Cisco set mtu on interface 3850

Cisco set mtu on interface 3850 DEFAULT

Interface and Hardware Components Configuration Guide, Cisco IOS XE Fuji 16.9.x (Catalyst 3850 Switches)

Standalone switch or switch stack

You can enter the system mtu command on a switch or switch stack, but system MTU value does not take effect on the switch. It affects the Fast Ethernet ports.

The range is from 1500 to 9198 bytes.

Use the ip mtu command.

The range is from 832 up to 1500 bytes.
Note 

The IP MTU value is the applied value, not the configured value.

Use the ipv6 mtu command.

The range is from 1280 to the system jumbo MTU value (in bytes).
Note 

The IPv6 MTU value is the applied value, not the configured value.

Sours: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/16-9/configuration_guide/int_hw/b_169_int_and_hw_3850_cg/configuring_system_mtu.html
conf t
system mtu 9198
wr
reload

Taken from cisco forums
The limitations of the platform in question would make no sense if system mtu routingwould apply to routed ports, but at the same time would not apply to SVIs. The point here seems to be that the model line cannot fragment in hardware.  So to avoid that, it just makes sure every L3 interface (routed port  and SVI alike) has the same IP MTU, so there is never any need to  fragment a packet.
I've tested this in the lab using two 3560E chassis, trivially set up with the default VLAN, connected through Gigabit (so system mtu jumbowill apply) and then pinging each other's SVIs. Now let's configure the  following (using the maximum frame size the platform supports, there's  not much sense in limiting it here):
system mtu jumbo 9198
After rebooting, all physical ports running 1000Base or better will allow payloads of up to 9198 bytes to be encapsulated. The routing mtuwill still be 1500 at this point. Try to ping one switch from the other like
ping 10.1.1.2 size 1500 df-bit
and it will succeed, but increasing size to 1501 will fail.
Now let's add:
system mtu routing 9000
to both switches and try again (no reboots needed). You will see that
ping 10.1.1.2 size 6000 df-bit
will suddenly work now, and the interface counters  will make clear that no fragmentation happens - it's really a single  6000 byte IP packet bouncing forth and back between the switches. That  works up to 9000, and starts failing at 9001, exactly as you would  expect.
Why is there a rumour that system mtu routingdoesn't apply to SVIs? Probably because show interfaceof an SVI will show you an MTU of 1500 (or whatever your system mtuis), while the same command applied to a routed port will show 9000.  This seems to be a glitch, as so often with interface MTU in show  commands. More specifically, the show interfaceMTU is  supposed to be the potential payload MTU of the underlying physical  interface of that routed interface, and there are other cases where it  displays rubbish. One should always compare to the IP MTUas given by show ip interface. Et voila: The IP MTU of our SVI (as given through show ip int vl1) displays as 9000. So the succeeding ping is not a mystery and system mtu routingdoes exactly what it intuitively states: Change the IP MTU of every L3 interface of the platform.
I know this won't help in cases like the one discussed  here, where supposedly two L3 interfaces running at different MTUs are  needed. In such cases, one should first reassure that what's needed is  really that and there's no way to redesign the setup to avoid that (by  placing L3 and L2 boundaries appropriately). If there is no way around  that, the 3560 will likely have to go for something that has  per-interface IP MTU, like the 49xx or 4500X platforms.
Discussions about MTU often mix up different problems  and lead to chaos. IMO this is because two things are often not regarded  to the necessary extent by the participants:
  1. There is not one MTU. When talking about MTU, always define  the layer you consider. That's often hard because you actually have to  think about a layer boundary, so two layers are involved. The mythical  1500 for instance is the L3 MTU on top of a classic L2 of the Ethernet  family. Things fundamentally change when you discuss L2 MTUs with regard  to some underlying L1 (but things are easier here, as L1+L2 are often  developed together as one technology, while the boundary between  technology layers and the network layer has more degees of freedom).
  2. MTU  doesn't exist as such, but is an emerging concept. In other words, MTU  is what everybody in a system of communicating nodes (typically a  broadcast domain) thinks it is, with the emphasis on everybody.  In that sense it's like the IP network that lives on top of a broadcast  domain - it doesn't exist per se, but by convention of everybody using  that broadcast domain as a bearer for IP packets sourced from  non-colliding adresses from the same visibility range (aka prefix, aka  network and netmask, in ancient times aka subnet/subnetmask). MTU is a  convention as well, a single node just changing its MTU (up or down from  the convention, that is, what everybody else uses) is a recipe for  disaster. That's why you don't do it except you know exactly what you  are doing, and have the might to change it everywhere in a broadcast  domain (every involved intermediate network device and every end node).  It's viral. Luckily it's not as widespread as the viral issue of people  configuring ports full duplex because that's what they always did,  introducing duplex mismatches up and down and then telling you that as  obviously auto negotiation doesn't work, they will continue this  practice. You see the common scheme: Breaking a convention is a bad  idea, unless you are the sole dictator.
Sours: http://tacocatsays.blogspot.com/2018/07/enable-jumbo-frame-on-cisco-3850.html
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MTU on Cisco Switches

