PPP-Max-Payload and IWF PPPoE Tag Support

Per the RFC, Accommodating an MTU/MRU Greater than 1492 in PPPoE, PPPoE peers can negotiate only MRUs with a maximum of 1492 octets so that the PPPoE header and PPP protocol ID can be inserted in the PPPoE session data packet. The maximum for an Ethernet payload is 1500 octets.

RFC 2516 defines a new tag to allow PPPoE peers to negotiate PPP MRU greater than 1492 if the underlying networks can support an Ethernet payload of greater than 1500 bytes. To enable processing of this new tag, a command has been defined in the Cisco IOS command-line interface as tag ppp-max-payload.The PPP-Max-Payload and IWF PPPoE Tag Support feature enhances the PPPoE component so the tag ppp-max-payloadcommand can process the new tag to influence the Link Control Protocol (LCP) MRU negotiations for the PPP session based on the MRU value specified in the tag from the PPPoE client.

The DSL Forum defined IWF to define the process for conversion of PPP over ATM (PPPoA) sessions to PPPoE sessions at the digital subscriber line access multiplexer (DSLAM) to the BRAS. This functionality was defined to help the migration of DSLAM networks from ATM to Ethernet media. So, essentially, the PPPoA session comes in to the DSLAM over ATM and is converted to a PPPoE session at the DSLAM, which is then connected to the BRAS as a PPPoE session. Each PPPoA session is mapped to a corresponding PPPoE session.

Typically, the BRAS is configured to limit PPPoE sessions originating from the same MAC address to protect itself from a DOS attack. This presents a problem for IWF PPPoE sessions because all PPPoE sessions originate from the same MAC address DSLAM. To overcome this issue, the IWF PPPoE tag is inserted at the DSLAM and read by the BRAS to distinguish the IWF PPPoE session from the regular PPPoE session during the PPPoE discovery frames.

For more information about this subject, refer to the DSL Forum Technical Report 101: Migration to Ethernet-Based DSL Aggregation.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    bba-group pppoe group-name

4.    virtual-template template-number

5.    tag ppp-max-payload [minimum value maximum value] [deny]

6.    sessions per-mac limit per-mac-limit

7.    interface ethernet slot / port

8.    pppoe enable group group-name

9.    virtual-template template-number

10.    ppp lcp echo mru verify [minimum value]

11.    end

12.    show pppoe session [all| packets]

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	bba-group pppoe group-name Example: Router(config)# bba-group pppoe pppoe-group	Enters BBA group configuration mode and defines a PPPoE profile.
Step 4	virtual-template template-number Example: Router(config-bba-group)# virtual-template 1	Configures a PPPoE profile with a virtual template to be used for cloning virtual access interfaces. The template-number argument is an identifying number of the virtual template that will be used to clone virtual-access interfaces.
Step 5	tag ppp-max-payload [minimum value maximum value] [deny] Example: Router(config-bba-group)# tag ppp-max-payload minimum 1200 maximum 3000	Specifies a range for the ppp-max payload tag value that will be accepted by the BRAS. Default values are 1492 for the minimum and 1500 for the maximum. The ppp-max-payload tag value accepted from the client cannot exceed the physical interface value for MTU minus 8.
Step 6	sessions per-mac limit per-mac-limit Example: Router(config-bba-group)# sessions per-mac iwf limit 200	Specifies a limit for IWF-specific sessions per MAC address (separate from session limits that are not IWF-specific). If this command is not entered, the normal MAC-address session limit is applied to IWF sessions. The per-mac-limit argument specifies the allowable number of IWF sessions. The default is 100.
Step 7	interface ethernet slot / port Example: Router(config-bba-group)# interface ethernet 1/0	Enters interface configuration mode for an Ethernet interface: The slot / port arguments identify the slot number and the port number to which this configuration applies. The slash mark is required.
Step 8	pppoe enable group group-name Example: Router(config-if)# pppoe enable group 1	Enables PPPoE sessions on an Ethernet interface or subinterface.
Step 9	virtual-template template-number Example: Router(config-if)# virtual-template 1	Configures a PPPoE profile with a virtual template to be used for cloning virtual access interfaces. The template-number argument is an identifying number of the virtual template that will be used to clone virtual-access interfaces.
Step 10	ppp lcp echo mru verify [minimum value] Example: Router(config-if)# ppp lcp echo mru verify minimum 1304	Verifies the negotiated MRU and adjusts the PPP virtual access interface MTU for troubleshooting purposes. If the optional minimum keyword is entered, the value can be from 64 to 1500. If the verification of minimum MTU succeeds, the PPP connection's interface MTU is set to that value. This reset is useful when you troubleshoot and need to adjust the sessions according to underlying physical network capability. After this command is configured, IP Control Protocol (IPCP) is delayed until verification of the MTU is completed at the LCP.
Step 11	end Example: Router(config-if)# end	Exits the current configuration mode and returns to privileged EXEC mode.
Step 12	show pppoe session [all| packets] Example: Router# show pppoe session all	Verifies the configuration and displays session information. all --Displays output indicating if a session is IWF-specific or if the PPP-Max-Payload tag is in the discovery frame and accepted. packets --Displays packet statistics for the PPPoE session.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    bba-group pppoe group-name

4.    tag ppp-max-payload deny

5.    end

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters interface configuration mode.
Step 3	bba-group pppoe group-name Example: Router(config-if)# bba-group pppoe pppoe-group	Enters BBA group configuration mode and defines a PPPoE profile.
Step 4	tag ppp-max-payload deny Example: Router(config-bba-group)# tag ppp-max-payload deny	Disables the processing of the ppp-max-payload tag value higher than the default of 1492 bytes.
Step 5	end Example: Router(config-bba-group)# end	Exits the BBA group configuration mode and returns to privileged EXEC mode.

The following configuration example shows the PPP-Max-Payload and IWF PPPoE Tag Support enabled to accept PPP-Max-Payload tag values from 1492 to 1892, limits the number of sessions per MAC address to 2000 when the IWF is present, and verifies that the PPP session can accept 1500-byte packets in both directions:

bba-group pppoe global
 virtual-template 1
 tag ppp-max-payload minimum 1492 maximum 1892
 sessions per-mac limit 1
 sessions per-mac iwf limit 2000
!
 interface Virtual-Template 1
!

The following configuration example disables the effect of the tag ppp-max-payload command:

bba-group pppoe global
 virtual-template 1
 tag ppp-max-payload deny