Conference 7.286::fddi

1226.0. "Path MTU Discovery Problem with DB620" by SOS6::GROSSETETE () Mon Feb 07 1994 05:35

    	Hi,
    
    	One of our customer has installed a Super Dragon system (Cray/Sun)
    on one of his FDDI rings. The network is basically divided in 2 rings,
    
    . a small one where are connected Super Dragon, Cray system's 
      through NSC boxes and a Cisco AGS+ router which has 2 FDDI interfaces.
    
    . a large one where the Cisco AGS+ is connected as well as about 20
      DECbridge 520/620 (v.1.2/v.1.3) which connect all Ethernet segments.
    
    	Super Dragon system is implementing the "Path MTU discovery"
    feature (RFC 1191), so it sends a IP frame with the "Don't Fragment"
    bit set to the Cisco with a MTU size equal its directly connected
    interface, in fact FDDI = 4352 bytes.
    
    	Cisco AGS+ forwards it on the large FDDI ring but this never
    goes through our DECbridge 520/620 since DF bit is set.
    According to the RFC, a router will have answer with an ICMP packet
    "Unreachable Destination report" where the code must have been
    "Fragmentation needed but DF bit was set".
    But our DECbridge 520/620 are not router's boxes...
    
    	So, as we are implementing IP fragmentation as a feature,
    how can we handle this problem ? Is this the same on DECbridge 900MX ? 
    
    	Patrick
    
    
    		Cray			Cray
    		NSC			NSC
    		=========================
    		|			|
    Small	|			|Super Dragon
    FDDI	|			|
    		=========================
    			Cisco AGS+
    		=========================
    Large	|			|
    FDDI	|			|
    		|			|
    		=========================
    			~ 20 DB520/620
    
    
        

DECbridge 5xx and 6xx implement fragmentation (including handling the DF bit)
but they do NOT implement RFC 1191.

On the other hand, DECbridge 900 MX DOES implement RFC 1191.  So if you upgrade
to the 900 MX, the problem will go away.

	paul

    	Any way to get a firmware update to support it ?
    
    Patrick