set MTU 4 bytes larger or add "vlan-tagging" option. Is this a bug in
Junos or documentation is incomplete? On EX4200 with interface-mode I
did not need to add/change anything.

Please take a closer look at ae1 interface:

root at KR-020-r1# show ae0
description "### KR-011-s1 ###";
vlan-tagging;
encapsulation extended-vlan-bridge;
aggregated-ether-options {
   lacp {
       active;
       periodic fast;
   }
}
inactive: unit 0 {
   family bridge {
       interface-mode trunk;
       vlan-id-list [ 708 206 ];
   }
}
unit 206 {
   vlan-id 206;
}
unit 708 {
   vlan-id 708;
}

[edit interfaces]
root at KR-020-r1# show ae1
description "### KR-020-s1 ###";
aggregated-ether-options {
   lacp {
       active;
       periodic fast;
   }
}
unit 0 {
   family bridge {
       interface-mode trunk;
       vlan-id-list [ 708 206 ];
   }
}

[edit interfaces]
root at KR-020-r1# show ae2
description "### KR-020-s2 ###";
mtu 1518;
aggregated-ether-options {
   lacp {
       active;
       periodic fast;
   }
}
unit 0 {
   family bridge {
       interface-mode trunk;
       vlan-id-list [ 708 206 ];
   }
}

[edit interfaces]
root at KR-020-r1# show ae3
description "### KR-020-s3 ###";
vlan-tagging;
aggregated-ether-options {
   lacp {
       active;
       periodic fast;
   }
}
unit 0 {
   family bridge {
       interface-mode trunk;
       vlan-id-list [ 708 206 ];
   }
}

[edit interfaces]
root at KR-020-r1# run show interfaces ae0 | grep MTU
 Link-level type: Extended-VLAN-VPLS, MTU: 1518, Speed: 2Gbps, BPDU
Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source
filtering: Disabled, Flow control: Disabled,
   Protocol bridge, MTU: 1518
   Protocol bridge, MTU: 1518
   Protocol multiservice, MTU: Unlimited

[edit interfaces]
root at KR-020-r1# run show interfaces ae1 | grep MTU
 Link-level type: Ethernet, MTU: 1514, Speed: Unspecified, BPDU
Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source
filtering: Disabled, Flow control: Disabled,
   Protocol bridge, MTU: 1514

[edit interfaces]
root at KR-020-r1# run show interfaces ae2 | grep MTU
 Link-level type: Ethernet, MTU: 1518, Speed: 2Gbps, BPDU Error:
None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering:
Disabled, Flow control: Disabled,
   Protocol bridge, MTU: 1518

[edit interfaces]
root at KR-020-r1# run show interfaces ae3 | grep MTU
 Link-level type: Ethernet, MTU: 1518, Speed: Unspecified, BPDU
Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source
filtering: Disabled, Flow control: Disabled,
   Protocol bridge, MTU: 1518
   Protocol multiservice, MTU: Unlimited

[edit interfaces]
root at KR-020-r1#

Regards,
     Dawid