Thursday, May 14, 2009

Does Different MTU Communicate

Does Different MTU Communicate

A dicey sitution occurs when MTU on different interfaces come in picture. Every is having a thought process that the lowest MTU on interface will negociate each other but in fact, it only works in case of path mtu is enabled. Assume a scenario where a R1 router is connected with R2 router. R1 interface is having MTU of 1500 bytes and latter is having MTU of 1600 bytes. If R1 sends a packets of 1501 bytes to R2 without DF bit set, R1 will fragment the packet and forwards it to R2 reassembles the packet and will forward a single packet of 1501 bytes to R1. R1 interface will drop the packet because the interface MTU is set to 1500 bytes and communiaction will never occur in case of packet size more than 1500 bytes.

shivlu jain

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1 comment:

deepika said...

In case OSPF is running among two routers and interfaces has different MTU set ,then sh ip ospf neighbor output will be stuck in exstart state.

When attempting to run OSPF between a Cisco router and another vendor's router, if the default interface MTUs do not match, the router with the higher MTU sends a packet larger than the MTU set on the neighboring router. This results in the neighboring router ignoring the packet. The solution is to change the MTU on one router to match the neighbor's MTU.

To set the MTU size of IP packets sent on an interface, issue the ip mtu command in interface configuration mode.

In some instances, disabling MTU mismatch detection may be necessary. To disable OSPF MTU mismatch detection on receiving Database Descriptor (DBD) packets, issue the ip ospf mtu-ignore command in interface configuration mode.