[j-nsp] Juniper Ethernet MTU question

[Harry Reynolds]

Ah, dfe mtus. A subject near to my heart theses days.

A few things.

1. The device mtu includes 14 bytes of ethernet header, the real mtu is
1500 bytes.

2. The inet mtu is automatically set to 1500, which matches real
ethernet mtu.

3. The mpls mtu is automatically computed to be 12 bytes less than real
mtu to accommodate up to 3 labels.

In you case it does not sound that inet will be configured. The vlan tag
does add an extra 4 bytes to the ethernet payload, resulting in need for
1504 byes of information and total device mtu of 1518. JUNOS software
automatically adjust the device mtu when you enable vlan tagging.

As for support of mpls, I assume this is a ce facing interface. As such
packets to and from this interface will not be labeled. Labels are
pushed as the packet enters the core. In a typical l2 vpn there would be
two labels added; vrf and outer transport. The core interfaces will need
an mtu of ethernet (1500) + vlan (4) + martini encap (0 | 4) + labels
(8) + what ever link encap is needed based on core interface type. If
the core interfaces are also ethernet then this is another 14 for total
of 1526-1530 (the latter is with optional martini control word). Pos
only add 4 bytes for link encap so mtu for core would be at least
1516-1520 for pos.

As you seem aware the max mtu on the dfe cards is only 1532. In order to
get this level of jumbo support you need to make sure you have the
latest fpga. See below for tips on that. I believe that you will be OK
with these cards as ce facing with the 1518 device mtu that results from
simply enabling vlan tagging, but should be safe to enable a larger mtu
as well. Technically the mtu of the CE devices should be used to
dimension your ce and core facing interfaces, as best I can tell you
will not need over 1518 device mtu if ce uses default ethernet mtu.

To determine if you have the needed rev "c" 12x dfe firmware, do this
(note hidden commands should only be used with Jtac guidance, and I'm
not in jtac):


Determine fpc/pic location. In this example its 0/2.

Go to a shell, become root and vty to the fpc (0 in this case). Or use
hidden command:

start shell pfe network fpc0 

[edit interfaces fe-0/2/0]
regress at boing# run start shell pfe network fpc0  


FPC platform (PPC 603e processor, 32MB memory, 256KB flash)

FPC0(vty)# 

Now display the dfpga rev:

FPC0(vty)# show dfe-pic 2 defpga    

PIC 2 DFE information:

 Dense FE PIC, 12 port(s) 1 GE links 1 cards
 Periodics are enabled
 Normal interrupts are enabled, total count is 406872
 IFD count is 12
Main Card DE FPGA: 
 Channel 0
   Transmit Packets :  4397, Transmit Bytes : 0 
   Receive Packets :  11090, Receive Bytes : 0 
   (0x850e1080)                  DFE CPU cntl  : 0x8004
   (0x850e1082)                    DFE Version : 0x000c <<<<< Rev c is
needed to get over 1500 mtu
   (0x850e1086)           DFE interrupt enable : 0x0010
Transmit Tagged Status:


HTHs


 

> -----Original Message-----
> From: juniper-nsp-bounces at puck.nether.net 
> [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of 
> Ihsan Junaidi Ibrahim
> Sent: Friday, September 29, 2006 7:02 AM
> To: Juniper-NSP
> Subject: [j-nsp] Juniper Ethernet MTU question
> 
> Hi all,
> 
> 2 questions:
> 
> 1) On a 12-port FE PIC, for vlan-cc encapsulation, should the 
> interface MTU be configured as 1518, 1522 or 1504? The 
> documentation below indicates that for vlan-ccc 
> encapsulation, it's a 4-byte field but I need to be certain.
> http://www.junipernetworks.nl/techpubs/software/junos/junos76/
> swconfig76-network-interfaces/html/interfaces-physical-config5
.html#1085070
> 
> 2) For a 12-port FE PIC, if I were to configure the maximum 
> physical MTU of 1532, will the PIC be able to accommodate MPLS MTU?
> 
> --
> Thank you for your time,
> Ihsan Junaidi Ibrahim
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