Difference between interface mtu and ip mtu

[sebastan_bach]

hi all can some pls tell me the difference between interface mtu and ip mtu. the confusion i feel is when we set the physical mtu it going to affect the ip packets anyway right then why do we have the ip mtu option. can someone pls explain the difference between them .

sebastan

[a.kiprawih · Accepted solution]

Hi,

The main difference is interface MTU defined max packet size supported by an interface, while IP MTU is used to set MTU size of IP PACKET.

1. Interface MTU - default MTU size for an interface:

Each interface has a default maximum packet size or MTU size. This number generally defaults to the largest size possible for that interface type. On serial interfaces, the MTU size varies, but cannot be set smaller than 64 bytes.

Media Type --------------- Default MTU (Bytes)

Ethernet --------------- 1500

Serial --------------- 1500

Token Ring --------------- 4464

ATM --------------- 4470

FDDI --------------- 4470

HSSI (HSA) --------------- 4470

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tir_r/int_m1ht.htm#wp999684

2. IP MTU:

To set the maximum transmission unit (MTU) size of IP packets sent on an interface. Minimum is 128 bytes; maximum depends on the interface medium.

Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu command.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tiap_r/apl_i2ht.htm#wp1202749

Pls rate useful post(s).

Rgds,

AK

[sebastan_bach]

hi thanks for ur reply. u there is one confusion i am having here. say the default mtu size is 1500 bytes. so the ip mtu is also 1500 right. the ip mtu is for ip protocol packets the mtu is 1500 bytes right. but nowadays we have only ip traffic.

another doubt is say i set the ip mtu to 500 bytes but the physical interface mtu is set to 1500 bytes only. so when ip packets are sent through that interface the router considers which mtu the interface mt or the ip mtu. can u pls tell me on thia. thanks a lot. waiting for ur reply.

regards

sebastan

[frankliney]

The smaller mtu is always used for the related packet. So with phys MTU 1500 and IP mtu 500 then an IP packet will consider the 500 mtu.

You say you only have IP packets, but that's not the same for all networks - even if the payload is always IP. Example, if you run ISIS as your IGP, ISIS uses CLNS packets not IP and you might want a different clns mtu to your IP mtu. (default clns mtu is 3 bytes smaller than phys).

Another cosideration even if the only L3 packet type you have is IP is if you have some L2 switching in the middle. Say you have a router trunked to a switch. You might set your physical (L2) mtu on each each of that trunk to 4470. But on each vlan within the trunk you have to consider the mtu that can be suppported at the far end device.

The first vlan might go through to another router which supports jumbo, so you'd be happy for the IP mtu to be 4470 same as the physical.

The second vlan might go through the switch to a FastEthernet interface on a device which doesn't support jumbo. So there you'd need to set the IP mtu down to 1500 so that IP packets larger than that are fragmented and not blackholed by the receiving interface.

Physical mtus in your core network might as well always be as large as the devices support on each physical network hop.

Your layer 3 (ip, clns, mpls) mtu's need to be thought of with more consideration, as this is going to affect where you cause packet fragmentation, but if the core is bigger than the edge life is typically good.

mpls mtu over ethernet is another chapter :-)

[sebastan_bach]

hi dude thanks a lot man. thanks for the detailed explaination. i also read that when using gre with ipsec is used it better to set the gre mtu smaller as compared to the physical mtu cause thsi will help the router avoid double fragmentation and especially fragmentation after ipsec encryption adn encapsulation. am i right. pls confirm. thanks a lot once again. waiting for ur reply.

regards

sebastan

[veeras455]

Hi All,

Can u please clarify my doubts regarding MTU/MRU.

1) What is the difference between OSPF MTU and IP MTU and what will happen when the OSPF MTU is lower than IP MTU and Vice versa.

2) How to verify the OSPF MTU/MRU related?

3) What are the different MTUs?

Please clarify the above scenarios.

Thanks in Advance ...

Regards,

Nani