Effect of MRU setting on EX Switch

MRU (Maximum Receive Unit) has a close relation to MTU but as far as I can see it has different effects in various active devices.
For example setting an MTU value of 1000 on an Ethernet interface of a Linux machine or an SRX box doesn’t prevent the larger packet from being accepted. However if the very same interface tries to return a similar size packet then it has to be fragmented. However on EX switch I saw something else. Let me explain;

I connected two PCs to my ex2200 test switch’s ge-0/0/8 and 9 port and assigned to the same vlan. Then I pinged from the PC connected to the port 8 to PC on 9. port with size 1000bytes and it worked. Then I set the interface MTU of port 8 to 900bytes and checked the MRU value.

As it can be seen MRU is 908 since Junos adds 8 bytes to calculate this value. After this setting I again tried to send a ping with 1000 size but no success. Here is the point because it is a switch and also it is MRU but not MTU your sender won’t get notified by any ICMP message even if you have PMTU discovery is turned on. You can literally beat the air if you have a small MRU setting on the switch this is my humble opinion:)

One thought on “Effect of MRU setting on EX Switch

  1. Laqzz

    Great article! Thank you!
    So it seems this issue exists only when both hosts are on the same VLAN right? As far as I can understand if there is router/fw between them it will reply with ICMP message and the source will fragment the packet.


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