How we can send packets over 1500 bytes in a bridge

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Jun 11th, 2007
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We are deploying a bridging escenario using 2 routers 3845 linked by a bundle of 6 e1 under PPP Multilink...when we want to pass ethernet frames over 1500 bytes they don't,when we test without bridging the frames pass without problems. Also we want to know if there is a IOS for 3845 to apply jumbo frames??

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sundar.palaniappan Mon, 06/11/2007 - 09:44
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Unfortunately, there's no way to get around the physical medium limitation of 1500 bytes. I don't think PPP links can handle packets that are bigger than 1500 bytes.


If the application is using DF bit then you may have to look for other ways to make the application send packets that are smaller than 1500 bytes or remove the DF bit itself.


HTH


Sundar

paolo bevilacqua Mon, 06/11/2007 - 10:12
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Hmm, no. Actually all serial interfaces can support bigger than 1500, consider for example that a requirement for frame-relay conformance is a minimum of 1600. Just change the MTU on the interface.

Same for the gigabits on board, no problem with jumbo frames.


How this helps, please rate post if it does!


sundar.palaniappan Mon, 06/11/2007 - 10:21
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Interesting!!


Even if the transport network can handle packets bigger than 1500 bytes how do you set the MTU on the LAN interface?


R5(config)#int f0/0

R5(config-if)#mtu ?

<64-17940> MTU size in bytes


R5(config-if)#mtu 1600

% Interface FastEthernet0/0 does not support user settable mtu.



HTH


Sundar

paolo bevilacqua Mon, 06/11/2007 - 10:26
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Hi Sundar,


the thing is that not all LAN hardware is made equal. The one used by 38x5 (broadcom) supports jumbo frames:

BA-CCME#sh int g0/0

GigabitEthernet0/0 is up, line protocol is up

Hardware is BCM1125 Internal MAC, address is 001b.54d6.f7c0 (bia 001b.54d6.f7c0)

Description:

Internet address is 10.110.0.1/24

MTU 16351 bytes, BW 1000000 Kbit, DLY 10 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive not set


sundar.palaniappan Mon, 06/11/2007 - 10:52
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Hi Paolo,


It appears gig interfaces can support jumbo frames. Yes, if the entire medium end-to-end can support jumbo frames the router can be configured accordingly. However, I personally haven't seen any Telco networks support jumbo frames on p-t-p link.


HTH


Sundar

paolo bevilacqua Mon, 06/11/2007 - 10:57
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The MTU of the Internet is by definition, 1500 and often, unfortunately, even smaller.


Then, what an enterprise customer does on his links is limited only by the equipment used. So if one can use of jumbo frames, I think it's a good thing.


datos_gye Mon, 06/11/2007 - 12:03
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Hi Sundar,

We had tried it change the mtu size, but the result it was the same, the packet over 1500 bytes don't pass, i think if you know other way i can set some feature in the bridge setting in order to change the mtu size...

paolo bevilacqua Mon, 06/11/2007 - 13:13
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Have you changed MTU on both physical, multilink (or dialer) and LAN interfaces ?

After you do that, can you ping bigger than 1500 using extended ping and DF set ?

Can you rule out it's a bridging issue using a regualr routing setup instead ?

sundar.palaniappan Mon, 06/11/2007 - 13:16
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I am not positive whether it would work but try configuring the command 'ip tcp adjust-mss 1460' under the LAN interface. I know this command would work if it's a routed interface but this being a bridged interface I am not so sure. BTW, this would only help if it's a TCP application.


HTH


Sundar

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