The MPLS-OPS Archive

[Roger Clark Williams <rogerw@nordlink.com>]

Tarun, your issue is one of the classic trip-wire situations for MPLS.
Since an MPLS/Ethernet frame is bigger than a normal Ethernet frame
without labels, that bigger frame can be seen by some systems as an
error, and the frame either dropped or fragmented, just as you say.

You may have to specifically set the MTU size for both the routers and
switches. Though the routers may be MPLS-aware, the LAN interfaces, ie.
Ethernet, don't automatically deal with jumbo frames. An MPLS frame would
be the 1500 bytes of Ethernet plus the label(s) at 4 bytes each. You have
to tell the routers what to do. Router WAN interfaces work differently.
It is my understanding that the new MPLS Ethertypes carried in WAN
headers inform the interfaces that a jumbo packet may be following, and
the WAN interfaces on MPLS-aware routers will then accept it. Many
non-MPLS-aware switches will discard these jumbo frames unless you tell
them explicitly to allow them. 

Look over this set of router commands....

interface fastethernet 0/0

mpls ip

mpls mtu 1512

and this switch command....

set port 1/3 jumbo enable

as possible candidates that may help.

Roger Williams




> Resent-Date: Fri, 21 Mar 2003
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> From: "Tarun Bhalla" <tarun.bhalla@wipro.com>
> To: <mpls-ops@mplsrc.com>
> Date: Fri, 21 Mar 2003 10:11:49 -0800
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> Subject: [MPLS-OPS]: MTU problems in EoMpls domain.. 
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>
> hi all,
>  
> I am new to this mailing list. so hi to all..
> now my problems :(( 
> Actually, I am trying to handle the problem of
> fragmentation in 
> Ethernet over MPLS network.
>  
> This is the existing network
> topology...
>  
> LAN A -------> MPLS Domain
> ----------> LAN B
>  
> To implement EoMPLS, I am capturing the packet in Ingress of the MPLS domain
> and trying to send this as a payload over the MPLS domain.. When I 
> am doing this the problem is that the frames which are 1500 bytes (normal
> ethernet frames) can not be sent across this MPLS domain as the payld
> of this domain can only be 1496 bytes... (4 bytes label gets added and leave
> the matter when multiple labels are added).. So, I am stuck as i am not
> able to send 1500 bytes frames.. 
>  
> These all things are being carried on test setups. So, I can not
> have the liberty or leisure to use SONET interfaces as my core n/w 
> interfaces.. also am not sure what to do in order to make use of jumbo
> ethernet frames... So, the only option left is to fragment the frames when i 
> capture the ethernet frames.. but for that i need ur advice.. 
>  
> I mean is there any standard methodology that says how to handle 
> fragmentation in ethernet layer cos I will have to anyway go to this 
> final resort.. or is there any other way arnd. ?? 
>  
> Thanks & Regards
> Tarun B
> _________________________________________________
> Tarun Bhalla
> Wipro Technologies
> 26 Hosur Main Road, 
> Bommanahalli,
> Bangalore- 560 068. India
> Tel +91-80-5732293, Extn 3139 <http://www.wipro.com/> 
>  
>  

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