10.2 mtu vs ip mtu command

Hi there,

While doing the QOS 10.2 WFQ section, I configured the serial with the following config :


r4  r5
int s0/1
 clock rate 128000
 bandwidth 128000
 hold-queue 256 out
 tx-ring-limit 1
 mtu 160
 fair-queue 16 128 8


I noticed that the solution configures "ip mtu 156" and not "mtu 160".

Is there a difference between the 2 commands ? Would "mtu 160" be an acceptable solution ?




  • Hi.

    The additional 4 bytes are for the Layer 2 HDLC encapsulation, which is not accounted for by the ip mtu command.

    Every frame sent out the link will have a maximum size of 160 bytes = 156 IP + 4 HDLC.

    Using ip mtu 160 would result in 164 bytes frame size.

    Hope this helps.

  • Thanks.

    I noticed after my previous post that using the "mtu 160" command also reduces the IP mtu automatically to 160.


    sh run int s0/1
    Building configuration...

    Current configuration : 113 bytes
    interface Serial0/1
     mtu 160
     ip address
     ip rip advertise 10
     clock rate 128000


    sh int s0/1 | i MTU
      MTU 160 bytes, BW 1544 Kbit/sec, DLY 20000 usec,


    sh ip int s0/1 | i MTU
      MTU is 160 bytes




  • Hi Netgeek, 

    Noticed the post is about an year old, but thgt I would chime in nevertheless.

    When you configure the IP MTU 156 command, that does NOT impact the MTU for the serial interface. It only effects the MTU for the IP protocol.

    However, when you configure the command MTU 156 it restricts the MTU of the interface to 156 as well as for the IP protocol. Add to that the 4 bytes of HDLC encapsulation which is not taking into a/c with this command and you get 160 bytes total.


  • mtu 160 would not be an acceptable, because this will set the L2 MTU to 156 while the IP MTU sets the size of the IP packet, in this case will remain a default of 1500



  • Greetings All,

    Believe RoadToCCIE is correct.

    mtu 160 -- mtu for L2 protocol is 160 bytes  -- a ceiling for all frames carried by L2 protocol

    Rest are per protocol mtu adjustments, which must be less than/ equal to ceiling.

    ip mtu 100 -- mtu for L2 protocol is 160 bytes for IP packets

    ipx mtu 99 -- mtu for L2 protocol is 160 bytes for IPX packets

    mpls mtu 160 -- mtu for L2 protocol is 160 for labeled packets

    (mtu is size of payload for your L2 protocol; so, doesn't include L2 header bytes)

  • MartinlMartinl ✭✭✭

    this topic is old; originated in 2012; I don't thnink those guys are around anymore.

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