MTU different sizes in router interfaces

[leoandino]

Hi. I have a confusion regarding the size of MTU.

What happens in a router with the packets if through the input interface it receives 1500 MTU but through the output interface it has two routes (OSPF equal cost) with 1400 MTU each ??

Fragment the packets in 1400 bytes and send it to an output interface?

fragments the packets to send it to 1400 bytes and sends them on both output interfaces?

The flow is:

INTERNET --- mtu1500 ---> ISP ---- mtu1500----> ROUTER ---- mtu1400 ---> DESTINATION 01

                                                                                                     ---mtu 1400--->DESTINATION 02

[Reza Sharifi]

Hi,

It will be fragmented to both destinations 1 and 2.

HTH

[Deepak Kumar]

Hi,

Fragmentation of packet will happen at the interface and load balancing is working on Source and destination IP address. 

Q:: What happens in a router with the packets if through the input interface it receives 1500 MTU but through the output interface it has two routes (OSPF equal cost) with 1400 MTU each ??

Ans: Fragment the packets in 1400 bytes and send it to an output interface.

Regards,

Deepak Kumar

[Joseph W. Doherty]

Fragmentation happens at the egress interface, so for the packet being fragmented, all its fragments should flow out the same interface.

However, further along the way, where fragmented packets are again ECMP routed, ECMP could send the fragments out different interfaces. Also, however, Cisco routers usually keep all of a flow's packets on the same interface.

[Joseph W. Doherty · Accepted solution]

BTW, 1500 to 1400 would generate two packets, the first being 1400 and the second being 100 payload plus overhead (which I recall [?] would only be another IP header).

[leoandino]

Thanks for your reply.

So,

if by the input interface the MTU is maintained at 1500. Then, the same two output interfaces are configured with MTU from 1400 to 3000 (for example);

- the received packet of 1500 MTU an interface of 3000 MTU will be sent to the destination in the original size (1500)?

- Is this possible to do?,

[Joseph W. Doherty · Accepted solution]

" the received packet of 1500 MTU an interface of 3000 MTU will be sent to the destination in the original size (1500)?"

Yes.

Also BTW, only the final receiver will reassemble the fragments.  I.e. if you go 1500 => 1400 => 1500, transit devices do not rebuild the 1500 byte packet.