SNMP v1 - the protocol
Network Mgmt/Sec.
Jim Binkley
1
Outline
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protocol overview
– UDP/polling/reliability
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protocol formats
– get, set, response, trap – get-next, and tables
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protocol examples
Jim Binkley
2
manager/agent
manager sends get database item (OID)
agent sends response
Jim Binkley
MIBS agent
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SNMP layered on top of UDP
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manager sends packet (snmp get) with:
– ip src = manager, ip dst = agent – udp src port = client port (>=1024), say 1046 – udp dst port = 161
response formed by client comes back from port 161 to manager client port (1046) ◆ traps sent to port 162 on manager
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UDP is unreliable protocol
collisions, congestion, noise may kill packets ◆ send mechanism will typically have N retries ◆ UDP does have checksum mechanism (needs to be on) ◆ in addition, typical manager does periodic poll (5 minutes/15 minutes or longer)
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polling, cont.
in general you need to consider how many agents exist ◆ how many MIB objects are accessed (e.g., in BGP router, routing table could be quite large) ◆ how "far" in link counts is manager from worst-case agent
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– less links the better; manager should be central and Jim Binkley should be on quality link
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Heisenberg ...
are you feeling "uncertain" ◆ question: how can you measure capacity usage at manager ◆ use HPOV on manager interfaces ◆ use MRTG to watch thruput on manager interfaces (or snmpget/snmpwalk) ◆ look at SNMP errors in snmp mib, interface errors in interface mib on manager
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traps are fundamentally unreliable
one way, agent to manager ◆ doesn't mean they aren't useful ◆ I rebooted (due to a crash) ◆ I lost an interface ◆ you get some redundancy here from syslog though
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