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flip-flopping between 2 internet gateways?

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Thu, 01/16/2020 - 22:38

#1

AE6XE

AE6XE's picture

flip-flopping between 2 internet gateways?

Does anyone have a situation where the selected internet gateway node is flip flopping between 2 gateways on the mesh with similar path distance? I have run into an OLSR parameter that will prevent the flip-flop and ensuing chaos. I'm looking for a test case to validate this config setting is working and solves the problem.

Joe AE6XE

Fri, 01/17/2020 - 12:16

#2

w6bi

Route flapping

Joe, I believe we may have it happening here routinely in Ventura County. I'm open to trying it.

Orv W6BI

Fri, 01/17/2020 - 12:29

#3

k1ky

Route Flopping

I believe I have a perfect test case for you as well. I'll message you direct.

Sat, 01/18/2020 - 11:55

#4

AE6XE

AE6XE's picture

I'm reviewing the code to

I'm reviewing the code to take the next step on this issue. Apparently, when using this setting with the ETX cost of 0.1 for dtdlinks, there is an increased change of a routing loop, so we will need to proceed with this in mind. Here's the write up of what this setting is supposed to do, below.

Joe AE6XE

The NatThreshold option was introduced by Sven Ola to suppress a very annoying

problem with OLSRd, switching default gateways. If a router is located between

two Internet gateways with similar path costs the default route (0.0.0.0/0)

will constantly switch between the two gateways due to normal fluctuations of

the link metrics. Whenever OLSRd decides that the other NAT gateway is

"better", then switching to this new gateway will result in termination of all

connected sessions (TCP and HTTP).

The user experience will be rather painful and users will experience hanging

SSH and HTTP sessions (or anything using TCP).

NatThreshold tries to help by introducing a hysteresis factor for

choosing the route to the default gateway. Only if the new gateway has

a lower cost than the current gateways path cost multiplied by

NatThreshold the node will switch the gateway.

In short:

if (cost(new_gateway) < cost(current_gw)*NatThreshold)) {

switch_gateway();

}

Practical experience shows that this leads to much better quality of default

gateway selection, even if (in theory) a small NatThreshold together with

Fisheye can lead to persistent routing loops.

Please note that even with NatThreshold enabled, some users will still

experience gateway switching. However, most users will not.

Mon, 01/20/2020 - 14:36

(Reply to #4) #5

k1ky

Reviewing the code to ....

Sounds reasonable to me. Do "we" the user get to choose the threshold number or will it be set to some "fixed" value?

Wed, 01/22/2020 - 14:30

(Reply to #5) #6

AE6XE

AE6XE's picture

Shifting gears to get the

Shifting gears to get the openwrt 19.07 upgrade into AREDN nightly builds. Will come back to this...

Joe AE6XE

Sat, 01/18/2020 - 13:07

#7

KL5T

Alaska flip-flopping

Hi Joe,

I think we've had this problem here. The problem manifested with our DMR repeater trying to maintain its connection with Brandmeister. Seems that once I stopped feeding our mesh with multiple internet fees the problem resolved. However, I'd like to get back to having redundant feeds.

Regards,

Kent, KL5T

Sat, 01/18/2020 - 14:22

#8

w6bi

DtD at ETX 0.1

Joe, this is what we were trying to work around by commenting out the " option Mode 'ether' " line in /etc/config/olsrd (which stops the link from being considered as being wired). Like this:

config Interface
        list interface 'dtdlink'
#        option Mode 'ether'

It's a bandaid, and only helps some.   I'm very much interested in whatever solution you come up with.

Orv W6BI

Tue, 01/21/2020 - 23:43

#9

Ai6bx

I do

Joe,

I dropped you an email with a couple nodes that may be test subjects.

Keith

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