Dave,
Thanks for chiming in - I was hoping you would...
Comments, questions inline...
> IS-IS PDUs can be fragmented, as the maximum LSP is 65535 bytes.
>
> This is incorrect; the maximum works out to about 381K bytes in total,
> but the "fragments" are idempotent (carrying separate sequence numbers
> and updated separately) and do not act very much at all like IP
> fragments.
Thanks for the clarification on the LSP size. Is this a value based on the
work done to increase the maximum LSP size by creating zero-cost adjacencies
to virtual ISs in the sending IS? I was basing my statement on the
1142/1195 specs, where the PDU Length field is 16 bits.
> The second point to make is that IIH PDUs are padded to
> maxsize - 1 octets.
> This ensures that mismatched MTUs on interfaces do not
> cause fragmentation
> issues.
>
> This is mostly deprecated due to operational problems in the field.
> The Juniper implementation pads LAN IIHs to the maximum LSP size
> (1492), which is the only thing that the protocol mechanism is
> attempting to ensure. I vaguely recall making the same change in the
> cisco implementation years back, but my memory is hazy.
It has been changed back! Mike Shand may know the reason. I saw this
yesterday where an adjacency would not form over an ATM subinterface that
had mismatched MTUs.
There used to
> be much excitement when people did silly things like bridge FDDI and
> Ethernet together.
>
> It should be noted that Cisco allows you to set the
> max-lsp size in the
> router configuration. In JUNOS, you have to set the
> lsp-size on the media
> itself, or use groups to set it globally on the router.
> Unless someone
> knows something else?
>
> You cannot adjust the LSP size in the Juniper implementation; it is an
> architectural constant in the protocol. There are proposals afoot to
> make this settable so that ISIS can run over somewhat arcane media
> with miniscule MTUs, but so far we have not seen any request for it.
Like the DCN? ;)
This is more of a problem on Cisco's, which as you know set DS3, HSSI, and
higher-speed media to an MTU of 4470, where as the T1s and V.35 type serials
and fast serials are still at 1500.
> The cisco implementation is settable as a side effect of ISIS sharing
> 99% of its code with IPX NLSP, which required smaller MTUs in case
> NLSP was being run over ARCnet. Changing this value will almost
> certainly bring suffering and recrimination in mixed networks, as the
> operational side effects can be somewhat subtle and
> topology-dependent.
This is what threw me. I know 10589 specifies a recommended maximum LSP
size of 1492. And all of my routers don't have enough TLV info to create an
LSP greater than 1492, but I wasn't sure. As an aside, I thought that the
LSP-MTU value was put there to interoperate with SONET gear, for which the
DCC specifies and MTU of 512 or 576, I can't remember. I remember the NLSP
hack, but I had always assumed it was done after the CLNS piece.
So, just for clarification and my own piece of mind (as this thread was
making me rethink my GigE deployments where POS links exist as well:)
1) JUNOS does not PAD IIHs.
2) MaxLSPFragmentSize is 1492, regardless of connected media MTU.
3) MaxLSPSize is 381KB
4) I have no business debating IS-IS implementation details with the primary
protocol implementor on the majority of IS-IS routers in the Internet. ;)
It's late...
./chris
>
> --Dave
>
This archive was generated by hypermail 2b29 : Mon Aug 05 2002 - 10:42:42 EDT