Cisco Jumbo MTU Notes

A jumbo frame is technically any Ethernet frame over 1500.

Keep in mind that the maximum data size for a standard MTU of 1500 is 1472 (MTU minus 20 bytes IP header and 8 bytes for ICMP header). For OTV, the maximum size will be 1430 because OTV adds 42 bytes for IP Frame overhead (1472 – 42 = 1430).

Now Fibre Channel frames have a max payload of 2112 and with headers the MTU size is 2148. So you can set regular MTU of 2148 or the max to 9036 (for most switches).

=====7K:

By default the 7K has this configured:
system jumbomtu 9216

show run all | inc jumbo
system jumbomtu 9216

Even though when you look at an interface, the MTU is 1500:

sh run all | sec  1/38
interface Ethernet1/38
  mtu 1500

At the end of the day, you still need to manually set the MTU.

GLOBAL:

system jumbomtu 9216

INTERFACE:

int eth1/17
  mtu 9216

=====5K:

GLOBAL ONLY VIA POLICY-MAP:

policy-map type network-qos jumbo
  class type network-qos class-default
    mtu 9216
    exit
  exit

system qos
  service-policy type network-qos jumbo

=====SVI:
Configure the MTU on the SVI Interface:

interface vlan 10
  mtu 9216

=====Port Channels:
5K:
Nothing to do since it uses “System Class”

7K:
Configure it on the Port-Channel.

=====VPC:
So on both the 5K and 7K, the peer-link is already done. There’s nothing to do for the peer-link.

NOTE:
VPC peer-link MTU is set to 9216 + padding.

=====FabricPath:
Nothing differenct for Fabric Path. Do the same as 5K and 7K above.

=====OTV:
Locally:
join-interface“: What ever interface is set here needs to be configured.

Neighbor:
extended-vlan“: What ever VLAN is configured here, that SVI needs to be configured on it’s neighbor.

N7K1:

interface Overlay1
  otv join-interface Ethernet1/25
  otv extend-vlan 10
  otv adjacency-server
  otv use-adjacency-server 169.254.2.2 unicast-only
  no shutdown

(Interface towards 5k)
int eth1/25
  mtu 9216

N5K1:

int vlan 10
  mtu 9216

=====FC:
GLOBAL:
mtu size vsan 10 9036

=====UCS:
1. Under “QOS System Class“, you need to set 9216 MTU. This has to be done and it’s very important. (the FI is based off the Nexus 5K)
2. Under the vNIC settings, there’s an MTU option. The max value allowed is 9000. This informs the OS that the NIC (vNIC) supports this particular MTU.

=====1Kv:
Only uplink interfaces need to be set.

TESTING USING PINGS:
From the switch:
Test Jumbo:
ping 169.254.1.1 df-bit packet-size 8000

Test Default:
ping 169.254.1.1 df-bit packet-size 1472

On a windows box, use this command:
Test Jumbo:
ping 10.1.1.1 -f -l 8000

Test Default:
ping 10.1.1.1 -f -l 1472

Please check out a great post by Peter Revill:
http://www.ccierants.com/2013/10/ccie-dc-definitive-jumbo-frames.html

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