Configure MTU Size on Catalyst 9500 Switch

Looking to increase the MTU size to configure jumbo frames on a Cisco Catalyst 9500 switch? Yes. Great. Let’s take a look at how we do that.

Let’s see what the current MTU size is.

CORE_9548-SWV#show system mtu
Global Ethernet MTU is 1500 bytes.

CORE_9548-SWV#

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Let’s check the MTU size on VLAN 20’s interface.

CORE_9548-SWV#show ip interface vlan 20
Vlan20 is up, line protocol is up
  Internet address is 10.5.20.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1500 bytes
  Helper address is not set
  Directed broadcast forwarding is disabled
  Multicast reserved groups joined: 224.0.0.1 224.0.0.2 224.0.0.22 224.0.0.13
  Outgoing Common access list is not set 
  Outgoing access list is not set
  Inbound Common access list is not set 
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF switching turbo vector
  IP Null turbo vector
  Associated unicast routing topologies:
        Topology "base", operation state is UP
  Associated multicast routing topologies:
        Topology "base", operation state is UP
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  Output features: MFIB Adjacency
  IPv4 WCCP Redirect outbound is disabled
  IPv4 WCCP Redirect inbound is disabled
  IPv4 WCCP Redirect exclude is disabled
CORE_9548-SWV#

Let’s see if we can change the MTU size to anything over 1500 on VLAN 20’s SVI.

CORE_9548-SWV(config)#interface vlan 20
CORE_9548-SWV(config-if)#ip mtu ?
  <256-1500>  MTU (bytes)

CORE_9548-SWV(config-if)#ip mtu

Nope. We can’t. We have to change the System MTU first. Here’s how we do it.

CORE_9548-SWV(config)#system mtu 9154
Global Ethernet MTU is set to 9154 bytes.
Note: this is the Ethernet payload size, not the total
Ethernet frame size, which includes the Ethernet
header/trailer and possibly other tags, such as ISL or
802.1q tags.

CORE_9548-SWV(config)#

Let’s verify whether the new system MTU size took place.

CORE_9548-SWV#show system mtu
Global Ethernet MTU is 9154 bytes.

CORE_9548-SWV#

Yes, it did. Now, let’s verify whether the new system MTU size increased the interface MTU range under an interface.

CORE_9548-SWV(config)#interface vlan 20
CORE_9548-SWV(config-if)#ip mtu ?
  <256-9154>  MTU (bytes)

CORE_9548-SWV(config-if)#ip mtu
CORE_9548-SWV(config-if)#exit
CORE_9548-SWV(config)#

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Yes, the MTU range goes up to 9154 bytes and now the Cisco switch supports jumbo frames as it matches the new system MTU of 9154; however, there’s no need to configure jumbo frames under the interface because the MTU value is already 9154.

CORE_9548-SWV#show ip interface vlan 20
Vlan20 is up, line protocol is down
  Internet address is 10.5.20.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 9154 bytes
  Helper addresses are 10.3.101.212
                       10.5.100.212
                       10.3.106.200
                       10.9.0.212
  Directed broadcast forwarding is enabled
  Multicast reserved groups joined: 224.0.0.1 224.0.0.2 224.0.0.22 224.0.0.13
  Outgoing Common access list is not set 
  Outgoing access list is not set
  Inbound Common access list is not set 
  Inbound  access list is not set
  Proxy ARP is enabled
  Local Proxy ARP is disabled
  Security level is default
  Split horizon is enabled
  ICMP redirects are always sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  IP fast switching is enabled
  IP Flow switching is disabled
  IP CEF switching is enabled
  IP CEF switching turbo vector
  IP Null turbo vector
  Associated unicast routing topologies:
        Topology "base", operation state is UP
  Associated multicast routing topologies:
        Topology "base", operation state is UP
  IP multicast fast switching is enabled
  IP multicast distributed fast switching is disabled
  IP route-cache flags are Fast, CEF
  Router Discovery is disabled
  IP output packet accounting is disabled
  IP access violation accounting is disabled
  TCP/IP header compression is disabled
  RTP/IP header compression is disabled
  Probe proxy name replies are disabled
  Policy routing is disabled
  Network address translation is disabled
  BGP Policy Mapping is disabled
  Input features: MCI Check
  Output features: MFIB Adjacency
  IPv4 WCCP Redirect outbound is disabled
  IPv4 WCCP Redirect inbound is disabled
  IPv4 WCCP Redirect exclude is disabled
CORE_9548-SWV#

Changing the system MTU changes the MTU value on the Layer 3 interfaces. And likewise, Layer 2 (switch port) interfaces also raise their MTU value to 9154 and therefore support jumbo frames.

