Comcast download speed fix for Linksys eSeries

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[Culled from DD-WRT Forum thread Critical DSCP bug Affecting WiFi Download Speeds on Comcast]

There is a critical bug that is likely crippling thousands of routers and pretty much only Comcast has this issue. It is an edge case interaction caused by bad DSCP information coming from Comcast and poor condition handling by the WMM driver in certain Linksys/Cisco eSeries routers (and possibly others as well). This issue can appear on both stock and modded routers. Comcast's network is misconfigured and has been for years and has likely caused many people to needlessly replace fully functional routers. IPv6 works because DSCP was configured correctly for that but it is incorrect for IPv4. Since the only packets with this bad information are the ones that are being downloaded, upload is not affected nearly as badly.

Cisco appears to have at some point released new WMM drivers in stock firmware that largely resolve the issue on current stock firmware.


Download speeds are normal with current stock firmware, and on wired connections with DD-WRT firmware, but are very low (less than 1 Mbps) on wireless connections with DD-WRT firmware.


To see if you are getting the bad DSCP, take a look at your downloaded IPv4 packets with Wireshark, preferably directly connected to WAN, but you should see them even on Wi-Fi. (See Add DSCP Column in Wireshark)

If you see the problem, open a Telnet or SSH connection to DD-WRT, and run the following commands:

insmod xt_DSCP.ko
iptables -t mangle -A PREROUTING -i vlan2 -j DSCP --set-dscp 0

Assuming there are no errors, check download speed on wireless.

You may need to change vlan2 depending on your particular setup.


To save the fix so it survives reboots, first navigate to Administration > Commands

In Command Shell, enter Commands:

insmod xt_DSCP.ko

and click Save Startup.

In Command Shell, enter Commands:

iptables -t mangle -A PREROUTING -i vlan2 -j DSCP --set-dscp 0

and click Save Firewall.


This fix corrects the bad DSCP header information as soon as a packet hits the WAN interface on a router.

DSCP as received from Comcast's network is 0x08. This is the lowest priority possible, which WMM interprets as being unimportant to transmit quickly.

The fix changes DSCP on all packets to 0x00, which essentially means unclassified, which WMM handles properly. This is how virtually every other ISP uses DSCP and is why the issues is specific to Comcast.