Default MTU size between CRS-1/IOS-XR and GSR/7600 is different

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Jun 14th, 2009
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Default Interface MTU size in CRS-1 is - 1514 & in GSR - 1500 bytes.


OSPF neighborship is coming up & working fine in the below scenarios,


· When both interfaces are left with the Default MTU settings (CRS tenGigE 0/0/2/0: 1514 bytes & GSR GigabitEthernet2/1/0: 1500 bytes)


· When CRS interface with 1570 bytes & GSR interface with 1556


OSPF is not working,

·When both interfaces are configured with equal MTU values


The same observation applies to ISIS also.


While configured with equal MTU (1556 bytes) in both side interfaces, we are getting the below errors


·CRS


·ospf[385]: Nbr 3.3.3.3 has larger interface MTU


·GSR


·OSPF: Nbr 30.30.30.30 has smaller interface MTU


The problem is resolved by using the "ip ospf mtu-ignore" command in interface configuration mode.


Summary:


Case -1

- ISIS & OSPF both down, when configured 1556 on both ends.


Case - 2


- When configured MTY 1556 in GSR & 1570 in CRS, both ISIS & OSPF.


Case -3


- When configured "mtu-ignore" OSPF is coming UP.


- ISIS session remain down.


One more important thing is...Both OSPF & ISIS coming up with default MTU settings ( 1500 on GSR & 1514 on CRS)


It may be the normal behavior, but what's the justification for this default large MTU on CRS-1/(IOS-XR), (apart from positioning the box in the backbone core, hence default larger MTU size).


Thanks!

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Laurent Aubert Sun, 06/14/2009 - 13:43
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  • Cisco Employee,

Hi


MTU definition on IOS-XR includes the L2 header which is not the case in the IOS.


That's why on an ethernet type connection, you will have a difference of 14 Bytes (18 if Dot1.Q is configured). It also explains why it works with default settings and why it doesn't if you have same value on both side.


HTH


Laurent.



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