Introduction
This document provides a sample configuration for Maximum Transmission Unit (MTU) sizes supported across all of the Cisco Catalyst-series switches on Ethernet-based ports.
Note: This document does not address interfaces such as Asynchronous Transfer Mode (ATM), Packet over SONET (POS), and Token Ring.
Prerequisites
Requirements
Ensure that you meet these requirements before you attempt this configuration:
Understand MTU
Have knowledge of jumbo and baby giants
Components Used
The information in this document is based on these software and hardware versions:
Catalyst 6x00 / Cisco 7600 OSR Series
Catalyst 4000/4500 Series
Catalyst 3750/3560 Series
Catalyst 3550 Series
Catalyst 2970/2960 Series
Catalyst 2950 Series
Catalyst 2940 / Catalyst Express 500 Series
Catalyst 8500 Series
Catalyst 5000 Series
Catalyst 2900XL/3500XL Series
Catalyst 2948-L3/4908G-L3 Series
Catalyst 1900/2800 Series
Note: In all the examples in this document, unless specifically mentioned, all values that quote MTU in bytes omit the 18 bytes for the Ethernet header and Frame Check Sequence (FCS).
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Conventions
Refer to the Cisco Technical Tips Conventions for more information on document conventions.
Background Information
This section describes the basic terminology that this document uses. This section also explains the background theory for the configurations in this document.
Term Definitions
MTU: MTU is short for Maximum Transmission Unit, the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packets before transmission.
Jumbo: Jumbo frames are frames that are bigger than the standard Ethernet frame size, which is 1518 bytes (including Layer 2 (L2) header and FCS). The definition of frame size is vendor-dependent, as these are not part of the IEEE standard.
Baby giants: The baby giants feature allows a switch to pass or forward packets that are slightly larger than the IEEE Ethernet MTU. Otherwise, the switch declares big frames as oversize and discards them.
Background Theory
In order to transport traffic across switched-networks, ensure that transmitted traffic MTU does not exceed the MTU that the switch platforms support. Here are the reasons why the MTU size of certain frames is truncated:
Vendor-specific requirements: Applications and some Network Interface Cards (NICs) can specify an MTU size outside of the standard 1500 bytes. Much of this drive has been due to studies undertaken, which prove that an increase in the size of an Ethernet frame can increase average throughput.
Trunking: In order to carry VLAN-ID information between switches or other network devices, trunking has been employed to augment the standard Ethernet frame. Today, the two most common forms of trunking are Cisco's proprietary InterSwitch Link (ISL) encapsulation and IEEE 802.1q. Refer to these documents for more information on trunking:
InterSwitch Link Frame Format
Basic Characteristics of 802.q Trunking
MultiProtocol Label Switching (MPLS): When you enable MPLS on an interface, MPLS can also augment the frame size of a packet, depending on the number of labels in the Label stack for an MPLS-tagged packet. The total size of a label is four bytes. The total size of a label stack is n x 4 bytes. If a label stack is formed, the frames can exceed the MTU.
Configure
In this section, you are presented with the information to configure the features described in this document.
Note: Use the Command Lookup Tool ( registered customers only) to obtain more information on the commands used in this section.
Configurations
This document uses these configurations:
Catalyst 6x00 / Cisco 7600 OSR Series
Catalyst 4000/4500 Series
Catalyst 3750/3560 Series
Catalyst 3550 Series
Catalyst 2970/2960 Series
Catalyst 2950 Series
Catalyst 2940 / Catalyst Express 500 Series
Catalyst 8500 Series
Catalyst 5000 Series
Catalyst 2900XL/3500XL Series
Catalyst 2948-L3/4908G-L3 Series
Catalyst 1900/2800 Series
Catalyst Support for Maximum Frame Sizes
The ability of the various Catalyst switches to support various frame sizes depends on many factors, including the hardware and software. Note that certain modules can support larger frame sizes than others, even within the same platform. Additionally, maximum frame size support can also change depending on the software version you use.
