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.
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. 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.
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.
--Dave
This archive was generated by hypermail 2b29 : Mon Aug 05 2002 - 10:42:42 EDT