Category:Cisco Systems -> Routing and Switching

There are 3 types of MTU that can be configured on a switch:

  • Layer 2 MTU that affects 10 and 100Mbit/s interfaces of a switch. Configured by system MTU {bytes} command in global config mode.
  • Layer 2 MTU that affects affects 1000Mbit/s and higher speed interfaces of a switch. Configured by system MTU jumbo {bytes} command in global config mode.
  • Layer 3 MTU that affects SVIs and routed interfaces of a switch with IP addresses on them and originating or transit IP traffic that uses these interfaces as GW for routing between networks. Configured by system mtu routing {bytes} command in global config mode.

The following MTU types can be configured on a Layer 3 switch 3750:

  1. System MTU
  2. System MTU jumbo
  3. System MTU routing

On 3550:

  1. System MTU

Let’s raise L2 MTU value to 1504 to accommodate Q in Q

3750:

If instead of jumbo mtu 1504 we used system mtu 1504 first and reloaded, jumbo mtu would automatically be raised to 1504 even if we didn’t touch its value before:

Following is the visual diagram of different possibilities that were tested by me:

MTU-on-Cisco-Switches-10.jpg

Although I didn’t bother to copy all my logs from troubleshooting and they were lost, but here’s a recovered fragment regarding system MTU investigation on switches:

Let’s see what happens if I changed system MTU of 3550:

If you turned on debug ip icmp you would have noticed that packets greater than 1500 bytes would not be allowed through without fragmentation.

By privilege15
Sours: https://evilttl.com/wiki/MTU-on-Cisco-Switches

Software Configuration Guide, Cisco IOS XE Denali 16.3.x (Catalyst 3850 Switches)

Information About the MTU

The default maximum transmission unit (MTU) size for frames received and sent on all device interfaces is 1500 bytes.

Restrictions for System MTU

When configuring the system MTU values, follow these guidelines:

  • The device does not support the MTU on a per-interface basis.

  • If you enter the system mtu global configuration command, the command affects all the switched and routed ports on the switch.

System MTU Value Application

In a switch stack, the MTU values applied to member switches depends upon the stack configuration. The following stack configurations are supported:

This table shows how the MTU values are applied.

Configuration

system mtu command

ip mtu command

ipv6 mtu command

Standalone switch

You can enter the system mtu command on a switch , but system MTU value does not take effect on the switch. It affects the Fast Ethernet ports.

The range is from 1500 to 9198 bytes.

Use the ip mtu command.

The range is from 832 up to 1500 bytes.
Note 

The IP MTU value is the applied value, not the configured value.

Use the ipv6 mtu command.

The range is from 1280 to the system jumbo MTU value (in bytes).
Note 

The IPv6 MTU value is the applied value, not the configured value.

The upper limit of the IP or IPv6 MTU value is based on the switch configuration and refers to the currently applied system MTU value. For more information about setting the MTU sizes, see the system mtu global configuration command in the command reference for this release.

How to Configure MTU Sizes

Configuring the System MTU

Follow these steps to change the MTU size for switched packets:

Procedure

 Command or ActionPurpose
Step 1

enable

Example:

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Enters global configuration mode.

Step 3

system mtu

Example:

(Optional) Changes the MTU size for all Gigabit Ethernet and 10-Gigabit Ethernet interfaces.

Step 4

end

Example:

Returns to privileged EXEC mode.

Step 5

copyrunning-config startup-config

Example:

Saves your entries in the configuration file.

Step 6

show system mtu

Example:

Verifies your settings.

Configuring Protocol-Specific MTU

To override system MTU values on routed interfaces, configure protocol-specific MTU under each routed interface.

Beginning in privileged EXEC mode, follow these steps to change the MTU size for routed ports:

Procedure

 Command or ActionPurpose
Step 1

configure terminal

Example:

Enters global configuration mode.

Step 2

interface

Example:

Enters interface configuration mode.

