Invalid MTU in IPv6 Router Advertisements

Hi @Kentzo Thanks for reaching out to our community for help with your internet. Are your services also affected? Can you tell us more about what's going on. Are you experiencing slow/ intermittent connection or packet loss? 

IPv6 connectivity is broken unless I manually alter the settings on my router to circumvent an incorrect value advertised by Comcast's upstream router.

Specifically, when Comcast's router sends an ICMPv6 message of type Router Advertisement, in that message it sets the MTU option to 9192. This is wrong because the IPv6 standard requires that value to be supported by segments. Since I'm connected to Comcast via a cable modem therefore it simply cannot handle this value.

Note that this behavior is new as of yesterday while proper behavior was observed for the past 2 years. It doesn't seem to be unique to me as while doing my research I found other users, primarily on the east coast, complaining about that since 2022. That included business users.

@Kentzo 

Could you please send our team a direct message with your full name and full address? Our team can most definitely take a further look at this issue.

To send a "Direct Message" to Xfinity Support:

Click "Sign In" if necessary

Click the "Direct Messaging" icon or https://comca.st/3EqVMu7

Click the "New message" (pencil and paper) icon

The "To:" line prompts you to "Type the name of a person". Instead, type "Xfinity Support" there

- As you are typing a drop-down list appears. Select "Xfinity Support" from that list

- An "Xfinity Support" graphic replaces the "To:" line

Type your message in the text area near the bottom of the window

Press Enter to send it

Just sent the message.

For some reason Xfinity is very adamant about scheduling the tech visit. I know for sure that they won't be able to fix this problem and might be responsible for the visit and diagnostics fee.

This upselling is unacceptable. My report contains all the necessary information (detailed problem description as well as reference to the incorrectly behaving equipment on your side via IPv6 and MAC addresses on the screenshot) for your administrators to verity and apply the fix remotely.

@bandito1200 Hello! Thank you for reaching out to us here on our Community Forums. Can you please send us a Private Message with your full name and address so that we can look into this for you? Here are instructions on how to send us a PM in case you need them:

Click "Sign In" if necessary
 • Click the "Peer to peer chat" 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

We look forward to hearing from you.

I'm in touch with Kentzo above -- since its the same issue, will wait to see his resolution, rather than initiate a separate stream.    

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