Hi All
Just trying to asses the risk of changing the MTU size on live N77k - N9k B2B VPC connections for supporting Jumbo Frames.
We have 2 x N77k - in VPC domain 11 - connecting to 2 x N9ks in VPC domain 21. Each N9k has a connection to each N7k so the entire B2B VPC is 4 x 40G links.
The MTU sizes are set to 1500 by default, and I have read a few articles on the considerations, the questions I have are:
1. on the 9372, I have read that the 40G ports ignore MTU settings anyway, so does it need setting here:
2. on the 77k, we dont have a system wide jumbo mtu so I suspect I would have to set it on the VPC member ports here
3. would changing the mtu size result in type 1 vpc inconsistency and affect either n9k?
Any comments would be welcome.....
Thanks,..
The following is taken from the N9k config guide from Cisco:
Port MTU Size
The maximum transmission unit (MTU) size specifies the maximum frame size that an Ethernet port can process. For transmissions to occur between two ports, you must configure the same MTU size for both ports. A port drops any frames that exceed its MTU size.
By default, each port has an MTU of 1500 bytes, which is the IEEE 802.3 standard for Ethernet frames. Larger MTU sizes are possible for more efficient processing of data with less overhead. The larger frames, called jumbo frames, can be up to 9216 bytes in size, which is also the default system jumbo MTU size.
On a Layer 3 interface, you can configure an MTU size between 576 and 9216 bytes.
Note
The global LAN port MTU size applies to the traffic through a Layer 3 Ethernet LAN port that is configured with a nondefault MTU size.
For a Layer 2 port, you can configure an MTU size that is either the system default (1500 bytes) or the system jumbo MTU size (initially 9216 bytes).
Note
If you change the system jumbo MTU size, Layer 2 ports automatically use the system default MTU size (1500 bytes) unless you specify the new system jumbo MTU size for some or all of those ports.
For the Cisco Nexus 9372 switch, the following applies:
The 10-G interfaces are mapped to specific hardware ports where the default MTU is 1500.
The 40-G interfaces are mapped as a Hi GiG port where the default MTU is 3FFF and the MTU limit check is disabled.
In the case of 40-G interfaces, since the MTU limit check is disabled, it ignores the packet size and traffic flows irrespective of its MTU.
For information about setting the MTU size, see the “Configuring the MTU Size” section.
