arp entry aging-out and becomes incomplete

[Andriy Sidko]

Hi guys.

I have following schema:

cisco router as DHCP server - vlan12 (guest wifi) <-> - dot11 0.70 - dot11 0 - bvi70- client cisco 881w

client got IP address 192.168.172.52 from server. DHCP lease expires in 1:41 PM (13:41)

Current time 11:41:21

+++++++++++++++++

gate#sh clo
11:41:21.714 EST Thu Sep 4 2019
gate#

gate#sh ip dhcp bin | i 172.5
192.168.172.52 0164.00f1.77b7.4a Sep 04 2019 1:41 PM Automatic
gate#

+++++++++++++++++

from router ARP cache I see ARP entry for 192.168.172.52 will expire in 32 minutes unless renewal (red bellow): 

+++++++++++++++++

gate#sh arp detail | s ARP entry for 192.168.172.5
ARP entry for 192.168.172.52, link type IP.
Dynamic, via Vlan12, last updated 1 minute ago.
Encap type is ARPA, hardware address is 6400.f177.b74a, 6 bytes long.
ARP subblocks:
* Dynamic ARP Subblock
Entry will be refreshed in 33 minutes and 37 seconds.
It has 2 chances to be refreshed before it is purged.
Entry is complete.
* IP ARP Adjacency
Adjacency (for 192.168.172.52 on Vlan12) was installed.

++++++++++++++++++++++

in 30 minutes router clear was trying to refresh arp and in to unsuccessfull attempts

++++++++++++++++++++++
gate#sh arp detail | s ARP entry for 192.168.172.5
ARP entry for 192.168.172.52, link type IP.
Dynamic, via Vlan12, last updated 36 minutes ago.
Encap type is ARPA, hardware address is 6400.f177.b74a, 6 bytes long.
ARP subblocks:
* Dynamic ARP Subblock
Entry will be purged in 0 minute and 1 second.
Entry is complete.
* IP ARP Adjacency
Adjacency (for 192.168.172.52 on Vlan12) was installed.
gate#
gate#
gate#
gate#sh arp detail | s ARP entry for 192.168.172.5
ARP entry for 192.168.172.52, link type IP.
Incomplete, via Vlan12, last updated 0 minute ago.
Encap type is ARPA, hardware address is 0000.0000.0000, 0 bytes long.
ARP subblocks:
* Dynamic ARP Subblock
Entry is incomplete.
gate#
gate#sh arp | i 172.5
Internet 192.168.172.52 0 Incomplete ARPA
gate#
++++++++++++++++++++++

how to syncronize DHCP lease with ARP cache?
Lease still ok for 2 hours but arp entry just for 32 minutes.

Could you guys suggest?

Thank you.

[marce1000]

                >how to syncronize DHCP lease with ARP cache?
>Lease still ok for 2 hours but arp entry just for 32 minutes.

 There's no correlation needed between these 2 factors, ARP cache will be updated automatically when the device

 tries to communicate again through the router.

 M.

[Richard Burts]

I don't fully understand the schema described. It has some Cisco router (no identification of what kind) at one end and an 881W at the other end. In between there is a vlan 12, a bvi 70, and some other stuff. I hope we can assume that all in the middle is just layer 2 stuff that would not affect either the DHCP information or the arp information. Then there are some observations about arp timeout and DHCP lease time. I can not tell whether there is some problem here or if this is just trying to understand the respective timers. I agree with the suggestion that there is no need to try to synchronize the arp timer and the DHCP lease timer. Each protocol acts independently. A Cisco router creates an arp entry when it has received an IP packet to forward to that IP address. Once the arp entry is created we do not want it to stay in the table for ever (at some point a device may power down, or move, or some way drop off the network). So the Cisco router has a timer for the entry. As the timer gets close to the end (but before the entry expires) the router will arp for the destination again. If the router receives a response then the device is still alive and connected to the network and the router keeps the entry and starts a new timer. If there is not a response to its arp request then the router changes the arp entry to incomplete (essentially this is a place holder for the device). If the router receives a delayed response then the entry is completed as normal and a new timer is started. If no delayed response is received after a short time the incomplete arp entry is removed from the table. I do not see any advantage in trying to match up the arp timer with the DHCP lease time. If there is some aspect of this situation that I do not understand correctly then please provide clarification.

HTH

Rick