[rbridge] Some comments about TRILL ESADI processing of CSNPs

[mike shand]

I'm sorry, I have only just noticed the following


In section 4.2.5.1 of the TRILL spec it says

"  The ESADI DRB sends TRILL-ESADI-CSNP frames on the ESADI virtual
    link. *For robustness, a participating RBridge that determines that
    some other RBridge should be ESADI DRB on such a virtual link but has
    not received or sent a TRILL-ESADI-CSNP in at least the ESADI DRB
    holding time MAY also send a TRILL-ESADI-CSNP on the virtual link.* A
    participating RBridge that determines that no other RBridges are
    participating in the ESADI protocol for a particular VLAN SHOULD NOT
    send ESADI information or TRILL-ESADI-CSNPs on the virtual link for
    that VLAN.
"

However, ISO/IEC 10589 second edition in clause 7.3.17 says

If an Intermediate system detects that the transmitter had more up to 
date information, the receiving Intermediate system shall multicast a 
Partial Sequence Numbers PDU (PSNP), containing information about LSPs 
for which it has older information. This serves as an implicit request 
for the missing information. Although the PSNP is multicast, *only the 
Designated Intermediate System of the appropriate level shall respond to 
the PSNP.*
NOTE 38 This is equivalent to the PSNP being transmitted directly to the 
Designated Intermediate System, in that it avoids each Intermediate 
System unnecessarily sending the same LSP(s) in response. However, it 
has the advantage of preserving the property that all routeing messages 
can be received on the multi-destination addresses, and hence by a LAN 
adapter dedicated to the multi-destination address.


Presumably in the ESADI case, the intention is that the non-DRB sending 
the CSNP should ALSO respond to the PSNP.

It probably therefore needs some words to clarify this.

However, I am more concerned by the following.

Presumably the intention is that the first non-DRB to notice the lack of 
CSNPs and send one itself will inhibit other RBridges from also sending 
CSNPs, hence avoiding a storm of CSNPs.  However it is possible that 
some number of other RBridges will start to transmit CSNPs before they 
receive the CSNP sent by the first RBridge. Since the CSNP will in most 
cases be a set of CSNPs, is the intention that reception of a CSNP 
should inhibit the sending of further members of the set, or should the 
complete set be transmitted once it has been started, even in the 
presence of CSNPs from another RBridge?

I think it has to be the latter, since two RBridges deciding to send a 
CSNP set at about the same time could mutually inhibit each other, 
resulting in only a partial CSNP set being transmitted by either of the 
RBridges, and thus negating the effect of the "backup" CSNP.

There would appear to be some need of clarification here.



     Mike



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