Catalyst 9500-40x missing mtu

[josunin75]

Hello, Cisco experts. 

We are in process of upgrading old cat 6500 to Catalyst 9500 switches. 

Most of the configuration have been mapped to 9500 without any issue. 

However, we are having problem with mtu commands. 

Normally, we had mtu set to 9216 on TenGig interfaces and Port-Channels. 

This MTU command under interface and port-channel are now missing. 

How can I set the MTU on the Catalyst to 9216 on this new platform?

Thanks in advance for your support. 

[Reza Sharifi]

Hi,

on the 9500 series, the MTU is a global command and not per interface.

system mtu xxxx

link to config guide:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9500/software/release/16-9/configuration_guide/int_hw/b_169_int_and_hw_9500_cg.pdf

HTH

[josunin75]

Raza, 

thanks for reply. 

Just had another question. 

For the link that utilize different mtu setting, what do I do if the 9500 sets the mtu for whole system?

I'm assuming that all the links will get the same mtu number? 

we have links that utilize different mtu size due to different vendor equipment.

thanks,

[Reza Sharifi]

For the link that utilize different mtu setting, what do I do if the 9500 sets the mtu for whole system?

That is correct. 

I'm assuming that all the links will get the same mtu number? 

That is correct.  When you change it globally, it applies to the whole switch.

we have links that utilize different mtu size due to different vendor equipment.

For operational consistency and ease of management, it would make sense to have the same type of MTU on both sides on each link, but overall it should not impact other links with different MTU sizes. The only thing is that other vendors equipment will not be able to send jumbo frames to the 9500 if they have smaller MTU setup.

HTH

[MARK BAKER]

I am in the same boat here. Issues I have seen with mismatched MTU is with EIGRP that can use the full MTU of the interface. I have seen where EIGRP neighbors flap when the MTUs don't match. I am going to be looking into the interface commands 'ip mtu' and 'mpls mtu' to see if I can match the MTU of attached routers. The 9500s are going to be used as core routers and not running BGP. If they were being used as PE routers, I could see issues with MTU mismatch between L2 tunnel endpoints.

[MARK BAKER]

I checked the interface MTU, IP MTU, and MPLS MTU on a 6807 port and found that they were all the same as configured under the interface mtu setting. It was 9174 in this case. I configured the 9500 system mtu to 9198 and the IP and MPLS MTU to 9174. I configured just 'ip mtu' first, but this didn't change the MPLS MTU. I used the command 'mpls mtu' to make it match the IP MTU.

6807 = MTU 9174, IP MTU 9174, MPLS MTU 9174

9500 = MTU 9198, IP MTU 9174, MPLS MTU 9174

I do have interfaces running MTU 9216 that I'll have to configure for 9198 or lower.

I connected the 9500 to an asr 920 and set the interface MTU on the 920 to 9174. The 9500 is set as it is above. I was able to ping up to but not over 9174 bytes. I also set the 9500 ip/mpls mtu to 1500 and left the 920 at mtu 9174. I could pint up to but not over 1500 bytes. EIGRP route MTU used the IP MTU setting of the interface as well.

By default all three of these match on the 6807:

show int te1/0/1= see global/interface MTU

show ip int te1/0/1 = ip MTU

show mpls int te1/0/1 detail = MPLS MTU

[alex.vagner1995@gmail.com]

Hi, upgrade your cisco IOS to  version 17.3.3 or higher