Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01

Shraddha Hegde <> Wed, 30 September 2015 16:32 UTC

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From: Shraddha Hegde <>
To: Manav Bhatia <>
Thread-Topic: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01
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Date: Wed, 30 Sep 2015 16:32:18 +0000
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Cc: Hannes Gredler <>, OSPF WG List <>, "Jalil, Luay" <>, Mohan Nanduri <>
Subject: Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01
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The draft is about a link going for maintenance and not about the node going for maintenance.
The draft talks about sending a “link overload” TLV associated with the link and the other end of the link
Setting metric to high value so that reverse traffic is also diverted away from the link.


From: Manav Bhatia []
Sent: Wednesday, September 30, 2015 7:32 PM
To: Shraddha Hegde <>
Cc: Acee Lindem (acee) <>om>; OSPF WG List <>rg>; Hannes Gredler <>at>; Jalil, Luay <>om>; Mohan Nanduri <>
Subject: Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01


I am probably joining the party quite late, so apologies in advance if the following has already been discussed and discarded.

Why cant the following be done:

Set the metric of all transit links/connections on an “overloaded” router to 0xffff in its Router LSAs. This will result this router to not be included as a transit node in its neighbors SPF tree.  Stub links can still be advertised with their normal metrics so that they are reachable even when a router is “overloaded”.

This way you can mimic IS-IS OL bit.

Cheers, Manav

On Mon, Sep 28, 2015 at 10:43 AM, Shraddha Hegde <<>> wrote:

Thanks for picking up the draft for adoption.

I believe this draft is very useful in automating the link upgrade process and software upgrade process in overlay deployments and
hence support WG adoption as co-author.

I would like to  take this opportunity to discuss  few of the points raised during Prague meeting.

1. Whether to keep the "Link overload" advertisement at area level or at link level.

In controller based deployments, it's useful to advertise the impending maintenance of the link to the controller so that controller can take
Special actions based on the information. The use case is described in sec 5.2 in  the draft.
The draft advocates increasing the metric to usable high metric on both ends of the link. This is for backwards compatibility and to avoid need of flag
Day upgrade on all nodes.

 Controller cannot assign special meaning to the metric  for ex: Metric XXXX means the link going for maintenance and take different actions based on metric.

For a completely automated upgrade process, controller would need a fine grained and specific information that the link is going for maintenance so that the services that use the particular link find a different path forcefully while keeping the entire process non-disruptive.

2. Use of high metric  on either side of the link  to divert the traffic.

As I already mentioned before, draft advocates raising the reverse metric to a high metric  but that is for backwards compatibility and to avoid
Need for flag-day upgrade. There were suggestions at the Prague meeting to use lower bandwidth advertisements as well as removal of
Link characteristics to force the services on different path. These mechanisms would be disruptive and defeats the purpose of the draft.

3.  Backward compatibility

"Link-overload"  is a new information attached to a link and is very similar to a new constraint being added to the link.
This information is non-invasive in the sense that services that do not want to look at the new constraint (link overload)
May depend only on the metric to take specific actions.

Whereas services that have specialized requirement of providing non-disruptive upgrades can do so by processing the new constraint.

Section 4 in the draft talks about backwards compatibility.
I'll add more clarifications in the coming days.


-----Original Message-----
From: OSPF [<>] On Behalf Of Acee Lindem (acee)
Sent: Saturday, September 26, 2015 6:05 AM
To: OSPF WG List <<>>
Subject: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01

In Prague, there was consensus in the room that this use case was not covered by existing mechanisms and that it was a problem the WG should solve. There were differing opinions as to the exact solution but that should not preclude OSPF WG adoption.

Please indicate your support (or concerns) for adopting this as a WG Document.


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