Catalyst 6000/6500 / Cisco 7600 OSR Series
The Catalyst 6000 series and 7600 Optical Services Router (OSR) platform can support jumbo frame sizes as of release 6.1(1) of CatOS, and 12.1(1)E for Native IOS. However, this is dependent on the type of line cards that you use. There are generally no restrictions to enable the jumbo frame size feature. You can use this feature with trunking/non-trunking and channeling/non-channeling.
The default MTU size is 9216 bytes after you enable jumbo frame support on the individual port. However, an Application Specific Integrated Circuit (ASIC) limitation requires that you limit the MTU size to 8092 bytes on these 10/100-based line cards:
WS-X6248-RJ-45
WS-X6248A-RJ-45
WS-X6248-TEL
WS-X6248A-TEL
WS-X6348-RJ-45
WS-X6348-RJ-45V
WS-X6348-RJ-21
The WS-X6516-GE-TX is also affected at 100 Mbps. At 10/1000 Mbps, up to 9216 bytes can be supported. However, the WS-X6548-RJ-45 line card is not affected, as this line card uses newer ASICs.
Note: Jumbo frame support is available on WS-X6101 ATM modules.
Configure in CatOS
Cat6509≶ (enable) set port jumbo
Usage: set port jumbo
Cat6509> (enable) set port jumbo 1/1 enable
Jumbo frames enabled on port 1/1.
Cat6509> (enable) 2002 May 29 12:34:35 %PAGP-5-PORTFROMSTP:
Port 1/1 left bridge port 1/1
2002 May 29 12:34:38 %PAGP-5-PORTTOSTP:Port 1/1 joined bridge port 1/1
Verify in CatOS
Cat6509> (enable) show port jumbo
Jumbo frames MTU size is 9216 bytes.
Jumbo frames enabled on port(s) 1/1,9/1.
Configure Native IOS
7609(config)#int gigabitEthernet 1/1
7609(config-if)#mtu ?
<1500-9216> MTU size in bytes
7609(config-if)#mtu 9216
Verify in Native IOS
7609#show interfaces gigabitEthernet 1/1
GigabitEthernet1/1 is up, line protocol is up (connected)
Hardware is C6k 1000Mb 802.3, address is 0007.0d0e.640a (bia 0007.0d0e.640a)
MTU 9216 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
The port ASICs on the Catalyst 6000 count as oversized, which are those frames greater than 1548 bytes but less than the configured jumbo MTU. It uses the rxOversizedPkts counter to track these frames in the output from the show counter command. In this case, the ifInErrors counter value increments and can equal the number of rxOversizedPkts packets value in the show counter command output. On a Catalyst 6000 that runs Cisco Integrated IOS (Native Mode), use the show interface command to check whether the input errors counter increments along with giant counters on the interface that receives these frames.
Note: There is no relationship between the MTU value that you can specify in the VLAN database and the mtu command in the interface configuration mode. With the VLAN database setting, the switch checks for an MTU value greater than 1500. If the switch detects a higher value, puts the VLAN in a non-operational state. Therefore, to support large frames, you only need to change the interface MTU value and not the VLAN database MTU value. If SVI is used, then in order to support large frames, you must increase the MTU on the SVI to route the traffic between VLANs . Refer to the Configure Jumbo Frame Support section of Configuring Interfaces for more information about Jumbo Frame support on the Catalyst 6500.
Catalyst 4000/4500 Series
You can classify the Catalyst 4000/4500 series switches into two groups in relation to jumbo or baby giant support:
Devices that run CatOS system software
Devices that run Cisco IOS system software
Devices running CatalystOS system software
This group includes the Catalyst 4000/4500 devices with Supervisor I and Supervisor II, WS-C2948G, WS-C2980G, and the WS-C4912G fixed-configuration switches also. Due to an ASIC limitation, there is no support for baby giants.
Workaround
As a workaround, you can enable a port for trunking in order to support baby giants. When you enable a port for 802.1q trunking, the switch automatically assumes that an extra four bytes of data are appended on, and increment the frame size of the L2 packet. Note that ISL encapsulation is not supported on these platforms.
