Layer 2 forwarding is a critical function in any switch or router. When troubleshooting L2 connectivity issues, the MAC address table and forwarding information base (L2FIB) are key areas to inspect. On Cisco Nexus switches, several useful commands provide visibility into MAC addresses and L2 forwarding:
Show MAC Address Table Commands
show mac address-table: Displays the basic MAC table summary per VLAN
switch# show mac address-table VLAN Mac Address Type Ports --------+-----------------+---------+------------ 1 00d0.ba86.1f3a dynamic Eth1/1 10 00d0.ba86.1f3b dynamic Eth1/2 100 00d0.ba86.1f3c dynamic Eth1/3
show mac address-table aging-time: Displays configured aging time per VLAN
switch# show mac address-table aging-time VLAN Aging Time ---- ---------- 1 300 sec 10 300 sec 100 300 sec 200 300 sec
show mac address-table count: Counts total MACs per VLAN
switch# show mac address-table count VLAN Unicast MAC Count ----- ----------------- 1 100 10 150 100 200 200 300 Total Unicast MAC Addresses in System (excluding CPU) = 750
show mac address-table dynamic: Shows only dynamic MAC addresses
switch# show mac address-table dynamic VLAN Mac Address Type Ports ---------+-------+----------------+------+ 1 0001.6400.1101 dynamic Eth1/1 10 0002.6400.1102 dynamic Eth1/2 100 0050.5678.9abc dynamic Eth1/3 200 0010.5678.9abd dynamic Eth1/4
Show Forwarding Information Commands
show forwarding distribution mac: Summarizes MAC addresses per module
switch# show forwarding distribution mac FWD Distribution table Info: Module 1: Vlan 1: MAC Count: 1000 MAC Miss: 0 Vlan 10: MAC Count: 800 MAC Miss: 0 Module 2: Vlan 1: MAC Count: 900 MAC Miss: 0 Vlan 10: MAC Count: 750 MAC Miss: 0
show forwarding distribution multicast: Multicast MAC forwarding info
switch# show forwarding distribution multicast FWD Distribution Multicast table Info: Module 1: Vlan 1: Multicast Group Count: 10 Group Miss: 0 Vlan 10: Multicast Group Count: 15 Group Miss: 0 Module 2: Vlan 1: Multicast Group Count: 8 Group Miss: 0 Vlan 10: Multicast Group Count: 12 Group Miss: 0
show platform fwmctrl l2: Forwarding manager L2 table details
switch# show platform fwmctrl l2 Info: Table size: 2048 MAC addresses: 1250 MAC address learning: enabled MAC address learning timeout: 300 sec Total Forwarded Packets: 134567890
Debug Commands
show system internal l2fm info: L2 forwarding manager summary
switch# show system internal l2fm info Information about L2FM (Layer 2 Forwarding Manager): MAC Move update interval: 300 sec MAC Move action: trap-and-log Total MAC Moves: 500 Current MAC table count: 1200 Total MAC entries since boot: 5000
show system internal l2fm l2dbg macdb: Detailed MAC address table
switch# show system internal l2fm l2dbg macdb MACDB Entries: -------------------- MAC: 0000.0c9f.f4c8, VLAN: 1 State: dynamic Interface: Eth1/1 Age: 10 sec Flags: local, learned MAC: 0000.0c9f.f4c9, VLAN: 10 State: dynamic Interface: Eth1/2 Age: 20 sec Flags: local, learned MAC: 0000.0c9f.f4ca, VLAN: 100 State: dynamic Interface: Eth1/3 Age: 30 sec Flags: local, learned
show system internal l2fm pfmac stats: Packet forwarding MAC stats
switch# show system internal l2fm pfmac stats Packet Forwarding MAC Statistics: -------------------------------------- Total MAC Additions: 2000 Total MAC Deletions: 1500 Total MAC Moves: 100 Total MAC Updates: 500 MAC Learning Drops: 10
Last but not least, show system internal l2fm l2dbg macdb address. The “show system internal l2fm l2dbg macdb address” command on Cisco Nexus switches provides detailed information about MAC address entries in the layer 2 forwarding information base (L2FIB or MAC address table).
