ISAKMP SA is not clearing even Remote Router is down IPSEC- LAN to LAN

[arun428750]

IPSEC_LAN-LAN: ISAKMP-SA is not clearing for some of the site even peer router is down. 
But still HO Router is sending--> ISAKMP<206> Identity Protection (Main Mode).

I have DPD: crypto isakmp keepalive 40 20 periodic

But still observing HO Router is keep sending packets for IKE1. Keepalive is configured globally to the HO

[MHM Cisco World]

Hi, I do the test, 
if you config keepalive after the ISAKMP is active the keepalive not effect, 
try clear isakmp and then check the keepalive again. 

[arun428750]

Hi Sir, Keepalive is set only in HO Router and its global configuration. Keepalive already configured in HO Router in beginning and we have more than 400 remotes. 

[MHM Cisco World]

""In order for ISAKMP keepalives to work, both VPN endpoints must support them.""

https://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/81824-common-ipsec-trouble.html#solution07