Alert for OSPF MTU-Ignore

Cisco Cisco Nexus
#1
  • Device Make/Model

Cisco Routers (I am not sure about Nexus, it might be as well)


  • Operating System

IOS


  • Description

Checking to ensure that OSPF MTU-Ignore command is NOT enabled. For OSPF neighbors to reach full state the interface MTU must match on both ends. If there is a MTU mismatch, a technician can enter the command “ip ospf mtu-ignore” on a Cisco device to force OSPF into full state. Yes, OSPF will now work, however, packet forwarding will have an issue. MTU issues can be a very nasty problem to detect. MTU issues can lead to intermittent packet drops. Small to large packets will not have an issue, but jumbo packets will be dropped. So, in other words, some web sites will work with no issues, and yet another web site might not work (because it is sending jumbo packets), or a file transfer will start, but after a couple seconds if will stop (due to ramping up to send jumbo packets). The bottom line is that if you have to add the command “ip ospf mtu-ignore”, you do have an interface MTU issue that you should address.


  • Remediaion Steps

Enter the command “no ip ospf mtu-ignore”


  • Command or Method

Enter the command “show running-config | include “ospf mtu-ignore”


  • Output Example

Building configuration….

ip ospf mtu-ignore


#2

Awesome, Deron! Have you checked this out?

#3

The proper solution in all cases is to configure the correct MTU on the interfaces. This MTU value must match on the IOS or NX-OS devices on either side of the link.


The "ip ospf mtu ignore" command is an option offered by OSPF , supported by both NX-OS and IOS and if is configured to a network device I dont believe that can not be used as a reason to raise an alert.


Check the command reference below:

https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/ospf/ip-ospf-mtu-ignore.html


#4

Vasileious, I am not sure that I am following you since you used a double negative. When you stated "I don't believe that it cannot be used as a reason to raise an alert", did you mean that "I believe that it can be used as a reason to raise an alert."