Troubleshooting VPN drops between 871 client and 2811

Question

My small company uses a 2811 ISR for VPN services (among other tasks such as internet access, p2p circuits to a second site, etc). I have a couple of remote users that have 871 routers that have occasional problems with their routers dropping their VPN tunnels to the 2811. I'm not really sure where to start with the troubleshooting. There are other clients (such as my own 871W) that seem to maintain a connection for weeks. These remote routers that do drop the connection usually reconnect at their next schedule attempt (180 seconds or so.)
Most of the previous questions I've seen similar to this involve software clients but these are hardware routers as the clients and as such I'm not sure how to enable or retrieve logs for the VPN sessions.

Answer 1

As expected, the isakmp lifetime is 86400, but for ipsec it merely reports how much time is left in the current sa.
For example:
router#show crypto isakmp policy
Global IKE policy
Protection suite of priority 3
encryption algorithm: Three key triple DES
hash algorithm: Secure Hash Standard
authentication method: Pre-Shared Key
Diffie-Hellman group: #2 (1024 bit)
lifetime: 86400 seconds, no volume limit
Default protection suite
encryption algorithm: DES - Data Encryption Standard (56 bit keys).
hash algorithm: Secure Hash Standard
authentication method: Rivest-Shamir-Adleman Signature
Diffie-Hellman group: #1 (768 bit)
lifetime: 86400 seconds, no volume limit
router#show crypto ipsec sa
interface: Virtual-Access4
Crypto map tag: Virtual-Access4-head-0, local addr 209.XXX.XXX.82
protected vrf: (none)
local ident (addr/mask/prot/port): (0.0.0.0/0.0.0.0/0/0)
remote ident (addr/mask/prot/port): (0.0.0.0/0.0.0.0/0/0)
current_peer 75.XXX.XXX.179 port 4500
PERMIT, flags={origin_is_acl,}
#pkts encaps: 681527, #pkts encrypt: 681527, #pkts digest: 681527
#pkts decaps: 670316, #pkts decrypt: 670316, #pkts verify: 670316
#pkts compressed: 0, #pkts decompressed: 0
#pkts not compressed: 0, #pkts compr. failed: 0
#pkts not decompressed: 0, #pkts decompress failed: 0
#send errors 1, #recv errors 0
local crypto endpt.: 209.XXX.XXX.82, remote crypto endpt.: 75.XXX.XXX.179
path mtu 1500, ip mtu 1500, ip mtu idb FastEthernet0/1
current outbound spi: 0xF0C2D65C(4039300700)
inbound esp sas:
spi: 0x2A7171E4(712077796)
transform: esp-3des esp-sha-hmac ,
in use settings ={Tunnel UDP-Encaps, }
conn id: 4093, flow_id: NETGX:2093, crypto map: Virtual-Access4-head-0
sa timing: remaining key lifetime (k/sec): (4577435/1047)
IV size: 8 bytes
replay detection support: Y
Status: ACTIVE
inbound ah sas:
inbound pcp sas:
outbound esp sas:
spi: 0xF0C2D65C(4039300700)
transform: esp-3des esp-sha-hmac ,
in use settings ={Tunnel UDP-Encaps, }
conn id: 4094, flow_id: NETGX:2094, crypto map: Virtual-Access4-head-0
sa timing: remaining key lifetime (k/sec): (4572865/1027)
IV size: 8 bytes
replay detection support: Y
Status: ACTIVE
outbound ah sas:
outbound pcp sas: