Hi,

While exploring certain knobs of OSPF I cam across overload bit.

When would you use OverloadBit ?

-> Your device has just started and you do not want traffic to pass through your device till all the protocols converge.

-> You are going into a Maintenance Window and want to auto-drain the traffic w.r.t IGP.

What does Overload-Bit do ?

-> Advertise Transit Networks with Max-Metric 0xFFFF 65535. Okay, I specifically said Transit Networks ? What about own loopbacks or connected Networks ? What is the behavior ? When do you call a connected Network stub / Transit ?

 

Lets see the below topology

topology_1

Lets have a quick look at Normal output and when configured with overload knob on r2 what it looks like on r3

 

1_normal_output_before 2_ospf_db_output_after_overload 3_r3_output_with_metric

Now, 9.9.27.0 network on r3 looks like having a metric of 2, hold that thought there as it is going to change now

 

4_looking_at_db

From the above, 9.9.27.0 network is mentioned as stub and going back to definition, anyone who is transit would get affected. What happens when i do peering over r2-r7, does the network 9.9.27.0 interface change from stub to transit, lets see how it gets altered.

 

5_final_output

The above image gives a Alteration in behavior when things change on R2. Hence the new metric for 9.9.27.0 network now is 6553x as it is treated as transit on Router-2

To make things simple any ospf routes from other neighbors will be sent out with highest-metric so that no other router will not use this routers path as the best one for forwarding traffic.

 

Regards

Rakesh M

 

 

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