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


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



From the above, 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 interface change from stub to transit, lets see how it gets altered.



The above image gives a Alteration in behavior when things change on R2. Hence the new metric for 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.



Rakesh M