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New Member

E-Mail Attachment problems with VLANs

User A was moved to a over to a VLAN on our network yesterday, which contains (6) 2950 switches trunked together via the GigabitEthernet ports, routing through a 2801, through a firewall, to a 2600 router and out to the Internet now he's having problems sending E-mail's with attachments larger than 40K. User A can get to the internet fine and use e-mail without attachments, but when sending e-mails with attachments, whether the service is web-based or POP3, the SMTP message time's out.


When User A is on the native VLAN he has no problems with E-mail attachments. MTU on both the router and firewall are at default 1500. Even if it was a MTU issue, which I don't think it is, that would effect the native VLAN users as well, or so I would think. Could this be an MSS problem?

We're using an Internet Web Filter so we do use SPAN to monitor ports

  • Other Network Infrastructure Subjects

Re: E-Mail Attachment problems with VLANs


the native VLAN does not use a dot1Q encapsulation, so it could be a MTU/MSS issue. Can you set the mtu on the trunkports to higher values f.e. 1600?

You could also check with ping -f -l 1472 what the path MTU for the client to the firewall is.



New Member

Re: E-Mail Attachment problems with VLANs

C:\Documents and Settings\Administrator>ping -f -l 1472

Pinging [] with 1472 bytes of data:

Reply from bytes=1472 time=39ms TTL=53

Reply from bytes=1472 time=47ms TTL=53

Reply from bytes=1472 time=37ms TTL=53

Reply from bytes=1472 time=37ms TTL=53

Ping statistics for

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 37ms, Maximum = 47ms, Average = 40ms

Looks ok.