CORE_9548-SWV#show int twe1/0/20
TwentyFiveGigE1/0/20 is down, line protocol is down (notconnect) 
  Hardware is Twenty Five Gigabit Ethernet, address is cc90.70d2.f514 (bia cc90.70d2.f514)
  MTU 9154 bytes, BW 25000000 Kbit/sec, DLY 10 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Auto-duplex, Auto-speed, link type is auto, media type is unknown
  Fec is auto
  input flow-control is on, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters 01:38:42
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 no buffer
     Received 0 broadcasts (0 multicasts)
     0 runts, 0 giants, 0 throttles 
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     0 packets output, 0 bytes, 0 underruns
     Output 0 broadcasts (0 multicasts)
     0 output errors, 0 collisions, 4 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out
CORE_9548-SWV#

Thanks for checking my post. I hope to see you around again.

Cheers.

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Alirio Zavarce, CCIE #28672, is a seasoned enterprise route-switch consultant with 30 years of experience with data networks. Alirio started this networking blog to help his peers become better network engineers and share all his everyday experiences and troubleshooting tips. More about me...

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10 thoughts on “Configure MTU Size on Catalyst 9500 Switch”

  1. Hello Alirio and thanks for the quick topic on mtu on Cisco 9500 series.
    Do you know if there is a effect when the command “system mtu 9154” is pass ?
    I don’t find clear answer for this.

    Reply
    • Hey Kevin. After you increase the MTU to 9154, you really just enable the switch to forward frames with a payload bigger than the default of 1500 Bytes. I hope this helps.

      Reply
    • I don’t think you need to reboot the Catalyst 9500 when changing the System MTU; however, the switch will tell you if a reboot is necessary. Changing the System MTU doesn’t change the MTU on individual interfaces.

      Reply
      • But for the all interfaces will be safe if this configure will be push, right? Because we have concern that all interfaces will be down for temporary, maybe a few seconds.

        Thank you.

        Reply
        • Hi Angga. I’ve never seen interface flaps when changing the MTU size; however, I recommend doing this during a maintenance window.

          Reply
  2. Great article thank you, finally it all makes sense.
    One question thou, when I do show vlan, the mtu still shows 1500 when the system mtu is 9216. Why is this? Thanks in advance.

    #sh system mtu
    Global Ethernet MTU is 9216 bytes.

    #sh vlan
    VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
    —- —– ———- —– —— —— ——– —- ——– —— ——
    99 enet 100099 1500 – – – – – 0 0
    100 enet 100100 1500 – – – – – 0 0
    101 enet 100101 1500 – – – – – 0 0
    102 enet 100102 1500 – – – – – 0 0

    Reply
    • I don’t know for sure, but I think the System MTU is the setting that matters when it comes to increasing the MTU size on the switch. I know, it’s a discrepancy, but if after changing the System MTU, you can pass jumbo frames, you should be good.

      Reply
  3. Hello,
    great article.
    If I want to enable jumbo frames just for VLAN X. Is it possible or you have to do it for the entire SW?
    And if it is posible I would enable it by setting the MTU on the actual physical interfaces that belong to VLAN X or is enough to just enable it on the SVI alone?
    And Would I use the “mtu” command or “ip mtu” command?
    Thank you!!
    Regars

    Reply
    • Hey Lucas. You need increase the MTU on the chassis first before increasing the MTU on the physical ports and VLAN interfaces. On the SVIs, you would use the “ip mtu” command; however, when you increase the chassis MTU, the SVIs and physical interfaces increase their MTU to match the value you set on the chassis.

      Reply

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