Jumbo MTU value on ASR9K

Answered Question
Mar 15th, 2010

Hi all,


was wondering if anyone could kindly point me to the right direction as to what is the maximum supportable MTU value on the ASR9010 for the following cards: A9K-4T-B & A9K-40GE-B?


Tx.

Correct Answer by Laurent Aubert about 6 years 11 months ago

Hi,


7600/6500: you can configure a MTU up to 9216 as the Hw supports frames size up to 9234 = 9216 + 14 (eth header) +4 (CRC).

4500/4900: you can configure a MTU up to 9198 as the Hw supports frames size up to 9216 = 9198 + 14 +4


Here are some links that could help as well:


http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c2


http://www.cisco.com/en/US/customer/products/hw/switches/ps663/products_tech_note09186a00801350c8.shtml


HTH


Laurent.

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Laurent Aubert Thu, 03/18/2010 - 17:55

Hi,


max MTU supported is 9216 Bytes. Be aware that in XR, MTU value take into account layer 2 header which is not the case with IOS. As an example on a ethernet interface:


XR MTU 1514 = IOS MTU 1500


HTH


Laurent.

frenzeus Thu, 03/18/2010 - 18:12

Hi Laurent,


Thanks for getting back.. 1 more question, since u've pointed out on the difference btw IOX & IOS on MTU.. on the ME4924 max supported is 9216 bytes as stated in the following link:

http://www.ciscorouters.biz/en/US/prod/collateral/switches/ps6568/ps7009/product_data_sheet0900aecd8052f36b.html

I understand in IOS that it does not include the link layer, therefore is the value of 9216 bytes already accounted for the 14 bytes of ethernet or its only IP mtu size which if include ethernet encap would effectively make it 9230 bytes? I have the same question regarding the 7600-S as well. But I would believe that in this case, ASR9K max ip MTU is 9202 (excluding 14bytes ethernet) while ME4924 & 7600-S max ip MTU is 9216 bytes.

Tx.

Correct Answer
Laurent Aubert Fri, 03/19/2010 - 06:29

Hi,


7600/6500: you can configure a MTU up to 9216 as the Hw supports frames size up to 9234 = 9216 + 14 (eth header) +4 (CRC).

4500/4900: you can configure a MTU up to 9198 as the Hw supports frames size up to 9216 = 9198 + 14 +4


Here are some links that could help as well:


http://www.cisco.com/en/US/customer/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c2


http://www.cisco.com/en/US/customer/products/hw/switches/ps663/products_tech_note09186a00801350c8.shtml


HTH


Laurent.

s.lachica Mon, 06/24/2013 - 20:18

But what if there is an OSPF peering between PE and CE, where MTU on ASR is 9234, and on the CE is 9216. Will this be an issue on MTU mismatch?


Thanks!

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