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Routing

jonk34567
Enthusiast
Enthusiast

pls look at the image attached. what might be the answer ?

1 Accepted Solution

Accepted Solutions

Deepak Kumar
Advocate
Advocate

Answer  D & E is correct. (Edited)

MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state. After hellos are exchanged and the routers become neighbours, each OSPF speaker advertises the IP MTU of its local interface in a Data Base Description (DBD) LSA. If there is a mismatch you’ll probably just adjust the configuration to be identical on both ends of the link and be done.

 

"OSPF runs directly over the Internet Protocol's network layer. OSPF packets are therefore encapsulated solely by IP and local data-link headers.

OSPF does not define a way to fragment its protocol packets and depends on IP fragmentation when transmitting packets larger than the network MTU. If necessary, the length of OSPF packets can be up to 65,535 bytes (including the IP header). The OSPF packet types that are likely to be large (Database Description Packets, Link State Request, Link State Update, and Link State Acknowledgment packets) can usually be split into several separate protocol packets, without loss of functionality. This is recommended; IP fragmentation should be avoided whenever possible"

 

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/116119-technote-ospf-mtu-00.html

 

Regards,

Deepak Kumar

 

 

Regards,
Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!

View solution in original post

5 Replies 5

Deepak Kumar
Advocate
Advocate

Answer  D & E is correct. (Edited)

MTU mismatches are the primary reason an OSPF adjacency becomes stuck in the EXSTART state. After hellos are exchanged and the routers become neighbours, each OSPF speaker advertises the IP MTU of its local interface in a Data Base Description (DBD) LSA. If there is a mismatch you’ll probably just adjust the configuration to be identical on both ends of the link and be done.

 

"OSPF runs directly over the Internet Protocol's network layer. OSPF packets are therefore encapsulated solely by IP and local data-link headers.

OSPF does not define a way to fragment its protocol packets and depends on IP fragmentation when transmitting packets larger than the network MTU. If necessary, the length of OSPF packets can be up to 65,535 bytes (including the IP header). The OSPF packet types that are likely to be large (Database Description Packets, Link State Request, Link State Update, and Link State Acknowledgment packets) can usually be split into several separate protocol packets, without loss of functionality. This is recommended; IP fragmentation should be avoided whenever possible"

 

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/116119-technote-ospf-mtu-00.html

 

Regards,

Deepak Kumar

 

 

Regards,
Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!

but answer says D,E

As deepak explained why D is correct, regarding E "ip ospf mtu-ignore"override the process of answer D and disable the checking of MTU mismatch when it receive the DBD packet so it is also possible solution and that is why it is correct.

 

For more you can visit the below page.

 

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

 

Thank you.

 

Kindest regards,

Uzzi

 

 



Kindest regards,
Uzair
CCENT, CCNA (R&S), CCNP (R&S).

Sorry, this is a typo in my last comment ... the correct answer is D & E...

Regards,
Deepak Kumar
Regards,
Deepak Kumar,
Don't forget to vote and accept the solution if this comment will help you!

what about "C " ? does cost need to be same on the both ends ? 

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