[j-nsp] Juniper Ethernet MTU question

Sat Sep 30 12:19:55 EDT 2006

Three labels are typically seen in carrier of carrier apps. Normal
l2/l3/vpls vpn is 2 labels (8 bytes), and as I mentioned the optional
martini control word (applicable to l2circuits), adds another 4 bytes
when enabled.

The frame leaving the pe towards the p router will be the ethernet frame
received from the CE minus preabmble/crc (may include a vlan tag), plus
control word (optional and applicable to l2circuit), plus vrf label,
plus mpls transport label, plus the L2 encap needed for that link. 

By way of example consider this l2vpn/circuit/vpls example. AFAIK they
all use same encap excepting the optional control world for l2 circuits:

Vlan
tagging

cpe-------------------pe---------------p

1518   		     +8       +14

		          vrf/mpls  eth encap

Here the PE must encapsulate up to 1518 bytes of data received from CE
(1500 + 4 (vlan) + 14 (Eth OH). Assuming no control word, the pe then
pushes two labels for a total payload size of 1526. It then encapsulates
for link-level transmission to p router, for ethernet this adds an
another 14 bytes, for a grand total of 1540. 

The dfe pics only support a device mtu of 1532, so I think you have an
issue. Loosing vlan tag reduces size by 4 bytes but the resulting 1536
still exceeds dfe jumbo support.

Only thing I can think of is to have the attached CEs reduce their Inet
mtus by ~ 8 bytes, i.e. to 1492 including ip/tcp headers. This way the
CEs can fragment as needed at ingress to the vpn. Not a very elegant
solution, but should work.

Regards

________________________________

	From: Ihsan Junaidi Ibrahim [mailto:ihsan.junaidi at gmail.com] 
	Sent: Saturday, September 30, 2006 5:34 AM
	To: Harry Reynolds
	Cc: Juniper-NSP
	Subject: Re: [j-nsp] Juniper Ethernet MTU question

	The interface will be used for a P-facing temporarily until a
better PIC can be POed. In what situation does an MPLS application
warrant up to 3 labels or an additional of 8 bytes over stock header of
4 bytes? How many labels does a VPLS transport require? 

	On 9/29/06, Harry Reynolds <harry at juniper.net> wrote: 

		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
<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
		> _______________________________________________
		> juniper-nsp mailing list juniper-nsp at puck.nether.net
<mailto:juniper-nsp at puck.nether.net> 
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		>

	-- 
	Thank you for your time,
	Ihsan Junaidi Ibrahim 