Hence, for implementations that require exactly one tag to be carried (either 802.1q or MPLS, but not both), you can configure the port as a trunk port to force the switchport to accept an extra four bytes of data. If the port were to carry multiple VLANs for VLAN-ID tagging or 802.1p prioritization, configure the port as a 802.1q trunk. However, even if VLAN tagging is not necessary, but you want the increased four-byte support, you can configure the port as a 802.1q trunk. Change the Native VLAN to be the one desired to carry the traffic. When you do so, you can accommodate an extra four bytes of data.
Devices running Cisco IOS system software
The Cisco Catalyst 4000/4500 devices with Supervisors, which run only Cisco IOS, currently support baby giants up to 1600 byte-sized frames and jumbo frames. Refer to Troubleshooting Baby Giant/Jumbo Frames in Catalyst 4000/4500 with Supervisor III/IV for more information.
Catalyst 3750/3560 Series
Catalyst 3750/3560 Series switches support an MTU of 1998 bytes for all 10/100 interfaces. All Gigabit Ethernet interfaces support jumbo frames up to 9000 bytes. The default MTU and jumbo frame size is 1500 bytes. You cannot change the MTU on an individual interface. You must set the MTU globally. Reset the switch afterwards for the MTU change to take effect.
Configure
Use the system mtu command to change the MTU for all 10/100 interfaces. This command only effects 10/100 interfaces.
3750(config)# system mtu 1546
3750(config)# exit
3750# reload
Use the system mtu jumbo command to change the MTU for all Gigabit Ethernet interfaces. This command only effects Gigabit Ethernet Interfaces.
3750(config)# system mtu jumbo 9000
3750(config)# exit
3750# reload
Note: Gigabit Ethernet ports are not affected by the system mtu command; 10/100 ports are not affected by the system mtu jumbo command. If you do not configure the system mtu jumbo command, the setting of the system mtu command applies to all Gigabit Ethernet interfaces.
Verify
Use the show system mtu command to view the mtu sizes after reload.
Switch# show system mtu
System MTU size is 1546 bytes
System Jumbo MTU size is 9000 bytes
Note: If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames that ingress on a Gigabit Ethernet interface and egress on a 10/100 interface are dropped.
Note: When you use dot1q on trunk interface on the Cat3750/3560, you can see runts in the show interface command output because Cat3750/3560 counts valid dot1q encapsulated packets that are 61-64 bytes including the q-tag as undersized frames, even when these packets are forwarded correctly. In addition, these packets are not reported in the appropriate category (unicast, multicast, broadcast) in receive statistics.
Catalyst 3550 Series
You can classify the Catalyst 3550 series Layer 3 (L3) switches into two major groups, where the Gigabit Ethernet versions support up to 2000 bytes and the Fast Ethernet versions support up to 1546 bytes. These models support up to 2000 bytes:
WS-C3550-12G
WS-C3550-12T
Configure
3550(config)#system mtu ?
<1500-2000> MTU size in bytes
3550(config)#system mtu 2000
Changes to the System MTU will not take effect until the next reload is done.
Verify
3550#show system mtu
System MTU size is 2000 bytes
In versions earlier than 12.1(9)EA1, an MTU of 2025 was configurable on these mentioned switches. Due to an ASIC limitation, the configurable MTU has been brought down to 2000 bytes.
These models support up to 1546 bytes:
WS-C3550-24
WS-C3550-24-DC-SMI
WS-C3550-24-EMI
WS-C3550-24-SMI
WS-C3550-48-EMI
WS-C3550-48-SMI
Configure
3550(config)#system mtu ?
<1500-1546> MTU size in bytes
3550(config)#system mtu 1546
Changes to the System MTU will not take effect until the next reload is done.
Verify
3550#show system mtu
System MTU size is 1546 bytes
Note: The MTU size of 1546 does not include the 18 bytes of the standard Ethernet header and FCS. Therefore, these switches actually support Ethernet frames up to 1564 bytes.
Catalyst 2970/2960 Series
The default maximum transmission unit (MTU) size for frames received and transmitted on all interfaces on the switch is 1500 bytes. You can increase the MTU size for all interfaces that operate at 10 or 100 Mbps with the system mtu global configuration command. You can increase the MTU size to support jumbo frames on all Gigabit Ethernet interfaces with the system mtu jumbo global configuration command.
