Mailing List Archive

ATM/L2TP/PPP MTU question
Here is the network diagram:

DSL ATM ATM ATM ETH
ADSL CPE ----- DSLAM ----- ATM SWITCH ----- BBRAS ----- LNS ----- INTERNET
|-------------- PPPoE/A------------------||----L2TP---|
|----------------------- PPP -------------------------|

What is the recommended MTU setting for the ip/atm connection between BBRAS/LNS (default
for ATM is 4470), if we have to :

1. Do not change anything on user's side
2. Do not load BBRAS/LNS with extra fragmentation/reassembly

Should i make it 1540 (1500 + 40 bytes of IP, UDP and L2TP headers) ?

--
***********************************
Chatzithomaoglou Anastasios
Network Design & Operations Center
FORTHnet S.A.
<achatz@forthnet.gr>
***********************************
Re: ATM/L2TP/PPP MTU question [ In reply to ]
Tassos Chatzithomaoglou [achatz@forthnet.gr] wrote:
>
> Here is the network diagram:
>
> DSL ATM ATM ATM ETH
> ADSL CPE ----- DSLAM ----- ATM SWITCH ----- BBRAS ----- LNS ----- INTERNET
> |-------------- PPPoE/A------------------||----L2TP---|
> |----------------------- PPP -------------------------|
>
> What is the recommended MTU setting for the ip/atm connection between
> BBRAS/LNS (default for ATM is 4470), if we have to :
>
> 1. Do not change anything on user's side
> 2. Do not load BBRAS/LNS with extra fragmentation/reassembly
>
> Should i make it 1540 (1500 + 40 bytes of IP, UDP and L2TP headers) ?

Yes, 1540 should do it if you are going Cisco to Cisco. If you are
going inter-vendor, it depends on the L2TP header options the other
vendor may use (eg. data sequencing would add additional
overhead). You may want to increase to 1550 to be safe.

Dennis

> --
> ***********************************
> Chatzithomaoglou Anastasios
> Network Design & Operations Center
> FORTHnet S.A.
> <achatz@forthnet.gr>
> ***********************************
>
> _______________________________________________
> cisco-nas mailing list
> cisco-nas@puck.nether.net
> http://puck.nether.net/mailman/listinfo/cisco-nas

--
-------------------------------------------------------------------------
|| || Dennis Peng
|| || Cisco Systems, Inc. Escalation Engineer
|||| |||| 170 West Tasman Drive Phone: (408) 526-6143
..:||||||:..:||||||:.. San Jose, CA 95134 Fax: (408) 232-2343
Cisco Systems Inc. dpeng@cisco.com
-------------------------------------------------------------------------
Re: ATM/L2TP/PPP MTU question [ In reply to ]
If i leave it on the default (4470 from our side, 8000+ from telco's side) will there be
any problem?

What are the possibilites of such a HUGE packet? Are there any disadvantages?

Dennis Peng wrote:
> Tassos Chatzithomaoglou [achatz@forthnet.gr] wrote:
>
>>Here is the network diagram:
>>
>> DSL ATM ATM ATM ETH
>>ADSL CPE ----- DSLAM ----- ATM SWITCH ----- BBRAS ----- LNS ----- INTERNET
>> |-------------- PPPoE/A------------------||----L2TP---|
>> |----------------------- PPP -------------------------|
>>
>>What is the recommended MTU setting for the ip/atm connection between
>>BBRAS/LNS (default for ATM is 4470), if we have to :
>>
>>1. Do not change anything on user's side
>>2. Do not load BBRAS/LNS with extra fragmentation/reassembly
>>
>>Should i make it 1540 (1500 + 40 bytes of IP, UDP and L2TP headers) ?
>
>
> Yes, 1540 should do it if you are going Cisco to Cisco. If you are
> going inter-vendor, it depends on the L2TP header options the other
> vendor may use (eg. data sequencing would add additional
> overhead). You may want to increase to 1550 to be safe.
>
> Dennis
>
>
>>--
>>***********************************
>> Chatzithomaoglou Anastasios
>>Network Design & Operations Center
>> FORTHnet S.A.
>> <achatz@forthnet.gr>
>>***********************************
>>
>>_______________________________________________
>>cisco-nas mailing list
>>cisco-nas@puck.nether.net
>>http://puck.nether.net/mailman/listinfo/cisco-nas
>
>

--
***********************************
Chatzithomaoglou Anastasios
Network Design & Operations Center
FORTHnet S.A.
<achatz@forthnet.gr>
***********************************
Re: ATM/L2TP/PPP MTU question [ In reply to ]
Anastassios Chatzithomaoglou [achatz@forthnet.gr] wrote:
> If i leave it on the default (4470 from our side, 8000+ from telco's side)
> will there be any problem?

Nothing immediately comes to mind.

> What are the possibilites of such a HUGE packet? Are there any
> disadvantages?

If it is only L2TP taffic between the BBRAS and the LNS, you shouldn't
see any packets larger than 1500 + the overhead of L2TP.

Dennis

> Dennis Peng wrote:
> >Tassos Chatzithomaoglou [achatz@forthnet.gr] wrote:
> >
> >>Here is the network diagram:
> >>
> >> DSL ATM ATM ATM ETH
> >>ADSL CPE ----- DSLAM ----- ATM SWITCH ----- BBRAS ----- LNS ----- INTERNET
> >> |-------------- PPPoE/A------------------||----L2TP---|
> >> |----------------------- PPP -------------------------|
> >>
> >>What is the recommended MTU setting for the ip/atm connection between
> >>BBRAS/LNS (default for ATM is 4470), if we have to :
> >>
> >>1. Do not change anything on user's side
> >>2. Do not load BBRAS/LNS with extra fragmentation/reassembly
> >>
> >>Should i make it 1540 (1500 + 40 bytes of IP, UDP and L2TP headers) ?
> >
> >
> >Yes, 1540 should do it if you are going Cisco to Cisco. If you are
> >going inter-vendor, it depends on the L2TP header options the other
> >vendor may use (eg. data sequencing would add additional
> >overhead). You may want to increase to 1550 to be safe.
> >
> >Dennis
> >
> >
> >>--
> >>***********************************
> >> Chatzithomaoglou Anastasios
> >>Network Design & Operations Center
> >> FORTHnet S.A.
> >> <achatz@forthnet.gr>
> >>***********************************
> >>
> >>_______________________________________________
> >>cisco-nas mailing list
> >>cisco-nas@puck.nether.net
> >>http://puck.nether.net/mailman/listinfo/cisco-nas
> >
> >
>
> --
> ***********************************
> Chatzithomaoglou Anastasios
> Network Design & Operations Center
> FORTHnet S.A.
> <achatz@forthnet.gr>
> ***********************************

--
-------------------------------------------------------------------------
|| || Dennis Peng
|| || Cisco Systems, Inc. Escalation Engineer
|||| |||| 170 West Tasman Drive Phone: (408) 526-6143
..:||||||:..:||||||:.. San Jose, CA 95134 Fax: (408) 232-2343
Cisco Systems Inc. dpeng@cisco.com
-------------------------------------------------------------------------