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FAQ-Why ME60 PPPoE subscriber MTU cannot be bigger than 1492 and how to be bigger

Publication Date:  2012-07-27 Views:  131 Downloads:  0
Issue Description
Q: ME60 version V6R2C06SPC100, why PPPoE online user MTU cannot be bigger than 1492, even change MTU and PPP MRU both bigger than 1492 in the virtual-Template?
For example, configuratino as below on ME60 cannot make the access-user MTU bigger than 1492.

[RAS01-HMAQ49-Virtual-Template1]disp this
interface Virtual-Template1
 ppp authentication-mode auto
 ppp mru 1503
 pppoe-server ac-name RAS01-HMAQ49
 mtu 1620

Alarm Information

Handling Process
According to RFC 4638 about PPPoE MTU,  if there is no PPP-Max-payload Tag in the PPPoE packets, then MRU cannot be bigger than 1492.
"If a PPPoE client wants to use an MTU/MRU higher than 1492 octets, then it MUST include an optional PPP-Max-Payload Tag in the PADI and PADR packets.  If the PPPoE server can support an MTU/MRU higher than 1492 octets, it MUST respond with an echo of the clients tag in the PADO and PADS packets when the PPP-Max-Payload tag is received from the client." This PPPoE tag type is ox0120.

For ME60, version V6R2 support this tag, its rule is as below.

If CPE does not send PPP LCP MRU actively, final online subscriber MTU will be the minimum(1492, VT MTU, Interface MTU-8).
If CPE sends PPP LCP MRU actively,
1.  When a "PPP-Max-Payload" tag is confirmed in PPPoE Discovery phase,
MTU will use the minimum(PPP-Max-Payload-Tag, CPE MRU, VT MTU, Interface MTU-8).
2.  When a "PPP-Max-Payload" tag is not received in PPPoE Discovery phase,
MTU will use the minimum (CPE MRU, VT MTU, Interface MTU-8).

Why need Interface MTU minus 8bytes? Because PPPoE link, PPP MTU need to exclude PPPoE header 6 bytes and PPP protocol type ID 2bytes.
So according to the rules above, if the CPE does not send MRU actively, then it have to send PPP-max-payload in PADI and PADR, otherwize, no matter ME60 MTU change to any big value,  online subscriber MTU can only reach 1492.
For example, if CPE send MRU 1600 actively, and ME60 VT MTU is 1500, then online user MTU will be 1500, if ME60 VT MTU is 1620, then online user MTU will be 1600.

Root Cause