show system internal l2fm l2dbg macdb address 6ae4.29df.f873 vlan 200
Legend
------
Db: 0-MACDB, 1-GWMACDB, 2-SMACDB, 3-RMDB, 4-SECMACDB
Src: 0-UNKNOWN, 1-L2FM, 2-PEER, 3-LC, 4-HSRP
5-GLBP, 6-VRRP, 7-STP, 8-DOTX, 9-PSEC 10-CLI 11-PVLAN
12-ETHPM, 13-ALW_LRN, 14-Non_PI_MOD, 15-MCT_DOWN, 16 - SDB
17-OTV, 18-Debounce Timer, 19-AM, 20-PCM_DOWN, 21 - MCT_UP
22-L2VPN, 23-EFP, 24-DRV 25-ELTM
Slot:0 based for LCS 19-MCEC 20-OTV/ORIB
VLAN: 200 MAC: 6ae4.29df.f873 FE ID: 2
Time If/swid Db Op Src Slot FE HW_ADDR Count
Tue Mar 6 14:18:57 2023 0x1b027000 0 AGE 3 0 2 0
VLAN: 200 MAC: 6ae4.29df.f873
Time If/swid Db Op Src Slot FE HW_ADDR Count
Sun Mar 4 09:37:12 2023 0x1b027000 0 UPDATE 3 0 1 0
Sun Mar 4 09:37:12 2023 0x1b027000 0 UPDATE 2 20 0 0 1
Sun Mar 4 09:37:14 2023 0x1b027000 0 UPDATE 3 0 5 0
Tue Mar 6 14:18:57 2023 0x1b027000 0 DELETE 0 0 15 0
Wed Mar 7 11:45:22 2023 0xffffffff 0 NOT_FOUND_INS_PC 2 20 0 0
Wed Mar 7 11:45:22 2023 0xffffffff 0 INSERT 2 20 0 0
Wed Mar 7 11:45:22 2023 0x1b027000 0 UPDATE 3 0 2 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 3 0 0 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 3 0 1 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 3 0 3 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 3 0 4 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 3 0 5 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 NOT_FOUND_INS_PC 2 20 0 0
Wed Mar 7 11:48:32 2023 0x1b027000 0 UPDATE 2 20 0 0 1
Wed Mar 7 11:48:33 2023 0x1b027000 0 UPDATE 2 20 0 0
Some main points about this command:
- It is a debug-level command for troubleshooting L2 forwarding issues and requires admin-level access.
- It shows detailed MAC address table entries, including MAC address, VLAN, logical and physical interfaces, aging timer, and flags.
- The “address” parameter allows specifying a specific MAC address to look up. If omitted, all MAC addresses are shown.
- It provides more details than the standard “show mac address-table” command, like logical vs physical interfaces and aging timer values.
- The table maps MAC addresses to logical interfaces like port-channels. The actual physical interface is also shown.
- Flags indicate details like whether it’s a static or dynamic entry, if remote vPC peer owns it, or if it’s mapped to an SVI.
- This command is useful when debugging issues like MAC flapping, incorrect forwarding, or flooding due to incomplete MAC tables.
- It can help validate synchronization and ownership of MAC addresses between vPC peers.
So in summary, the show system internal l2fm l2dbg macdb command displays the detailed contents of the L2 forwarding MAC address table on Cisco Nexus switches for troubleshooting purposes. It provides more details than the standard show MAC command.
The show system internal l2fm l2dbg macdb command provides the most comprehensive view of the L2FIB. It displays every MAC entry with details like interface, aging timer, VLAN, flags, etc. This can be useful for debugging MAC flapping, flooding, and learning issues.
Combining the standard show commands with debug-level forwarding info and L2FIB detail allows you to analyze the complete L2 forwarding behavior on Nexus switches. Always start with common show commands first before using debug commands.
Proactively monitoring MAC tables and L2 forwarding is also recommended to baseline expected behavior and more easily detect abnormalities.