Gigabit Ethernet ports are not affected by the system mtu command; 10/100 ports are not affected by the system mtu jumbo command. If you do not configure the system mtu jumbo command, the setting of the system mtu command applies to all Gigabit Ethernet interfaces.
You cannot set the MTU size for an individual interface; you set it for all 10/100 or all Gigabit Ethernet interfaces on the switch. When you change the system or jumbo MTU size, you must reset the switch before the new configuration takes effect.
Frame sizes that can be received by the switch CPU are limited to 1998 bytes, no matter what value was entered with the system mtu or system mtu jumbo commands. Although frames that are forwarded are typically not received by the CPU, in some cases, packets are sent to the CPU, such as traffic sent to control traffic, SNMP, or Telnet.
If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames received on a Gigabit Ethernet interface and sent on a 10/100 interface are dropped.
Configure
Use the system mtu command to change the MTU for all 10/100 interfaces. This command only affects 10/100 interfaces.
2970(config)# system mtu 1998
2970(config)# exit
2970# reload
Use the system mtu jumbo command to change the MTU for all Gigabit Ethernet interfaces. This command only affects Gigabit Ethernet Interfaces.
2970(config)# system mtu jumbo 9000
2970(config)# exit
2970# reload
Verify
Use the show system mtu command to view the MTU sizes after reload.
2970# show system mtu
System MTU size is 1998 bytes
System Jumbo MTU size is 9000 bytes
Catalyst 2950/2955 Series
You can classify the Catalyst 2950/2955 Series switches into two major groups, where one supports baby giants (up to 1530 bytes), but the other does not. However, this refers to traffic that flows through the switch. Packets destined to the management (VLAN) interface can support only 1500 bytes.
These models of 2950 switches support only 1500 bytes:
WS-C2950-12
WS-C2950-24
WS-C2950-48
WS-C2950C-24
WS-C2950T-24
These models of 2950/2955 switches support up to 1530 bytes:
WS-C2950G-12-EI
WS-C2950G-24-EI
WS-C2950G-24-EI-DC
WS-C2950G-48
WS-C2950G-48-EI
All models of 2950 LRE Series switches
All models of 2955 Series switches
For those switches that support up to 1530 bytes, the default MTU value is 1500. If you want to change this, use the global configuration command that is available for versions of software above 12.1(6)EA2. Here is sample configuration and verification:
Configure
2950G(config)#system mtu ?
<1500-1530> MTU size in bytes
2950G(config)#system mtu 1530
Verify
2950G#show system mtu
System MTU size is 1530 bytes
Catalyst 2940 / Catalyst Express 500 Series
The System MTU can only be set to 1500 bytes, the default. You cannot set the MTU on a per-interface basis.
Catalyst 8500 Series
Jumbo frame support is available only on some of the two-port enhanced Gigabit Ethernet modules. Additionally, jumbo frame support requires software revision 12.1(7)EY and hardware revision 6.0. Only this hardware revision has a new ASIC that enables you to change the MTU value. In order to identify whether the enhanced two-port Gigabit Ethernet module can support jumbo frames, issue the show controller command.
Verify whether the command output contains one of these values:
Sys Status Register --> 0x15—This value indicates that the E-2PGE port supports jumbo frames.
Sys Status Register --> 0x03—This value indicates that the port does not support jumbo frames.
