Show fdb command

From NetworkCommands

A command is available on most Extreme switch platforms; used to display the device's layer 2 MAC address table (the forwarding database (fdb),) which contains MAC address to port mappings (in other words, which port a particular MAC address can be reached through.)

These mappings are continuously 'learnt' by the device through the inspection of the Ethernet header (which contains source and destination MAC addresses) of all frames entering the switch's ports. These are used by the device to switch frames between specific ports rather than broadcast them to all ports (as a hub does.) However, frames with a destination MAC addresses not in this table are broadcast or 'flooded' to all ports.

Usage Syntax

show fdb [options]

Common options include;

• mac-address - display only the fdb entry for the specified MAC address
• ports port - display only those fdb entries learnt on the specified port
• vlan vlan - display only those fdb entries learnt on ports in the specified vlan
• permanent - display only permanent or static fdb entries (usually the switch's MAC addresses)
• stats - display fdb statistics

Usage Examples

show fdb - display all fdb entries

show fdb ports 1:2 - display only those fdb entries learnt on port 1:2

'show fdb vlan VLAN_ONE - display only those fdb entries learnt on ports in vlan VLAN_ONE

show fdb 00:12:34:56:78:AB - display the fdb entry for MAC address 00:12:34:56:78:AB

Usage Notes

This command is obviously very useful in discovering which switch port a particular host is connected to, if you have it's MAC address. If you only have a host's IP address, you may be able to find it's MAC address using the show iparp command. If the port a MAC address is associated with is connected to another switch, you will have to repeat this step on that switch (and possibly others.)

Frames with a destination MAC addresses not in this table are flooded to all ports.

Typical Output

SWITCH-ONE:show fdb
Index              Mac              Vlan       Age    Use  Flags  Port List
--------------------------------------------------------------------------
001e0-ffb 00:E0:3B:00:00:00 VLAN_104  (0104)  0000000 0000  s m      CPU
003b0-10d 00:0D:70:1E:E6:75 VLAN_104  (0104)  0000000 0001  d mi     14
00500-109 00:0D:70:1E:DB:53 VLAN_103  (0103)  0000015 0000  d m      10
006a0-ffb 01:80:D2:00:00:0E VLAN_101  (0101)  0000000 0000  s m      CPU
007e0-01f 00:A0:9E:0D:F7:AB VLAN_92   (0092)  0000000 0023  d mi     32
00810-ff0 00:04:A6:18:57:C4 VLAN_127  (0127)  0000000 0001  s m      CPU
00970-10d 00:0D:70:1E:DC:7D VLAN_104  (0104)  0000000 0001  d mi     14
00a30-105 00:0D:70:1E:E6:FD VLAN_102  (0102)  0000000 0000  d m      6

SWITCH-ONE:show fdb stats
Total: 248 Static: 92 Perm: 0 Locked: 0 Secure: 0 Dynamic: 156 Dropped: 0
Questionable: 0 Remapped: 0 Locked with timeout: 0
FDB Aging time: 300 seconds

Related Commands

Use the configure fdb command to specify the fdb aging time; how long a MAC address to port mapping stays in the forwarding database.

The Extreme show iparp command can be used to display IP address to MAC address mappings.

Command Equivalents

The Cisco command equivalent is the show mac-address-table command

Related Articles

You may find our OSI Model General Diagram useful in understanding where layer 2 MAC addressing fits into the OSI model. Alternatively browse all our available Reference Diagrams.

Information on other Extreme commands

Information on Cisco commands

Information on Linux commands

Information on F5 BigIP commands

Information on Blue Coat SGOS commands

Information on Nokia IPSO commands

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