OSPF

[abdul.qadir5001]

Q1: How Inter-Area routes are subject to routing loop in OSPF.

Q2:“IP ospf mtu ignore” What is the potential threat of this command .

[Georg Pauwen · Accepted solution]

Hello,

not sure what you are after, but OSPF prevents inter-area routing loops by implementing a split-horizon mechanism. Have a look at the documents linked below.

OSPF inter-area and intra-area routing rules

https://cciethebeginning.wordpress.com/2015/01/21/ospf-inter-area-and-intra-area-routing-rules/

Loop Prevention in OSPF

https://drive.google.com/file/d/17BProJiVrhHuZ-hcIDnL5c2wyT3HVh5M/view

'ip ospf mtu-ignore' is often used for OSPF compatibility between vendors. There have been quite a few cases on this forum here where that command is required for Cisco and Palo Alto devices. As for caveats, I am not aware of any, I did find he blog post below, but that is quite old, so I am not sure if that is still applicable.

https://blog.ipspace.net/2009/11/ip-ospf-mtu-ignore-is-dangerous-command.html

[abdul.qadir5001]

Dear Georg Pauwen,

I understood the ospf loop mechanism.

Also IP mtu link is very useful.

Thanks for your guidence.

[Vishnu Vardhan S]

When talking about the inter area routes the OSPF is no more a link state protocol.

Because inter-area OSPF is distance vector, it is vulnerable to routing loops. It avoids loops by mandating a loop-free inter-area topology, in which traffic from one area can only reach another area through area 0.

MTU ??

When you have two different vendor device trying to form a ospf neighborship then they may end up in extart state, since the interface MTU may vary between them, hence we use ignore MTU to eliminate the rule of having same MTU in the ospf neighbor interfaces. 

Try this gns3 lab you will understand easily :

R1(Fa0/0-MTU 1400)--------------(Fa0/0-MTU 1500)R2

Try forming neighborship between them it will stuck in Extart state, now use the ignore mtu command in them it will become FULL