Here is a sample output:
8500#show controller g3/0/0
IF Name: GigabitEthernet3/0/0
Port Status UP
FPGA Rev : 0.2
Gigabit Ether Status : 0xF (Optical Detect,Rx Sync,Link UP)
Mode Parallel Register : 0x0
Serial Mode Register : 0x0
Link Interrupt Enable : 0x1
Tx Disable : 0x0
Internal Reset Trigger Count : 0
Slicer registers
SMDR 0xFF78 SSTR 0x1202 SSMR 0x4002 EVER 0x3001
SIMR 0x0000 MBXW 0x0000 MBXR 0x0000 SPER 0xF000
F000 chan0 chan1 chan2 chan3 sstr 1202
0006 0006 0006 0006
task0 61 61 61 61
task1 789 789 789 789
task2 61 61 61 61
task3 789 789 789 789
GCR = 0x4 GICR = 0x2403
MII registers:
Direct Access:
Control Register (0x0): 0x1140
Status Register (0x1): 0x16D
Auto Neg. Advt. Register (0x4): 0x1A0
Auto Neg. Partner Ability Reg (0x5): 0x4020
TR_IPG_TIME Register (0x10): 0x7
PAUSE_TIME Register 1 (0x11): 0x100
PAUSE_TIME Register 2 (0x12): 0x18
PAUSE_SA1 Register (0x13): 0x0
PAUSE_SA2 Register (0x14): 0x0
PAUSE_SA3 Register (0x15): 0x0
PAUSE_DA1 Register (0x16): 0x180
PAUSE_DA2 Register (0x17): 0xC200
PAUSE_DA3 Register (0x18): 0x1
Pause Upper Watermark Reg. (0x19): 0x7800
Pause Lower Watermark Reg. (0x1A): 0x8C00
TX FIFO Watermark Register (0x1B): 0x40
Memory Address Register (0x1C): 0xF009
Sync Status Address Register (0x1D): 0x40
Sys Status Register (0x1E): 0x15
Sys Control Register (0x1F): 0xFFDA
Indirect Access:
Pause Frame Sent Counter(L)(0xF000): 0x0
Here are the model numbers that support these jumbo frames:
C85EGE-2X-16K
C85EGE-2X-64K
C85EGE-2X-256K
Additionally, the ATM Router Module 2 (C8540-ARM2) also supports a configurable MTU. The maximum configurable MTU on the ARM 2 is 17976 bytes.
Configure
8500(config)#int atm 12/0/0
8500(config-if)#mtu ?
<64-17976> MTU size in bytes
8500(config-if)#mtu 17976
Verify
8500#show int ATM 12/0/0
ATM12/0/0 is up, line protocol is up
Hardware is arm2_port, address is 0090.2141.b077 (bia 0090.2141.b077)
SVC idle disconnect time: 300 seconds
MTU 17976 bytes, sub MTU 17976, BW 1000000 Kbit,
DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255
Catalyst 5000 Series
As of version 6.2(1) of the CatOS software, all ports or modules (with the exceptions stated in this section), support the default jumbo frame size up to and including 9216 bytes. Here is an example to enable jumbo frames:
Configure
Cat5000> (enable) set port jumbo
Usage: set port jumbo
Cat5000> (enable) set port jumbo 5/1 enable
Jumbo frames enabled on port 5/1.
Cat5000> (enable) set port jumbo 3/1 enable
Feature not supported on port 3/1.
Verify
Cat5000> (enable) show port jumbo ?
Usage: show port jumbo
Cat5000> (enable) show port jumbo
Jumbo frames enabled on port(s) 6/1-2,7/1-8.
If you cannot enable the jumbo frame feature on some ports at system startup time, the Command Line Interface (CLI) displays a line that the system failed to enable the jumbo frame feature on those ports. This means that the feature is user enabled in NonVolatile RAM (NVRAM), but operationally disabled on the line card.
Console> (enable) show port jumbo
Jumbo frames enabled on port(s) 6/1-2.
Enabling of jumbo frames failed on port(s) 7/1-8.
Other Considerations
Trunking: When a port is in trunking mode, the jumbo frame feature is automatically enabled on that port. When the port is not in trunking mode, the jumbo frame setting on that port returns to the original setting that you set. If you try to disable the jumbo frame feature on a trunk port, the port still passes jumbo frames until you turn off trunking.
Port channeling: Channeling ports need to have the same jumbo frame setting on each port.
Cat5000> (enable) show port jumbo
Jumbo frames enabled on port(s) 6/1-2,7/1-8.
Cat5000> (enable) set port jumbo 6/1 disable
Jumbo frames disabled on port 6/1.
Cat5000> (enable) set port chan 1/1-2 on
Port(s) 1/1-2 are assigned to admin group 9.
Port(s) 1/1-2 channel mode set to on.
Cat5000> (enable) set port jumbo 1/1 enable
Jumbo frames enabled on port 1/1-2.
