Subject: [Isis-wg] How does IS-IS handle jumbo Hello messages?

From: Radia Perlman <[email-masked]>
Date: Thu, 16 Aug 2007 14:09:55 -0700

1) How big can an IS-IS Hello be?

2) If the IS-IS Hello is bigger than that, is there anything in IS-IS 
today that fragments
Hellos? (like CSNPs and LSPs can be fragmented).

I'm asking because there are some potentially scary big things that 
TRILL might want to
add to IS-IS Hellos. For instance, you might want to list, for each 
neighbor, all the
VLAN tags that you can hear that neighbor on. If Hellos can't get bigger 
than some size,
then this might influence the TRILL design.

Thanks!

Radia

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From: "Les Ginsberg \(ginsberg\)" <[email-masked]>
Date: Thu, 16 Aug 2007 15:36:43 -0700

> -----Original Message-----
> From: Radia Perlman [mailto:[email-masked]]
> Sent: Thursday, August 16, 2007 2:10 PM
> To: [email-masked]
> Subject: [Isis-wg] How does IS-IS handle jumbo Hello messages?
> 
> 

The MTU size of the link.

> Hellos? (like CSNPs and LSPs can be fragmented).

IS-IS does not support fragmentation/reassembly of any PDUs.
What you are referring to as regards CSNPs/LSPs involves specifying a
different range of IDs(SNP case) or a different LSP # (in the LSP case).
There is no comparable encoding for IIHs.

> then this might influence the TRILL design.

No doubt you will bring your requirements and the rationale for them to
the WG for consideration. :-)

   Les

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From: Tony Li <[email-masked]>
Date: Thu, 16 Aug 2007 17:10:46 -0700

> The MTU size of the link.

In case it helps: I know of no implementations that are currently  
supporting less than an MTU of a full Ethernet MTU for Ethernet  
interfaces.  I'm not going to try to do the accounting for you, but  
this should give you 1400 odd bytes to play with.

This should be an ample number of bytes to encode any rational and  
sane number of VLANs as a simple list.

Of course, if you want to go beyond that, you could instead send a  
bitmap of the full VLAN space.

Tony

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From: mike shand <[email-masked]>
Date: Fri, 17 Aug 2007 10:34:18 +0100

> 1) How big can an IS-IS Hello be?
Only as big as the MTU. Indeed they are often padded to max MTU
> Hellos? (like CSNPs and LSPs can be fragmented).
No. And I wouldn't be very happy with massive hellos because of the work 
of parsing them all.
> VLAN tags that you can hear that neighbor on. If Hellos can't get 
> bigger than some size,
How big are you thinking they might get?

    Mike