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waforbes100
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DHCP Scope Limitation for Cisco 851

Note:
1. I am using a Cisco 851 router for DHCP
2. I am leasing a 25-bit subnet mask (126 available addresses)
3. The Cisco851 router refuses to lease more than 48 addresses for this scope
4. Additional devices can only get an IP address when one current host is powered off.
5. Note that "sh ip dhcp pool" shows only 48 leases from a pool of 126 IP addresses.
Is this a default behavior that can be fixed by configuration?
RoutersDHCP

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waforbes100

8/22/2022 - Mon
memo_tnt

Hi

be sure you DHCP configuration looks like this one::


ip dhcp pool sdm-pool
  import all
   network 10.10.10.0 255.255.255.128
   default-router 10.10.10.1
   dns-server 8.8.4.4
   lease 0 2
rochey2009

hi,

please post the relevant part of the configuration.

Thanks.
waforbes100

ASKER
Is the following true: without the "import all" command, a DHCP scope's IP range will only be partially used?
That is the symptom I am having: only ~40 percent of the scope is being used.
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waforbes100

ASKER
Please note: I am not using a DHCP server database. I just found out about it!
How does this fact change the 'equation'?
memo_tnt

it imports (DHCP) option parameters into the DHCP server database ..

waforbes100

ASKER
I am deducing that the "import all" command is not related to the issue I am having:
1. My Cisco DHCP server is not giving more than 48 out of 126 IP addresses.
2. When I restart host1, host2 immediately grabs the IP address of the host1 (which is being restarted).
3. Once the restarted host1 comes back up, it cannot get an IP address.
4. When I now restart host2, then host3 (another different host), grabs the IP address of host2.
5. Host2 is no longer able to get an IP address ... Cycle repeats.
Conclusion: There is serious IP contention even though there 78 more available addresses.
My apologies if I was not clear on the nature of my problem.
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rochey2009

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memo_tnt

may one of your PC is configured to get ip manually that conflicts with other ..
send
#sh arp

waforbes100

ASKER
Note the following:
1. I am unable to provide "show ip dhcp binding" results (taken during troubleshooting) because of potential for company exposure.
2. Similar is true for "show ip dhcp conflict" results
3. However, note that the number of automatic bindings appearing after router restarts changed from 180 (original count) to 22 (after 1st restart), and then to 24 (after 2nd restart).
4. No hosts are configured for manual IP addressing in the IP range (manual IP hosts are in different  ranges and subnets).
5. I can't provide "show arp" results because this was not taken during troubleshooting: the routers had been removed from production network (powered off) and replaced with the pre-existing solution on the same  day of project 'failure'.
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rochey2009

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waforbes100

ASKER
Are these debugs dangerously processor intensive?
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rochey2009

The debug will occur when a dhcp request happens. DHCP requests aren't frequent so it shouldn't impact the router. Saying that, debugging makes me anxious, so you could always try this when your user community has gone home.
waforbes100

ASKER
Hosts had IP addresses but router did not show all addresses in results of "show ip dhcp binding" when restarted: I need a dhcp server database to store the bindings (leases) so that I can centrally keep track of all hosts with an IP.
Additionally, some hosts were holding onto the old IP addresses they had from the previous DHCP server.