[c-nsp] high CPU with snmp IS THERE A REAL FIX

[Jeff Fitzwater]

We are running 12.2.SXI on sup-720-cxl

We use snmp getnext and getbulk to get the ARP table from a router  
that has ~16K entries and it takes about 10min to complete, with  
ROUTER CPU at 100%.   Our other routers have the same hardware and IOS  
but have <10K entries and work fine.

This is an old problem with CISCO and I thought they might have  
finally fixed it in SXI, but NOT.

The problem is the ARP entries are stored internally in a hashed  
format and SNMP needs them in an ordered format, so for every snmp PDU  
the CPU must get the entire ARP table and crunch it to create the  
ordered PDU response.  This will take place for as many PDUs as it  
takes to retrieve the entire table.   There does appear to be a point   
(size of ARP table) at which it takes very long to retrieve the whole  
table which seems to be around 12K but with a 16K table it takes  
forever.

In the attached PDF from CISCO they explain the problem and also state  
the if you turn on CEF (has always been on for long time) that it is  
much faster since the FIB is already in a lexical order that snmp  
likes.   Since CEF is always on, why does it still take so long.

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At this point we basically cannot do any retrieval of the ARP  tables.



Their must be many other groups that have this same problem or I have  
really missed something.


I will open up a ticket with CISCO if nobody out there has an answer.

Currently we use an expect script to get the table via CLI which is  
much faster but it doesn't help tools that must use snmp.


Thanks for any help.


Jeff Fitzwater
OIT Network Systems
Princeton University