Have you ever watched the holdtime tick tock down so that your FDP neighbors age out? Here is a scenario where there was a loop in the network. I shut down one of the interfaces where the loop was (because I was not onsite) so that the loop would at least be logically taken out. However, with FDP, until the holdtime ages down to '0', it keeps the entry in the results of the "show fdp neighbors" command. See below.
telnet@BrocadeSwitch1(config)#sh fdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
(*) indicates a CDP device
Device ID Local Interface Holdtm Capability Platform Port ID
-------------- ---------------- ------ ---------- ----------- -------------
BrocadeSwitch2 ethernet1/1/3 22 Switch ICX6430-24P ethernet1/1/4
BrocadeSwitch2 ethernet1/1/24 142 Switch ICX6430-24P ethernet1/1/24
CoreSwitch ethernet1/2/1 167 Router FastIron SX ethernet4/21
telnet@BrocadeSwitch1(config)#sh fdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
(*) indicates a CDP device
Device ID Local Interface Holdtm Capability Platform Port ID
-------------- ---------------- ------ ---------- ----------- -------------
BrocadeSwitch2 ethernet1/1/3 15 Switch ICX6430-24P ethernet1/1/4
BrocadeSwitch2 ethernet1/1/24 135 Switch ICX6430-24P ethernet1/1/24
CoreSwitch ethernet1/2/1 160 Router FastIron SX ethernet4/21
telnet@BrocadeSwitch1(config)#sh fdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
(*) indicates a CDP device
Device ID Local Interface Holdtm Capability Platform Port ID
-------------- ---------------- ------ ---------- ----------- -------------
BrocadeSwitch2 ethernet1/1/3 5 Switch ICX6430-24P ethernet1/1/4
BrocadeSwitch2 ethernet1/1/24 125 Switch ICX6430-24P ethernet1/1/24
CoreSwitch ethernet1/2/1 150 Router FastIron SX ethernet4/21
telnet@BrocadeSwitch1(config)#sh fdp neigh
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
(*) indicates a CDP device
Device ID Local Interface Holdtm Capability Platform Port ID
-------------- ---------------- ------ ---------- ----------- -------------
BrocadeSwitch2 ethernet1/1/24 120 Switch ICX6430-24P ethernet1/1/24
CoreSwitch ethernet1/2/1 145 Router FastIron SX ethernet4/21
Notice in the last 'show fdp neighbors', the first entry is gone.
