Ubuntu: How can I get the NIC to respond to jumbo packets when MTU is set to 4000



Question:

I have a mythbuntu 14.04 server.

# uname -a  Linux KISE-055 3.16.0-55-generic #74~14.04.1-Ubuntu SMP Tue Nov 17 10:15:59 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux  

I have the MTU set to 4000 vice 1500 to get jumbo packets...

# ip link  ...  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 4000 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000  link/ether 90:fb:a6:eb:d7:f3 brd ff:ff:ff:ff:ff:ff  

I set the MTU in /etc/network/interfaces

# cat /etc/network/interfaces  auto lo  iface lo inet loopback  #primary wired interface  auto eth0  iface eth0 inet dhcp  post-up /sbin/ifconfig eth0 mtu 4000  

Which can be seen in the output of 'ifconfig'...

~$ ifconfig  eth0      Link encap:Ethernet  HWaddr 90:fb:a6:eb:d7:f3        inet addr:192.168.0.55  Bcast:192.168.0.255  Mask:255.255.255.0        inet6 addr: fe80::92fb:a6ff:feeb:d7f3/64 Scope:Link    ⇛ ⇛ UP BROADCAST RUNNING MULTICAST  MTU:4000  Metric:1  ⇚ ⇚        RX packets:43654 errors:0 dropped:0 overruns:0 frame:0        TX packets:41807 errors:0 dropped:0 overruns:0 carrier:0        collisions:0 txqueuelen:1000        RX bytes:29877609 (29.8 MB)  TX bytes:21276131 (21.2 MB)    lo        Link encap:Local Loopback        inet addr:127.0.0.1  Mask:255.0.0.0        inet6 addr: ::1/128 Scope:Host        UP LOOPBACK RUNNING  MTU:65536  Metric:1        RX packets:2010 errors:0 dropped:0 overruns:0 frame:0        TX packets:2010 errors:0 dropped:0 overruns:0 carrier:0        collisions:0 txqueuelen:0        RX bytes:279223 (279.2 KB)  TX bytes:279223 (279.2 KB)  

But the system does not respond to jumbo packets from other computers on the LAN. So all it's clients must use an MTU of 1500 to talk to the server.

Here's the nic...

02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 03)      Subsystem: Foxconn International, Inc. Device 0e22      Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx+      Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-      Latency: 0, Cache Line Size: 64 bytes      Interrupt: pin A routed to IRQ 47      Region 0: I/O ports at e800 [size=256]      Region 2: Memory at fafff000 (64-bit, prefetchable) [size=4K]      Region 4: Memory at faff8000 (64-bit, prefetchable) [size=16K]      Expansion ROM at fdfe0000 [disabled] [size=128K]      Capabilities: [40] Power Management version 3              Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA PME(D0+,D1+,D2+,D3hot+,D3cold+)              Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-      Capabilities: [50] MSI: Enable+ Count=1/1 Maskable- 64bit+              Address: 00000000fee0200c  Data: 4142      Capabilities: [70] Express (v2) Endpoint, MSI 01              DevCap: MaxPayload 256 bytes, PhantFunc 0, Latency L0s <512ns, L1 <64us                      ExtTag- AttnBtn- AttnInd- PwrInd- RBE+ FLReset-              DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-                      RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop-                      MaxPayload 128 bytes, MaxReadReq 512 bytes              DevSta: CorrErr+ UncorrErr- FatalErr- UnsuppReq+ AuxPwr+ TransPend-              LnkCap: Port #0, Speed 2.5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s <512ns, L1 <64us                      ClockPM+ Surprise- LLActRep- BwNot-              LnkCtl: ASPM Disabled; RCB 64 bytes Disabled- CommClk+                      ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-              LnkSta: Speed 2.5GT/s, Width x1, TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-              DevCap2: Completion Timeout: Not Supported, TimeoutDis+, LTR-, OBFF Not Supported              DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-, LTR-, OBFF Disabled              LnkCtl2: Target Link Speed: 2.5GT/s, EnterCompliance- SpeedDis-                       Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS-                       Compliance De-emphasis: -6dB              LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete-, EqualizationPhase1-                       EqualizationPhase2-, EqualizationPhase3-, LinkEqualizationRequest-      Capabilities: [ac] MSI-X: Enable- Count=4 Masked-              Vector table: BAR=4 offset=00000000              PBA: BAR=4 offset=00000800      Capabilities: [cc] Vital Product Data              Unknown small resource type 00, will not decode more.      Capabilities: [100 v1] Advanced Error Reporting              UESta:  DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-              UEMsk:  DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-              UESvrt: DLP+ SDES+ TLP- FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-              CESta:  RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr+              CEMsk:  RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr+              AERCap: First Error Pointer: 00, GenCap+ CGenEn- ChkCap+ ChkEn-      Capabilities: [140 v1] Virtual Channel              Caps:   LPEVC=0 RefClk=100ns PATEntryBits=1              Arb:    Fixed- WRR32- WRR64- WRR128-              Ctrl:   ArbSelect=Fixed              Status: InProgress-              VC0:    Caps:   PATOffset=00 MaxTimeSlots=1 RejSnoopTrans-                      Arb:    Fixed- WRR32- WRR64- WRR128- TWRR128- WRR256-                      Ctrl:   Enable+ ID=0 ArbSelect=Fixed TC/VC=ff                      Status: NegoPending- InProgress-      Capabilities: [160 v1] Device Serial Number 16-00-00-00-68-4c-e0-00      Kernel driver in use: r8169  


Solution:1

I have a question about your environment, What kind of switch do you have? and is the switch configured to Jumbo Frames.


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