Restrictions
10/100 UTP-based line cards support a maximum frame size of only 8092 bytes due to an ASIC limitation.
The nine-port Gigabit Ethernet line card (WS-X5410) does not support the jumbo frame feature due to an ASIC limitation. For a workaround to enable support for MPLS-tagged frames, see the Workaround in the Catalyst 4000 Series section of this document.
Jumbo frame support is currently not available on ATM modules for the Catalyst 5500 series switches.
Catalyst 2900XL/3500XL Series
The Catalyst 2900XL/3500XL series switches are able to support up to the MTU size of 2018 bytes as of version 12.0(5.2)XU. There is no support for a full jumbo frame. You can configure the MTU size on a per-interface basis. You can perform this configuration on all 10/100/1000 interfaces.
Configure
3500XL(config)#int fastEthernet 0/3
3500XL(config-if)#mtu ?
<1500-2018> MTU size in bytes
3500XL(config-if)#mtu 2018
Verify
3500XL#sh interfaces fastEthernet 0/3
FastEthernet0/3 is up, line protocol is up
Hardware is Fast Ethernet, address is 0007.85b8.6983 (bia 0007.85b8.6983)
MTU 2018 bytes, BW 0 Kbit, DLY 100 usec, reliability 255/255,
txload 1/255, rxload 1/255
Note: Currently, the Catalyst 2900LRE-XL switches do no support frame sizes larger than 1536 bytes signaling connection due to limitations on the CPE device.
A Catalyst 2900XL reports oversized frames when Catalyst 2900XL receives a legal max-size Ethernet frame encapsulated or tagged for ISL/802.1Q but cannot forward the frame to any other ports. There are many valid reasons why a port receives a packet but does not forwarded the packet to any other ports. For example, packets that a port blocked by Spanning Tree Protocol (STP) receives are not forwarded. This issue is a cosmetic bug with Cisco bug ID CSCdm34557 ( registered customers only) .
Catalyst 2948G-L3/4908G-L3 Series
The Catalyst 2948G-L3 and 4908G-L3 series switches do not support a configurable MTU on either 10/100 or 1000. The MTU is therefore the default, which is 1500 bytes.
Configure
2948G-L3(config)#int gig 49
2948G-L3(config-if)#mtu 2000
% Interface GigabitEthernet49 does not support user settable mtu.
Verify
2948G-L3#sh interfaces gigabitEthernet 49
GigabitEthernet49 is up, line protocol is up
Hardware is xpif_port, address is 0004.6e3b.b507 (bia 0004.6e3b.b507)
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec, rely 255/255, load 1/255
Encapsulation ARPA, loopback not set, keepalive set (10 sec)
Full-duplex, 1000Mb/s, 1000Base-SX, Auto-negotiation
Catalyst 1900/2820 Series
The ability of Catalyst 1900/2820 series switches to support baby jumbo frames depends on the revision of the switch in question. The older models of the 1900/2820 series switches that were manufactured with a metal casing can support a larger MTU of 1508 bytes to pass through. These frames are also be logged as giant frames in the statistics report, as shown here:
Catalyst 1900 - Port B Statistics Report
Receive Statistics Transmit Statistics
------------------------------------- -------------------------------------
Total good frames 120 Total frames 262767
Total octets 10041 Total octets 16840696
Broadcast/multicast frames 49 Broadcast/multicast frames 262664
Broadcast/multicast octets 5000 Broadcast/multicast octets 16825351
Good frames forwarded 107 Deferrals 0
Frames filtered 13 Single collisions 3
Runt frames 0 Multiple collisions 0
No buffer discards 0 Excessive collisions 0
Queue full discards 0
Errors: Errors:
FCS errors 0 Late collisions 0
Alignment errors 0 Excessive deferrals 0
Giant frames 5 Jabber errors 0
Address violations 0 Other transmit errors 0
The newer models of the 1900/2820 series switches that were manufactured with a plastic casing can support only a maximum MTU of 1500 bytes. Larger frames are dropped.
Troubleshoot
There is currently no specific troubleshooting information available for this configuration.
