Overload Bit – 0XFFFF -> Advertisement Behavior Stub vs Transit

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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



Quick Scenario -3 – Bgp Local-as

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3rd post in the quick series is about BGP local-as

BGP Local-AS

“The local-AS feature allows a router to appear to be a member of a second Autonomous systems (AS), in addition to its real AS. This feature can only be used for true eBGP peers. You cannot use this feature  for two peers that are members of different confederation sub-ASs”


Requirement – R1 needs to peer with R2 with As number 400 while all other Routers to peer with R2 should use a AS number of 200



As we can see, R2 is acting as Local-as 400 for R1 and AS 2 for R3






Quick Scenario -2 – Bgp Communities – (no-advertise)

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2nd post in the quick series is about BGP community no-advertise

Community – No-Advertise

“No-Advertise is similar to No-export, While no-export does not export routes to another AS, this goes further and it will not export route to both ebgp or ibgp neighbors as well.”

Requirement – Make sure R1 advertises its routes to R2, and R2 should not advertise the routes to either EBGP or IBGP neighbors



Before confiuring anything





access-list 99 permit host

route-map NO-ADV-COMM permit 10
match ip address 99
set community no-advertise

router bgp 100
nei route-map NO-ADV-COMM out










Quick Scenario -1 – Bgp Communities – (no-export)

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This will be a series of posts in which i shall be posting some quick scenario labbings. Mostly i have designed these so that I can relate to some stuff which i do not often use, nevertheless will blog all small topics

Community – No-export

“NO-EXPORT is commonly used within an AS to instruct routers not to export a prefix to eBGP neighbors. For instance, subnets of a larger block can be advertised to influence external AS best-path selection, and those not required for this traffic engineering purpose may be tagged NO-EXPORT to prevent them from being leaked to the Internet (and thus contributing to unnecessary global routing table growth). If a neighboring AS accepts this community, it can be used to selectively leak more specifics for traffic engineering but limit their propagation to just one AS.”

Requirement – R1 has its loopback advertised to R3 via BGP. Make sure the advertisement stays within the AS and should not go to another AS Router in this Case R3

no-export scenario

Before community addition – Routing Table of R3


AS we can see below route is not seen R3.



Rakesh M

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