Step 3

ip mtu

Example:

Changes the IPv4 MTU size

Step 4

ipv6 mtu

Example:

(Optional) Changes the IPv6 MTU size.

Step 5

end

Example:

Returns to privileged EXEC mode.

Step 6

copyrunning-config startup-config

Example:

Saves your entries in the configuration file.

Step 7

show system mtu

Example:

Verifies your settings.

Configuration Examples for System MTU

This example shows how to set the maximum packet size for a Gigabit Ethernet port to 7500 bytes:

If you enter a value that is outside the allowed range for the specific type of interface, the value is not accepted. This example shows the response when you try to set Gigabit Ethernet interfaces to an out-of-range number:

Configuration Examples for System MTU

Example: Configuring Protocol-Specific MTU

Example: Configuring the System MTU

Additional References for System MTU

MIBs

MIB MIBs Link

All the supported MIBs for this release.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

Technical Assistance

Description Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/support

Feature Information for System MTU

Release

Modification

Cisco IOS XE 3.2SE

This feature was introduced.

Information About the MTU

The default maximum transmission unit (MTU) size for frames received and sent on all device interfaces is 1500 bytes.

Restrictions for System MTU

When configuring the system MTU values, follow these guidelines:

  • The device does not support the MTU on a per-interface basis.

  • If you enter the system mtu global configuration command, the command affects all the switched and routed ports on the switch.

System MTU Value Application

In a switch stack, the MTU values applied to member switches depends upon the stack configuration. The following stack configurations are supported:

This table shows how the MTU values are applied.

Configuration

system mtu command

ip mtu command

ipv6 mtu command

Standalone switch

You can enter the system mtu command on a switch , but system MTU value does not take effect on the switch. It affects the Fast Ethernet ports.

The range is from 1500 to 9198 bytes.

Use the ip mtu command.

The range is from 832 up to 1500 bytes.
Note 

The IP MTU value is the applied value, not the configured value.

Use the ipv6 mtu command.

The range is from 1280 to the system jumbo MTU value (in bytes).
Note 

The IPv6 MTU value is the applied value, not the configured value.

The upper limit of the IP or IPv6 MTU value is based on the switch configuration and refers to the currently applied system MTU value. For more information about setting the MTU sizes, see the system mtu global configuration command in the command reference for this release.

How to Configure MTU Sizes

Configuring the System MTU

Follow these steps to change the MTU size for switched packets:

Procedure

 Command or ActionPurpose
Step 1

enable

Example:

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Enters global configuration mode.

Step 3

system mtu

Example:

(Optional) Changes the MTU size for all Gigabit Ethernet and 10-Gigabit Ethernet interfaces.

Step 4

end

Example:

Returns to privileged EXEC mode.

Step 5

copyrunning-config startup-config

Example:

Saves your entries in the configuration file.

Step 6

show system mtu

Example:

Verifies your settings.

Configuring Protocol-Specific MTU

To override system MTU values on routed interfaces, configure protocol-specific MTU under each routed interface.

Beginning in privileged EXEC mode, follow these steps to change the MTU size for routed ports:

Procedure

 Command or ActionPurpose
Step 1

configure terminal

Example:

Enters global configuration mode.

Step 2

interface

Example:

Enters interface configuration mode.

Step 3

ip mtu

Example:

Changes the IPv4 MTU size

Step 4

ipv6 mtu

Example:

(Optional) Changes the IPv6 MTU size.

Step 5

end

Example:

Returns to privileged EXEC mode.

Step 6

copyrunning-config startup-config

Example:

Saves your entries in the configuration file.

Step 7

show system mtu

Example:

Verifies your settings.

Configuration Examples for System MTU

This example shows how to set the maximum packet size for a Gigabit Ethernet port to 7500 bytes:

If you enter a value that is outside the allowed range for the specific type of interface, the value is not accepted. This example shows the response when you try to set Gigabit Ethernet interfaces to an out-of-range number:

Configuration Examples for System MTU

Example: Configuring Protocol-Specific MTU

Example: Configuring the System MTU

Additional References for System MTU

MIBs

MIB MIBs Link

All the supported MIBs for this release.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

Technical Assistance

Description Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/support

Feature Information for System MTU

Release

Modification

Cisco IOS XE 3.2SE

This feature was introduced.

Sours: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3850/software/release/16-3/configuration_guide/b_163_consolidated_3850_cg/b_163_consolidated_3850_cg_chapter_01001.html

Interface cisco on set 3850 mtu

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Cisco Switch basic Configuration - Cisco Switch Configuration Step by Step

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