Any constraints on MTU size with IPSec?

ITEM: RTA000153071

Topic thread:                                                                   
RALEIGH NETWORKING SUPPORT CENTER (RALY - NA/ATS)                               
 IBM COMMERCEPOINT                                                              
  IBM eNetwork Firewall V3.2 for AIX                                            
The customer believes he read that there is a constraint on MTU size            
when using VPN where it would have to be set to 512 bytes. I could              
not find this documented in my research.  Is there such a constraint            
or recommendation?                                                              
The customer was also concerned that certain Internet Service Providers         
might not allow passage of VPN sessions.  Are you aware of any                  
Providers for whom this might be true?  Thanks.                                 
Earlier in the products lifecycle, we sometimes told customers to set           
MTU size for the Win 95 Secure Remote client to 512, in order to                
always guarantee that it would work. However, we haven't had any                
problems like this in a while, so normal MTU sizes (1500 range or so)           
should work for the secure remote client as well.                               
For tunnels between Firewalls, we've never had any MTU related size             
problems. The customer should not have to modify the existing MTU sizes         
in his network.                                                                 
We are not aware of any specific ISPs that will not allow VPN's.                
Obviously, if this is a concern for the customer, they should consult           
with the ISPs sales department or technical support before engaging            
a specific ISP to determine if they can support VPNs or not.                    
Thanks for using ASQ                                                            

WWQA: ITEM: RTA000153071 ITEM: RTA000153071
Dated: 10/1998 Category: FIREWAIX
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