Arista EOS MAC Address Table: Show Commands and Aging

When a host "is on the wrong port" or traffic to a quiet server suddenly floods, the Arista EOS MAC address table (FDB) is where the truth lives: which MAC was learned on which VLAN and port, whether it is dynamic or static, and how long since it moved. Two platform facts frame every FDB problem: EOS ages dynamic MACs after 300 seconds by default, while the ARP timeout runs 4 hours - so a quiet host can lose its MAC-to-port mapping while its IP-to-MAC entry persists, producing intermittent "unreachable" behavior. This guide covers the essential show mac address-table commands, aging/static management, and the common failure modes.

Reading the MAC Address Table

switch# show mac address-table
Mac Address Table
------------------------------------------------------------------
Vlan    Mac Address       Type        Ports      Moves   Last Move
----    ------------      ----        -----      -----   ---------
10      0000.0000.0001    DYNAMIC     Po67       1       0:00:56 ago

Purpose-built queries:

show mac address-table vlan 10
show mac address-table address aaaa.bbbb.cccc
show mac address-table interface Ethernet1
show mac address-table | include 

The Moves/Last Move columns are gold for troubleshooting: a MAC that keeps moving between two ports (or shows a high move count) indicates a loop, a bridge misconfiguration or two devices sharing one MAC.

Dynamic Entries, Aging and the FDB/ARP Gap

• Dynamic entries are learned from source MACs and age out (default 300 s on EOS). A host that sends nothing for 5 minutes is removed from the table; the switch then floods its unicast traffic until the host sends again - which matters for servers with aggressive power-saving NICs.
• The ARP cache on the router/host side can still point at the same IP for hours (4-hour ARP timeout is common), so from the host's perspective nothing changed while the switch no longer knows where the MAC lives. Cross-check show ip arp <IP> against show mac address-table address <MAC> when "endpoint location" seems wrong.
• Change aging globally with mac address-table aging-time <seconds> (and disable with 0 only deliberately - entries then never expire until the table fills).

Static and Blackhole Entries

switch(config)# mac address-table static aaaa.bbbb.cccc vlan 10 interface Ethernet1
switch(config)# mac address-table aging-time 600
switch# clear mac address-table dynamic        ! flush learned entries
switch# clear mac address-table dynamic interface Ethernet1
switch# clear mac address-table dynamic address aaaa.bbbb.cccc

Static entries survive reboots and are unaffected by aging - use them sparingly for infrastructure MACs (or for security, where Cisco's equivalent tooling is port security/sticky MAC, covered in our Cisco port security guide). Clearing the dynamic table is the standard move after moving a cable or fixing a loop: it forces re-learning and drops stale forwarding state immediately.

Troubleshooting Scenarios

• MAC missing on the expected switch: it aged out (host silent > 300 s) or moved - check show mac address-table address across the path and the ARP table for the IP.
• MAC flapping between ports: look for an L2 loop or two switches bridging the same segment; the Moves counter quantifies it.
• Traffic flooding to all ports: the destination MAC is not in the FDB (aged out, or the host is behind a different VLAN than expected - verify with show vlan and the VLAN/SVI configuration guide).
• MAC on a port-channel: the port column shows PoNN - the MAC learned through the bundle, confirming the port-channel/LACP config carries that VLAN.

原文链接：https://netdata.cloud/guides/network/network-fdb-mac-staleness · Arista EOS User Manual: MAC Address Tables