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corentin
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Sunday, October 26th, 2025 5:33 PM

Invalid MTU 9192 from xfinity for IPv6 (router advertisements?)

I'm trying to set up my IPv6 setup and I keep getting MTU packets of 9192 from Xfinity. Presumably these are router advertisements.

It seems my router learned the address for the gateway anyway, but it seems the upstream equipment might be misconfigured? The MTU of ethernet isn't 9192, so maybe this is the MTU that is supported over DOCSIS? I have my xfinity gateway configured in bridge mode to go directly to my Mikrotik router at home.

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XfinityJorge

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21 days ago

 

corentin Hello! Thank you so much for using our Forums and welcome. We are happy to work together to make sure we help get your modem set up properly to ensure you are getting the best connection while trying to enjoy your internet. In order to get started can I please get your full name and service address? 
Here's the detailed steps to direct message us: 
• Click "Sign In" if necessary 
• Click the "Direct Message" icon (upper right corner of this page) 
• Click the "New message" (pencil and paper) icon 
• Type "Xfinity Support" in the to line and select "Xfinity Support" from the drop-down list 
• Type your message in the text area near the bottom of the window 
• Press Enter to send your message
 

 

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user9641

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@XfinityJorge​ I had an Xfinity outage this morning/afternoon starting around 10am CST (Chicago area North suburbs). At almost exactly the same time, my (Cisco LinkSys) router started logging the following message every 5-6 seconds that was never seen before today 17 Nov 2025:

"NDISC router announcement with mtu = 9192"

My router log is clogged with these messages, and this hurts my router performance as all the system resources are directed to logging these router error messages.

The Xfinity outside network interface for my neighborhood is right in front of my house across the street. Two Xfinity bucket trucks responded to restore service and then drove away after about 60 minutes. I never received notice of a scheduled outage.

Is Xfinity switching to IPv6 and abandoning IPv4?

Please let me know what is going on here.

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2 hours ago

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EG

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114.5K Messages

19 days ago

@corentin @XfinityJorge 

Please circle back here and post any possible solutions for the issue here in these open public forums so that all readers here may benefit from the exchange / info. This is in keeping with the spirit for which these public help forums were originally intended. Thank you.

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