Opened 6 years ago

Last modified 4 years ago

#11347 new enhancement

Baby Jumbo Frames (MTU 1508) on WAN port for WNDR3700

Reported by: ah@… Owned by: developers
Priority: normal Milestone: Barrier Breaker 14.07
Component: base system Version: Trunk
Keywords: Cc:


Does the mention of

#define AG71XX_TX_MTU_LEN 1540


imply that eth1 (wan) on a WNDR3700 should be able to support baby jumbo frames e.g. mtu 1508?

I know that gigabit style jumbo frames of mtu 9000 are not possible, but would an mtu 1508 be possible?

Currently, issuing

ifconfig eth1 mtu 1508

results in ifconfig: SIOCSIFMTU: Invalid argument

Attachments (0)

Change History (6)

comment:1 follow-up: Changed 6 years ago by anonymous


comment:2 in reply to: ↑ 1 Changed 6 years ago by ah@…

Replying to anonymous:


Indeed it isn't, which is why I've taken the time to ask if the issue I've spent time researching is a valid bug, which to me it appears to be.

If the hardware isn't capable of this for some reason I don't fully understand then please close the bug with a brief explanation.

comment:3 Changed 6 years ago by ah@…

During a discussion on CeroWRT-devel list, Robert Bradley kindly put together this patch which enables larger MTU values than 1500 for ag71xx. In conjunction with the latest pppd rp-pppoe plugin code, it is able to succesfully utilise RFC 4638 (PPPoE MTU of 1500 with appropriate hardware).

--- C:/Users/robert/AppData/Local/Temp/ag71x-revBASE.svn000.tmp.c Mon
May 28 03:55:59 2012
+++ C:/Users/robert/Desktop/ag71xx/ag71xx_main.c Thu Jun 21 13:58:44 2012
@@ -1042,13 +1042,25 @@


+ * Copied from eth_change_mtu and modified so that baby jumbo packets
+ * may be used. This has not been tested!
+ */
+int ag71xx_change_mtu(struct net_device *dev, int new_mtu)

+ if (new_mtu < 68
new_mtu > (ETH_DATA_LEN + 8))

+ return -EINVAL;
+ dev->mtu = new_mtu;
+ return 0;

static const struct net_device_ops ag71xx_netdev_ops = {

.ndo_open = ag71xx_open,
.ndo_stop = ag71xx_stop,
.ndo_start_xmit = ag71xx_hard_start_xmit,
.ndo_do_ioctl = ag71xx_do_ioctl,
.ndo_tx_timeout = ag71xx_tx_timeout,

  • .ndo_change_mtu = eth_change_mtu,

+ .ndo_change_mtu = ag71xx_change_mtu,

.ndo_set_mac_address = eth_mac_addr,
.ndo_validate_addr = eth_validate_addr,


comment:4 Changed 6 years ago by ah@…

Robert has now improved his patch and posted to the dev list

comment:5 Changed 5 years ago by anonymous

Is there any prospect of this getting merged?

comment:6 Changed 4 years ago by jow

  • Milestone changed from Attitude Adjustment 12.09 to Barrier Breaker 14.07

Milestone Attitude Adjustment 12.09 deleted

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