Using SNMP to Find a Port Number from a MAC Addres

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.

 
 
Readers of this document should have knowledge of these topics:

How to get VLANs from a Catalyst switch with use of SNMP

How to use community string indexing with SNMP

General use of the SNMP get command and walk command

 
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:

Catalyst 3524XL that runs Cisco IOS Software Release 12.0(5)WC5a

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.

 
For more information on document conventions, refer to the Cisco Technical Tips Conventions.

 
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.

 
 

.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 }
 
Complete the steps in this section in order to use SNMP to get the port number on which a MAC address has been learned.

Note: In the commands in this section:

public is the read community string.

@1 is the VLAN 1 part of the read community string.

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.

Issue this command in order to use VLAN 1 to obtain the MAC address table:

snmpwalk -c 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
…The command lists all MAC addresses that have been learned on all ports that belong to VLAN 1.

This command uses community string indexing. The command also uses dot1dTpFdbAddress, which has OID .1.3.6.1.2.1.17.4.3.1.1. If you have loaded the MIBs onto your network management system (NMS), you can use the object name instead of the OID. Issue this command instead:

snmpwalk -c crumpy dot1dTpFdbAddress
Note: You can also use the object names in Steps 2–5.

Issue this command to determine the bridge port number for VLAN 1:

snmpwalk -c 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 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 = 5This command queries the dot1dBasePortIfIndex, which has OID .1.3.6.1.2.1.17.1.4.1.2.

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 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
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.8 = Fa0/7
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.9 = Fa0/8
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.10 = Fa0/9
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.11 = Fa0/10
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.12 = Fa0/11
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.13 = Fa0/12
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.14 = Fa0/13
ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.15 = Fa0/14
…Link a MAC address to the port on which the address was learned.

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

From Step 2, the bridge port tells that the MAC address belongs to bridge port number 13:

13 17.4.3.1.2.0.0.12.7.172.8 =
13
 From Step 3, the bridge port number 13 has ifIndex number 2:

17.1.4.1.2.13 =
2

From Step 4, the ifIndex 2 corresponds to port Fast Ethernet 0/1:

ifMIB.ifMIBObjects.ifXTable.ifXEntry.ifName.2 =
Fa0/1
 

 
The MAC address 00 00 0C 07 AC 08 is learned on port Fa0/1.

Compare this conclusion with output from:

The show cam dynamic command for CatOS switches

The show mac command for Cisco IOS Software switches

Here is 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