Using SNMP to Find a Port Number from a MAC Address on a Catalyst Switch
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Contents
Introduction
This document describes how to use Simple Network Management Protocol (SNMP) to obtain the port number on a Cisco Catalyst switch from which you know the MAC address.
Prerequisites
Requirements
Readers of this document should have knowledge of these topics:
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How to get VLANs from a Catalyst switch with use of SNMP
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How to use community string indexing with SNMP
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General use of the SNMP get command and walk command
Components Used
This document applies to Catalyst switches that run regular Catalyst OS (CatOS) or Cisco IOS® Software. The software supports the BRIDGE-MIB and the IF-MIB.
The information in this document is based on these software and hardware versions:
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Catalyst 3524XL that runs Cisco IOS Software Release 12.0(5)WC5a
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Net-SNMP version 5.0.6
Note: To obtain this software, refer to Net-SNMP
.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
Background
For more information on how to query the content-addressable memory (CAM) table, VLANs, and all related MIBs, such as the CISCO-VTP-MIB and the BRIDGE-MIB, refer to the Background section of the document How To Get Dynamic CAM Entries (CAM Table) for Catalyst Switches Using SNMP.
Details of the MIB Variables, Which Includes Object Identifiers (OIDs)
.1.3.6.1.2.1.17.4.3.1.1
dot1dTpFdbAddress OBJECT-TYPE
-- FROM BRIDGE-MIB
-- TEXTUAL CONVENTION MacAddress
SYNTAX OCTET STRING (6)
MAX-ACCESS read-only
STATUS Mandatory
DESCRIPTION "A unicast MAC address for which the bridge has forwarding
and/or filtering information."
::= { iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) dot1dBridge(17) dot1dTp(4)
dot1dTpFdbTable(3) dot1dTpFdbEntry(1) 1 }
.1.3.6.1.2.1.17.4.3.1.2
dot1dTpFdbPort OBJECT-TYPE
-- FROM BRIDGE-MIB
SYNTAX Integer
MAX-ACCESS read-only
STATUS Mandatory
DESCRIPTION "Either the value "0", or the port number of the port on which
a frame having a source
address equal to the value of the corresponding instance of
dot1dTpFdbAddress has been seen.
A value of "0" indicates that the port number has not been learned,
but that the bridge does
have some forwarding/filtering information about this address (that is,
in the StaticTable).
Implementors are encouraged to assign the port value to this
object whenever it is
learned, even for addresses for which the corresponding value of
dot1dTpFdbStatus is not learned(3)."
::= { iso(1) org(3) dod(6) internet(1) mgmt(2) mib-2(1) dot1dBridge(17) dot1dTp(4)
dot1dTpFdbTable(3) dot1dTpFdbEntry(1) 2 }
.1.3.6.1.2.1.2.2.1.1
ifIndex OBJECT-TYPE
SYNTAX InterfaceIndex
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A unique value, greater than zero, for each interface. It
is recommended that values are assigned contiguously
starting from 1. The value for each interface sub-layer
must remain constant at least from one re-initialization of
the entity's network management system to the next re-
initialization."
::= { ifEntry 1 }
.1.3.6.1.2.1.17.1.4.1.2
dot1dBasePortIfIndex OBJECT-TYPE
SYNTAX INTEGER
ACCESS read-only
STATUS mandatory
DESCRIPTION
"The value of the instance of the ifIndex object,
defined in MIB-II, for the interface corresponding
to this port."
::= { dot1dBasePortEntry 2 }
.1.3.6.1.2.1.31.1.1.1.1
ifName OBJECT-TYPE
SYNTAX DisplayString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The textual name of the interface. The value of this
object should be the name of the interface as assigned by
the local device and should be suitable for use in commands
entered at the device's `console'. This might be a text
name, such as `le0' or a simple port number, such as `1',
depending on the interface naming syntax of the device. If
several entries in the ifTable together represent a single
interface as named by the device, then each will have the
same value of ifName. Note that for an agent which responds
to SNMP queries concerning an interface on some other
(proxied) device, then the value of ifName for such an
interface is the proxied device's local name for it.
If there is no local name, or this object is otherwise not
applicable, then this object contains a zero-length string."
::= { ifXEntry 1 }
Get the Port Number on Which a MAC Address Has Been Learned
Step-by-Step Instructions
Complete the steps in this section in order to use SNMP to get the port number on which a MAC address has been learned. Consider that the port number is in VLAN1.
Note: In the commands in this section:
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public is the read community string.
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@1 is the VLAN 1 part of the read community string.
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crumpy is the device host name.
Note: You can also use the IP address for this host name.
Note: The Conclusion section uses the values that appear in italics in the command output.
