[j-nsp] juniper mpls mtu

Cyn D. cynthia_dal at yahoo.ca
Tue Dec 2 14:47:04 EST 2008


Juniper default physical layer MTU 1514, family inet (IP) MTU 1500, family 
MPLS 1488 (=1514-14-3X4) counted most tags possible (3).

To pass big IP packet (1460+20+20), make MPLS MTU 1500, therefore interface 
physical MTU is set to 1500+3x4+14=1526. this also makes family inet 
mtu=1526-14=1512.



C.

----- Original Message ----- 
From: "Andreas Friedrich" <andreasf at colt.net>
To: "Phil Bedard" <philxor at gmail.com>
Cc: <juniper-nsp at puck.nether.net>
Sent: Tuesday, December 02, 2008 2:04 PM
Subject: Re: [j-nsp] juniper mpls mtu


One more question regarding this MPLS MTU.
Are the 12 byte (3 labels) reserved by Juniper for there purpose and I
have to calculate my labels additional? So is MPLS MTU  than 1512 if I
want to use also 3 labels?

Thanks,
Andreas

Am 04.06.2008 um 01:54 schrieb Phil Bedard:

> I think with most of the forwarding engines, the maximum number of  labels 
> that can be pushed at one time is 3, hence the 12 bytes.
>
> Phil
>
> On Jun 3, 2008, at 9:15 AM, cp wrote:
>
>> I hoping someone can provide insight into why by default juniper
>> calculates mpls mtu at 12 bytes less than the ip mtu?  I've been  testing
>> using l3vpn and it seems that mpls mtu pads 8 bytes to its mpls mtu
>> reducing the ip mtu the inside packet by 8 bytes. Quick example. So  if
>> mpls mtu is 1508 the ip mtu of the packet inside is 1500. That  means the
>> ip mtu of the mpls interface is 1520. The only reason I can see
>> additional bytes needed on top of the l3vpn two is for fast reroute
>> bypass.  I assume it's for the safety factor although I am probably
>> missing something. Any information is appreciated.
>>
>>
>>
>> -Chip
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> _______________________________________________
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>
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Andreas Friedrich
Network Engineer

COLT Telecom GmbH
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