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 an edge case interaction between Comcast IPv4 DSCP and improper condition handling in the WMM driver in certain Linksys/Cisco eSeries routers (and possibly others as well) which results in very low download speeds. This issue can appear in both stock and third-party (DD-WRT) firmware, although Cisco appears to have at some point released new WMM drivers that largely resolve the issue in current stock firmware.

Unlike other service providers, IPv4 packets from the Comcast network are configured with DSCP 8, Priority higher than Default/Routine (0).


Download speeds are normal with current stock firmware, and on wired connections with DD-WRT firmware, but are much lower (possibly even less than 1 Mbps) on wireless connections with DD-WRT firmware even with strong signal.

To see if you are getting the problematic DSCP, 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)


To test this fix, open a Telnet or SSH connection to DD-WRT, and run the following commands (copy and paste to avoid typos):

insmod xt_DSCP.ko
iptables -t mangle -A PREROUTING -i `nvram get wan_ifname` -j DSCP --set-dscp 0

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


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

In Command Shell, enter Commands (copy and paste to avoid typos):

insmod xt_DSCP.ko

and click Save Startup.

In Command Shell, enter Commands (copy and paste to avoid typos):

iptables -t mangle -A PREROUTING -i `nvram get wan_ifname` -j DSCP --set-dscp 0

and click Save Firewall.


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

DSCP as received from Comcast's network is 8. This is actually higher than Default priority, but the WMM driver interprets it incorrectly.

The fix changes DSCP on all packets to 0 (Default priority), which the WMM driver handles properly. (This is how other ISPs set DSCP and is why the issue is specific to Comcast.)