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Retrieve the VLANs. Use the snmpwalk command on the vtpVlanState object (.1.3.6.1.4.1.9.9.46.1.3.1.1.2 ):
%snmpwalk -c public crumpy .1.3.6.1.4.1.9.9.46.1.3.1.1.2 CISCO-VTP-MIB::vtpVlanState.1.1 = INTEGER: operational(1) CISCO-VTP-MIB::vtpVlanState.1.3 = INTEGER: operational(1) CISCO-VTP-MIB::vtpVlanState.1.7 = INTEGER: operational(1) CISCO-VTP-MIB::vtpVlanState.1.10 = INTEGER: operational(1) ...
Note: This command uses community string indexing. The command also uses vtpVlanState, which has OID .1.3.6.1.4.1.9.9.46.1.3.1.1.2. If you have loaded the MIBs to your network management system (NMS), you can use the object name instead of the OID. Issue this command instead:
%snmpwalk -c public@1 crumpy vtpVlanState
Note: You can also use the object names in steps 2 through 6.
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Issue this command in order to obtain the MAC address table by considering that the port belongs to VLAN1:
snmpwalk -c public@1 crumpy .1.3.6.1.2.1.17.4.3.1.1 17.4.3.1.1.0.0.12.7.172.8 = Hex: 00 00 0C 07 AC 08 17.4.3.1.1.0.1.2.27.80.145 = Hex: 00 01 02 1B 50 91 17.4.3.1.1.0.1.3.72.77.90 = Hex: 00 01 03 48 4D 5A 17.4.3.1.1.0.1.3.72.221.191 = Hex: 00 01 03 48 DD BF ...
Note: Provide the appropriate VLAN number after the community string. In this example, it is VLAN1.
The command lists all MAC addresses that have been learned on all ports that belong to VLAN 1.
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Issue this command to determine the bridge port number for VLAN 1:
snmpwalk -c public@1 crumpy .1.3.6.1.2.1.17.4.3.1.2 17.4.3.1.2.0.0.12.7.172.8 = 13 17.4.3.1.2.0.1.2.27.80.128 = 13 17.4.3.1.2.0.1.2.27.80.145 = 13 17.4.3.1.2.0.1.2.163.145.225 = 13 ...
Note: VLAN 1 is dot1dTpFdbPort , or .1.3.6.1.2.1.17.4.3.1.2.
-
Issue this command to map the bridge port to the ifIndex, OID .1.3.6.1.2.1.2.2.1.1:
snmpwalk -c public@1 crumpy .1.3.6.1.2.1.17.1.4.1.2 17.1.4.1.2.13 = 2 17.1.4.1.2.14 = 3 17.1.4.1.2.15 = 4 17.1.4.1.2.16 = 5
This command queries the dot1dBasePortIfIndex, which has OID .1.3.6.1.2.1.17.1.4.1.2.
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Use the walk command with ifName in order to correlate the ifIndex value with a correct port name.
Issue this command:
Note: The ifName has OID .1.3.6.1.2.1.31.1.1.1.1.
snmpwalk -c public@1 crumpy .1.3.6.1.2.1.31.1.1.1.1 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.1 = VL1 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.2 = Fa0/1 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.3 = Fa0/2 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.4 = Fa0/3 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.5 = Fa0/4 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.6 = Fa0/5 ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.7 = Fa0/6 ...
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Link a MAC address to the port on which the address was learned.
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From Step 1, the MAC address is:
17.4.3.1.1.0.0.12.7.172.8 = Hex: 00 00 0C 07 AC 08
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From Step 2, the bridge port tells that the MAC address belongs to bridge port number 13:
17.4.3.1.2.0.0.12.7.172.8 = 13
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From Step 3, the bridge port number 13 has ifIndex number 2:
17.1.4.1.2.13 = 2
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From Step 4, the ifIndex 2 corresponds to port Fast Ethernet 0/1:
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.2 = Fa0/1
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Conclusion
The MAC address 00 00 0C 07 AC 08 is learned on port Fa0/1.
Compare this conclusion with output from:
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The show cam dynamic command for CatOS switches
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The show mac command for Cisco IOS Software switches
Here is the sample output:
crumpy# show mac Dynamic Address Count: 58 Secure Address Count: 2 Static Address (User-defined) Count: 0 System Self Address Count: 51 Total MAC addresses: 111 Maximum MAC addresses: 8192 Non-static Address Table: Destination Address Address Type VLAN Destination Port ------------------- ------------ ---- ------------------- 0000.0c07.ac08 Dynamic 1 FastEthernet0/1 0001.021b.5091 Dynamic 1 FastEthernet0/1 0001.0348.4d5a Dynamic 1 FastEthernet0/1 0001.0348.ddbf Dynamic 1 FastEthernet0/1 0001.972d.dfae Dynamic 1 FastEthernet0/1 0002.55c6.cfe7 Dynamic 1 FastEthernet0/1 0002.7d61.d400 Dynamic 1 FastEthernet0/1 …
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