[j-nsp] juniper mpls mtu

Tue Dec 2 15:48:53 EST 2008

I have seen that but it does not make clear if the 12 byte are used by  
Juniper or if it describe my 3 possible label.

But I see that we maybe did the mistake by setting the IP MTU to 1500.

Sound that IP MTU has a higher priority to the MPLS calculation as the  
phys. interface MTU:
MPLS MTU = physical interface MTU - L2 encapsulation overhead - 12
if IP MTU is already set, so MPLS MTU = IP MTU + 20 bytes

Thanks,
Andreas

Am 02.12.2008 um 21:20 schrieb Ricardo Tavares:

> Hi!
>
> Old topic:
>
> http://puck.nether.net/pipermail/juniper-nsp/2007-June/008349.html
>
>
> On Tue, Dec 2, 2008 at 6:05 PM, Andreas Friedrich  
> <andreasf at colt.net> wrote:
> That sounds as we have interpreted it. But it does not seam to work  
> for us. We have following setup
> CPE - PE (E-Series) - P (Cisco CRS-1)
> We send from the CPE ipv4 ping with 1500 byte DF set
> We get at the PE 2 labels on top. Only difference for E-Series is 18  
> bytes for Ethernet.
> PE int to P config:
> mtu 1534
> ip mtu 1500
>
> Calculation: 1534 - Ethernet header 18 byte - 4 byte .1q - 12 byte  
> labels = 1500
>
> We use only 2 labels.
>
> If we configure mtu 1542 it works. If I calculate back I get
>
> 1500    IP packet
> 4               .1q
> 8               2 MPLS labels
> 18              Ethernet
> ------
> 1530 byte used
>
> So I need to configure 12 byte more. And that sounds for the 12 byte  
> (3 labels).
>
> What do you think?
>
> Thanks,
> Andreas
>
>
> Am 02.12.2008 um 20:47 schrieb Cyn D.:
>
> 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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> Andreas Friedrich
> Network Engineer
>
> COLT Telecom GmbH
> Herriotstr. 4
> 60528 Frankfurt
>
>
> Tel: + 49 (0)69 56606 6005    Internal: 8 491 6005
> Fax: + 49 (0)69 56606 6350
> Email: andreasf at colt.net
>
>
>
> www.colt.net
>
>
>
>
>
> Data | Voice | Managed Services
>
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>
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>
> _______________________________________________
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>
> Andreas Friedrich
> Network Engineer
>
> COLT Telecom GmbH
> Herriotstr. 4
> 60528 Frankfurt
>
>
> Tel: + 49 (0)69 56606 6005    Internal: 8 491 6005
> Fax: + 49 (0)69 56606 6350
> Email: andreasf at colt.net
>
>
>
> www.colt.net
>
>
>
>
>
> Data | Voice | Managed Services
>
> Schütze Deine Umwelt | Erst denken, dann drucken
>
> *****************************************
> COLT Telecom GmbH, Herriotstraße 4, 60528 Frankfurt/Main,  
> Deutschland • Tel +49 (0)69 56606 0 • Fax +49 (0)69 56606 2222 •
>
> Geschäftsführer: Albertus Marinus Oosterom (Vors.), Rita Thies •  
> Amtsgericht Frankfurt/Main HRB 46123 • USt.-IdNr. DE 197 498 400
>
>
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Andreas Friedrich
Network Engineer

COLT Telecom GmbH
Herriotstr. 4
60528 Frankfurt

Tel: + 49 (0)69 56606 6005    Internal: 8 491 6005
Fax: + 49 (0)69 56606 6350
Email: andreasf at colt.net

www.colt.net

Data | Voice | Managed Services

Schütze Deine Umwelt | Erst denken, dann drucken

*****************************************
COLT Telecom GmbH, Herriotstraße 4, 60528 Frankfurt/Main, Deutschland  
• Tel +49 (0)69 56606 0 • Fax +49 (0)69 56606 2222 •

Geschäftsführer: Albertus Marinus Oosterom (Vors.), Rita Thies •  
Amtsgericht Frankfurt/Main HRB 46123 • USt.-IdNr. DE 197 498 400