MTU on PIX using DSL

Answered Question
Feb 5th, 2009
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Hi all,


I have a PIX where the outside interface is connected to a DSL modem. The PIX is configured for PPPoE on the outside interface. I know that the typical MTU size is 1500 for Ethernet and 1492 for PPPoE.


Should I set the MTU to 1492 on the outside interface of the PIX, or does the MTU only need to be set properly on the DSL modem?


Any help is very much appreciated.


Thanks,

- Steve


Correct Answer by eddie.mitchell@... about 8 years 3 months ago

According to this configuration guide, the MTU is automatically configured for 1492 when the default route is added:


http://www.cisco.com/en/US/docs/security/pix/pix63/configuration/guide/pixclnt.html#wp1031522


"The PPPoE client is only supported on the outside interface of the PIX Firewall. PPPoE is not supported in conjunction with DHCP because with PPPoE the IP address is assigned by PPP. The setroute option causes a default route to be created if no default route exists. The default router will be the address of the access concentrator. The maximum transmission unit (MTU) size is automatically set to 1492 bytes, which is the correct value to allow PPPoE transmission within an Ethernet frame."


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Correct Answer
eddie.mitchell@... Thu, 02/05/2009 - 11:16
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According to this configuration guide, the MTU is automatically configured for 1492 when the default route is added:


http://www.cisco.com/en/US/docs/security/pix/pix63/configuration/guide/pixclnt.html#wp1031522


"The PPPoE client is only supported on the outside interface of the PIX Firewall. PPPoE is not supported in conjunction with DHCP because with PPPoE the IP address is assigned by PPP. The setroute option causes a default route to be created if no default route exists. The default router will be the address of the access concentrator. The maximum transmission unit (MTU) size is automatically set to 1492 bytes, which is the correct value to allow PPPoE transmission within an Ethernet frame."


rstevek Thu, 02/05/2009 - 11:56
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Hi Eddie,


Thanks very much! I did not realize that this was being done automatically for me. I am much less confused now.


Thanks,

- Steve


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