[j-nsp] ISIS over GRE (1500 MTU)

[Matt Hite]

Wanted to throw this out there to see if anyone else has solved this
problem before me.

I'd like to set up an IS-IS adjacency over an Internet based GRE
tunnel. The tunnel comes up fine in my tests but the IS-IS adjacency
never forms. One can probably safely presume this is due to an MTU
issue -- the 1492 PDU size for IS-IS + 24 for GRE doesn't fit in the
1500 MTU this is all riding over.

Juniper has a technical note which touches upon the issue:

http://kb.juniper.net/InfoCenter/index?page=content&id=KB7848

... so I have some hope the problem is solvable...

I'm using Trio linecards on MX platform. Technically speaking, I don't
want to enable fragmentation of my IP traffic. I'd like for path MTU
discovery to do its thing with that. Not quite sure I actually
understand (from the tech note) what I am supposed to do
configuration-wise to solve this...

Here's a rough outline of my basic config (real simple):

== R1 ==

set chassis fpc 2 pic 3 tunnel-services bandwidth 10g

set interfaces gr-2/3/0 unit 0 tunnel destination x.x.x.3
set interfaces gr-2/3/0 unit 0 tunnel source x.x.y.3
set interfaces gr-2/3/0 unit 0 family inet
set interfaces gr-2/3/0 unit 0 family iso

set protocols isis interface gr-2/3/0.0 point-to-point
set protocols isis interface gr-2/3/0.0 level 1 disable
set protocols isis interface gr-2/3/0.0 level 2 metric 1000

== R2 ==

set chassis fpc 2 pic 3 tunnel-services bandwidth 10g

set interfaces gr-2/3/0 unit 0 tunnel source x.x.y.3
set interfaces gr-2/3/0 unit 0 tunnel destination x.x.x.3
set interfaces gr-2/3/0 unit 0 family inet
set interfaces gr-2/3/0 unit 0 family iso

set protocols isis interface gr-2/3/0.0 point-to-point
set protocols isis interface gr-2/3/0.0 level 1 disable
set protocols isis interface gr-2/3/0.0 level 2 metric 1000


--

Thanks for any thoughts or ideas.

-M