Visitor
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19 Messages
Invalid IPv6 MTU
Hello, I am having the same issue as the person in this thread: https://forums.xfinity.com/conversations/customer-service/invalid-mtu-in-ipv6-router-advertisements/64ae3b8fd92de332576d7911
Comcast is sending out a DHCPv6 reply with an MTU of 9192. I suspect this is not intentional, and I should not have to reconfigure my router to support such an MTU?
Please advise.
XfinityRaul
Official Employee
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1.5K Messages
28 days ago
Hello, @train_wreck. I appreciate you making us aware that you're experiencing this with your IPv6 connectivity. Can you please share with me what has been your experience with this MTU error? Has it helped to reconfigure your router?
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train_wreck
Visitor
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19 Messages
27 days ago
Hi support folks. So what's happening is a race condition between the ICMPv6 router advertisements and the DHCPv6 offer. The router at Comcast's end is sending RAs with an invalid MTU of 9192. When my DHCPv6 expires, or I reboot my router or modem, sometimes it will receive the invalid RA before it receives the DHCPv6 ACK. Once this happens, all IPv6 addresses are removed and my router (and everything behind it) loses IPv6 connectivity. This is what my logs show when this happens:
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: Router Advertisement from fe80::21c:73ff:fe00:99
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: advertised MTU 9192 is greater than link MTU 1500
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: adding route to fd00:0:101:21::/64
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: adding route to 2001:558:1032:2c::/64
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: adding route to fd00:0:d:2::/64
Aug 23 14:42:18 Gateway daemon.info dhcpcd[19138]: eth4: adding default route via fe80::21c:73ff:fe00:99
After several minutes, my router restarts the DHCPv6 process and regains connectivity:
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: eth4: ADV 2001:558:6016:44:----:----:----:----/128 from 2001:558:4013:44::10
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: eth4: REPLY6 received from 2001:558:4013:44::10
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: eth4: adding address 2001:558:6016:44:----:----:----:----/128
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: eth4: renew in 3499, rebind in 5029, expire in 6287 seconds
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: lo: adding reject route to 2601:---:---:---0::/60
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: eth4: delegated prefix 2601:---:---:---0::/60
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan20: adding address 2601:---:---:---0::1/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan21: adding address 2601:---:---:---7::1/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan30: adding address 2601:---:---:---1::1/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan40: adding address 2601:---:---:---2::1/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan20: adding route to 2601:---:---:---0::/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan21: adding route to 2601:---:---:---7::/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan30: adding route to 2601:---:---:---1::/64
Aug 23 14:43:00 Gateway daemon.info dhcpcd[19138]: vlan40: adding route to 2601:---:---:---2::/64
This has been happening for at least several months. I would request that NOC reconfigure the CMTS or head-end router with the default 1500 byte MTU. I am able to do that on my device, but the vast majority of customers would not know anything about the technical processes, and even if they did they would very likely be unable to change the MTU on their gateway or router, as many do not have a way to change this.
Hope this was helpful. Thanks.
(edited)
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