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This appendix explains the User Registrar application's limited function as an SNMP manager, in that it sends SNMP get and set requests to SNMP agents. There is an SNMP agent in each DOCSIS modem. This SNMP agent answers SNMP get and set requests from SNMP managers, according to the DOCSIS MIBs and other MIBs (Management Information Bases).
A MIB describes the objects that can be managed by SNMP. A typical MIB contains the common name (which must be unique) of each object, the value of the OID (object IDs, which are hierarchical and must be unique), and the textual description of the syntax and semantics of each object.
User Registrar uses SNMP only to set the value of one object: docsDevResetNow. By setting this object value to true, User Registrar resets the modem. Note that this has been erroneously called doing "an SNMP reset of the modem."
The MIB definition of this object is:
docsDevResetNow OBJECT-TYPE
SYNTAX TruthValue
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"Setting this object to true(1) causes the device to reset.
Reading this object always returns false(2)."
::= { docsDevBase 3 }
The SNMP OID for this object is .1.3.6.1.3.83.1.1.3. It is constructed by appending the value 3 to the OID for docsDevBase - .1.3.6.1.3.83.1.1.
The tricky part is knowing the community string that would allow the DOCSIS modem to be managed (that is, be reset). Some DOCSIS modems are pre-configured to allow SNMP gets with the community string public, and to allow SNMP sets with the community string private. This method is unreliable, and some customers are asking modem vendors not to ship with these defaults.
The most reliable mechanism is to configure the SNMP community strings in the DOCSIS configuration file through Modem Registrar.
The DOCSIS configuration file may optionally include a TLV option, "SNMP MIB Object" (type 11), which can be used to set the values of one or more MIB objects. When the DOCSIS modem processes a configuration file with this option, it sets the MIB object value as if it had received an SNMP set request with the correct community string.
In addition, the DOCSIS modem has MIB objects that control access to its SNMP agent—within a MIB table called docsDevNmAccessTable. Therefore, you can construct a DOCSIS configuration file that contains the community strings that the modem should use to restrict network management access.
In Modem Registrar, you can set up multiple type 11 tuples with the following implied SNMP sets.
docsDevNmAccessIp.1 = <IP address of User Registrar server> docsDevNmAccessIpMask.1 = 255.255.255.255 docsDevNmAccessCommunity.1 = <User Registrar community string> docsDevNmAccessControl.1 = readWrite(3) docsDevNmAccessStatus.1 = active(1) docsDevNmAccessIp.2 = <IP subnet of network management stations> docsDevNmAccessIpMask.1 = <IP netmask of subnet above> docsDevNmAccessCommunity.1 = <Network Manager community string> docsDevNmAccessControl.1 = rwWithTraps(5) docsDevNmAccessStatus.1 = active(1)
The following is the MIB definition for the Network Management Access table:
--
-- The following table provides one level of security for access
-- to the device by network management stations.
-- Note that access is also constrained by the
-- community strings and any vendor-specific security.
--
docsDevNmAccessTable OBJECT-TYPE
SYNTAX SEQUENCE OF DocsDevNmAccessEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table controls access to SNMP objects by network
management stations. If the table is empty, access
to SNMP objects is unrestricted."
::= { docsDevMIBObjects 2 }
docsDevNmAccessEntry OBJECT-TYPE
SYNTAX DocsDevNmAccessEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Controls access to SNMP objects by a particular network
management station. For each entry in this table, the
contents are not readable unless the management station
has read-write permission."
INDEX { docsDevNmAccessIndex }
::= { docsDevNmAccessTable 1 }
DocsDevNmAccessEntry ::= SEQUENCE {
docsDevNmAccessIndex INTEGER,
docsDevNmAccessIp IpAddress,
docsDevNmAccessIpMask IpAddress,
docsDevNmAccessCommunity DisplayString,
docsDevNmAccessControl INTEGER,
docsDevNmAccessInterfaces OCTET STRING,
docsDevNmAccessStatus RowStatus
}
docsDevNmAccessIndex OBJECT-TYPE
SYNTAX INTEGER (1..2147483647)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"Index used to order the application of access entries."
::= { docsDevNmAccessEntry 1 }
docsDevNmAccessIp OBJECT-TYPE
SYNTAX IpAddress
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The IP address (or subnet) of the network management
station. The address 255.255.255.255 is defined to mean
any NMS. If traps are enabled for this entry, then the
value must be the address of a specific device."
DEFVAL { 'ffffffff'h }
::= { docsDevNmAccessEntry 2 }
docsDevNmAccessIpMask OBJECT-TYPE
SYNTAX IpAddress
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The IP subnet mask of the network management stations.
If traps are enabled for this entry, then the value must
be 255.255.255.255."
DEFVAL { 'ffffffff'h }
::= { docsDevNmAccessEntry 3 }
docsDevNmAccessCommunity OBJECT-TYPE
SYNTAX DisplayString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The community string to be matched for access by this
entry. If set to the null string then any community string
will match."
DEFVAL { "public" }
::= { docsDevNmAccessEntry 4 }
docsDevNmAccessControl OBJECT-TYPE
SYNTAX INTEGER {
none(1),
read(2),
readWrite(3),
roWithTraps(4),
rwWithTraps(5),
trapsOnly(6)
}
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Specifies the type of access allowed to this NMS. Setting
this object to none(1) causes the table entry to be
destroyed. Read(2) allows access by 'get' and 'get-next'
PDUs. ReadWrite(3) allows access by 'set' as well.
RoWithtraps(4), rwWithTraps(5), and trapsOnly(6)
control distribution of Trap PDUs transmitted by this
device."
DEFVAL { read }
::= { docsDevNmAccessEntry 5 }
-- The syntax of the following object was copied from RFC1493,
-- dot1dStaticAllowedToGoTo.
docsDevNmAccessInterfaces OBJECT-TYPE
SYNTAX OCTET STRING
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Specifies the set of interfaces from which requests from
this NMS will be accepted.
Each octet within the value of this object specifies a set
of eight interfaces, with the first octet specifying ports
1 through 8, the second octet specifying interfaces 9
through 16, etc. Within each octet, the most significant
bit represents the lowest numbered interface, and the least
significant bit represents the highest numbered interface.
Thus, each interface is represented by a single bit within
the value of this object. If that bit has a value of '1'
then that interface is included in the set.
Note that entries in this table apply only to link-layer
interfaces (e.g., Ethernet and CATV MAC). Upstream and
downstream channel interfaces must not be specified."
-- DEFVAL is the bitmask corresponding to all interfaces
::= { docsDevNmAccessEntry 6 }
docsDevNmAccessStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Controls and reflects the status of rows in this table."
::= { docsDevNmAccessEntry 7 }
The following RFCs represent the current specification of SNMPv2. There are many SNMP and network management textbooks available.
Posted: Tue Sep 26 04:47:25 PDT 2000
Copyright 1989-2000©Cisco Systems Inc.