Description

The following article describes how the Juniper Networks EX/QFX Series switches calculate the MTU for the IRB interfaces on a given VLAN.

Symptoms

You need to modify the IRB MTU for a specific VLAN.

Solution

The MTU for the IRB logical interface is inherited from the lowest MTU of the layer two interfaces that are part of the VLAN. If the lowest MTU is 1514, the IRB unit MTU will be 1514 regardless of the user-configured MTU applied, or if other interfaces within that VLAN have higher MTU. For example, the IRB unit 10 below has a configuration of 9192, but the underlying interfaces have lower MTU:

root@switch# show interfaces irb 
mtu 9192;
unit 10 {
    family inet {
        address 10.10.10.1/30;
}

root@switch# run show interfaces irb.10 extensive | match mtu 
    Protocol inet, MTU: 1500

root@switch# run show vlans 

Routing instance        VLAN name             Tag          Interfaces
default-switch          V10                   10       
                                                           et-0/0/48.0*
                                                           xe-0/0/0.0*


As seen above, the MTU for irb.10 will be 1500 regardless of the configured value. The interfaces below have the default MTU for xe-0/0/0 (1500) and jumbo frames for et-0/0/48 (9192).

root@switch# show interfaces 
xe-0/0/0 {
    unit 0 {
        family ethernet-switching {
            vlan {
                members V10;
            }
        }
    }
}
et-0/0/48 {
    mtu 9192;
    unit 0 {
        family ethernet-switching {
            vlan {
                members V10;
            }
        }
    }
}


When the MTU of xe-0/0/0 is changed to 9192 to match that of et-0/0/48, then the irb.10 MTU will change as well.

root@switch# set interfaces xe-0/0/0 mtu 9192 
{master:0}[edit]
root@switch# commit 

configuration check succeeds
commit complete

{master:0}[edit]
root@switch# run show interfaces irb.10 extensive | match mtu 
    Protocol inet, MTU: 9178


In the examples above, MTU set on interfaces is 14 bytes less than the configured MTU because Ethernet header of 14 bytes is accounted for while setting MTU.

Note: When the irb has an interface UP participating on its same vlan with a lower MTU value, the interface will take its value since it is lower, the irb will always take the mtu value of the lowest mtu of one of the interfaces participating in the same vlan.

Modification History

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