Router - Cisco IPv6 IOS Versions

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TCP window scaling Bug

Taken from: http://aa.net.uk/kb-broadband-ipv6-tech.html

Some customers running native IPv6 on a Cisco 877 router have seen a huge disparity between IPv4 and IPv6 speeds. IPv6 file transfers are about 10 times slower than IPv4 transfers!

It turns out to be a Cisco IOS bug; once the IPv6 stateful firewall (inspect) has been disabled, IPv6 should run at full speed.

It is Cisco bug CSCtb10776 (Inspection drops NIC-segmented packets when WScale is on). It can also be worked around by disabling TCP window scaling on the affected hosts, but that can have performance implications of its own. It has been fixed in IOS versions:

  • 15.1(0.18)T
  • 12.4(25b)M0.13
  • 15.0(1)M1.2
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