MTU size subinterface

[JPC11]

Hi,

I have two 93180 switches that will be used for WAN connectivity between two sites. The switches will each use one physical interface to the WAN divided into two sub interfaces, one sub is used for inter site WAN traffic between sites A and B and the other subinterface will be used as an IPN for ACI, again between the same two sites A and B.  My question is will these differing MTU sizes per subinterface have any negative affect on traffic?

Many thanks,

Jonathan

[Joseph W. Doherty]

Could you clarify your question as MTU is tied to physical interface?  Are you planning to use IP MTU per subinterface?

[JPC11 · Accepted solution]

Hi Joseph, thanks for replying.  Hopefully this answers your question.  Th email

interface E1/1

has the MTU set to 9150,

E1/1.1

does not have an MTU setting so is using an MTU of 1500, finally

E1/1.4

has a setting of 9150

interface Ethernet1/1
mtu 9150

interface Ethernet1/1.1
encapsulation dot1q 3
ip address 192.168.195.23/31
no shutdown

interface Ethernet1/1.4
mtu 9150
encapsulation dot1q 4
vrf member IPN-1
ip address 192.168.195.17/31
ip ospf network point-to-point
ip router ospf IPN area 0.0.3.142
!

Show interface reports that the interfaces are as configured 

Ethernet1/1 is up
admin state is up, Dedicated Interface
Hardware: 100/1000/10000/25000 Ethernet, address: f4ee.3111.95af (bia f4ee.3111.95b0)
MTU 9150 bytes

show int e1/1.1
Ethernet1/1.1 is up
admin state is up, Dedicated Interface, [parent interface is Ethernet1/1]
Hardware: 100/1000/10000/25000 Ethernet, address: f4ee.3111.95af (bia f4ee.3111.95b0)
Internet Address is 192.168.195.23/31
MTU 1500 bytes

!

show int e1/1.4
Ethernet1/1.4 is up
admin state is up, Dedicated Interface, [parent interface is Ethernet1/1]
Hardware: 100/1000/10000/25000 Ethernet, address: f4ee.3111.95af (bia f4ee.3111.95b0)
Description: Connection to YH_IPN1
Internet Address is 192.168.195.17/31
MTU 9150 bytes,

Thanks again,

Jonathan

[Joseph W. Doherty · Accepted solution]

Never tried what your doing.

Physically, MTU represents the largest frame size that can be handled by the NIC.  You cannot use a size larger than the hardware can process, but you can set the MTU lower.  In the latter case the software will impose a logical limit.

So, what you're doing should work fine, except I understand Cisco will often accept (ingress) a frame larger than a logical MTU setting.  I.e. your subinterface with the 1500 MTU will not send a larger frame but might accept a 9k frame.

[JPC11]

Seeing as traffic did not need to be routed between the subinterfaces with differing MTU I didn't think it was going to cause prblmes with excessive fragmentation but I just was not sure as I have never done it either.  Thank you for taking the time to answer.

Jonathan

[guruvignesh · Accepted solution]

Hello JPC,

No, having different MTU sizes on subinterfaces won't have a negative effect on traffic. MTU is like a size limit for data packets. As long as the MTU is set correctly on each subinterface and devices can handle it, data will flow fine. Just make sure all devices along the path can handle the MTU you set for each subinterface.

[JPC11]

Hi, thanks for taking the time to answer.  I'd just never had to do this configuration previously.

Jonathan