On Mon, Mar 08, 2010 at 12:55:29PM -0500, Chuck Anderson wrote:
> On Mon, Mar 08, 2010 at 11:57:55AM -0500, Eric Van Tol wrote:
> > Both MTUs are consistent and always have been. I started out with
> > 9216 physical MTU and 1500 inet MTU, but have since just deleted the
> > custom MTU and went with the defaults. I am quite sure now that
> > this is not an MTU issue, but rather a deficiency with the EX2500.
> > I've opened up a JTAC case and will let the list know what the
> > problem turns out to be.
>
> But you didn't have the iso mtu set. Which means iso was probably
> using 9216-6-6-4-2-3 = 9195 (or 9192-6-6-4-2-3 = 9171) by default for
> a VLAN encapsulated 802.2 LLC frame. Changing the inet MTU doesn't
> affect any non-IP protocols' MTUs.
Just to confirm this I tested with a physical mtu 9192, family inet
mtu 1500 and no family iso mtu configured:
interfaces {
fe-1/0/0 {
vlan-tagging;
mtu 9192;
unit 999 {
vlan-id 999;
family inet {
mtu 1500;
}
family iso;
}
}
}
You can see that iso is using an MTU of 9171:
lab at main> show interfaces fe-1/0/0 extensive | match mtu
Link-level type: Ethernet, MTU: 9192, Speed: 10m, Loopback: Disabled,
FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0
Protocol inet, MTU: 1500, Generation: 283, Route table: 3
Flags: User-MTU
Protocol iso, MTU: 9171, Generation: 282, Route table: 3
But even so, the Hellos don't look to be padded out to the MTU
(ethernet length is only 75):
0:90:69:bc:2c:7e > 1:80:c2:0:0:15, ethertype 802.1Q (0x8100),
length 75: vlan 999, p 6, LLC, dsap OSI (0xfe) Individual, ssap OSI
(0xfe) Command, ctrl 0x03: OSI NLPID IS-IS (0x83): length 54
L2 Lan IIH, hlen: 27, v: 1, pdu-v: 1, sys-id-len: 6 (0), max-area: 3 (0)
This contradicts what it says in the JNCIA Study Guide, page 291:
"Therefore, each interface must support the transmission of the
maximum IS-IS PDU of 1492 bytes. To enforce this requirement, the
IS-IS Hello PDUs are padded to this maximum value. If the hello gets
to the neighboring router, the connecting interface supports the
maximum PDU size. Should the hello not be received by the neighboring
router, no adjacency forms and this link is not used by IS-IS."
I'm not seeing the IIH's being padded at all (JUNOS 8.4R3.3).
-Chuck, in the middle of preparing for the JNCIE exam.