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This is one of multiple videos discussing dynamic host configuration protocol or DHCP, and specifically

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how to troubleshoot DHCP. In this topology

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we've got a client connected to a router connected to a DHCP server, and we've been told that the client

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is not receiving IP addresses from the DHCP server. In this example, rather than using physical PCs

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I've got iOSV Routers acting as our DHCP server and DHCP client.

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So here's the client, show IP interface brief.

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Notice the client hasn't got an address assigned to gigabit00 DHCP is being used

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but no address has been assigned. So let's shut the interface down and then no shut it

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to test what's happening.

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Interfaces come up, show IP interface, brief interfaces, up up no address has been assigned through

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DHCP. So on the DHCP server, let's run a debug, so debug IP DHCP

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server, and I'm going to go for packets to see detailed information. We're already seeing something 

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here.

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Says clients VPN is something no option 125 DHCP Discover received from a client through a relay of 10.

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1.1.254.

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So a DHCP message is being relayed by the router.

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We can see in this example that router 2 has this IP address on the interface facing the client,

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and that's the address that we see on the DHCP server

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but we're told that option 125 is not present in the message.

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There's a problem with option 125.

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You can find information about option 125 in this RFC, vendor identifying vendor options for DHCP.

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The problem here is that option 125 is vendor-dependent.

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So we're seeing a lot of messages stating that Option 125 is not present in the message.

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So let's have a look at the relay agent show run interface Gigabit00.

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Now, we do see the IP helper address on the relay agent, which is router 2 but we're still seeing problems

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on the DHCP server.

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We're getting information at the DHCP server based on this configuration. So we can ping the DHCP server,

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DHCP messages are being forwarded to the server, but there's a problem on the DHCP server.

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So let's have a look at the DHCP server configuration.

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We have a pool configured.

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This looks right.

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Network default gateway or default router.

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That needs to be in the same subnet as the interface where the client is configured.

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So that looks right.

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DNS server is 10.1.2.1 

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that is the IP address of the DHCP server, so that looks right, there are no excluded addresses in

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the pool.

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So we've got a very basic pool and an IP address configured on the gigabit interface.

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Everything else looks standard, but I'm going to go through the show run slowly to see if you can

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find the problem.

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What is the issue in this configuration?

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Pause the video if you need to, but there's a problem in this config show, IP, DHCP, Pool.

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We have a pool next address to be allocated is 10.1.1.3 show IP, DHCP binding.

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There is an address allocated to a client somewhere, but no address has been allocated to this client.

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Now, this is something you need to remember for DHCP notice what happens when I type show IP route.

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We're told that there's no gateway of last resort, there's no routing on the DHCP server.

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The DHCP server has to have a route back to the subnet of the relay

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agent, router 2 is receiving a DHCP broadcast on Gigabit 00.

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But the DHCP server doesn't know how to get back there, so notice here, it can't get to that subnet because

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it doesn't have a route in the routing table.

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So IP route and I'll simply create a default route and point it to this IP address on router 2 which

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is gigabit01.

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So on the client

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show IP interface brief, no IP addresses allocated yet, I'll force this by shutting the interface

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down and then no shutting it.

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Show IP interface brief

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let's see if we get an IP address now and there we go, we've just been allocated an IP address by the

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DHCP server.

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This address has been allocated to the client.

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Now, the reason that the binding was shown previously is that this address was allocated previously

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to the client and then I broke the network.

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So the same address has been allocated back to the client now since we fixed the network.

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So this is the thing about DHCP servers on Cisco.

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Make sure that they have a route back to the client.

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In other words, they need to know how to get to the relay agent's IP address, otherwise they will

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not allocate addresses to clients.

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So now

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we have a default route, and if we look at the binding, we should be able to ping the client and there

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you go, we can ping 10.1.1.2.

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So that was an example of how to troubleshoot DHCP.

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Don't forget that your DHCP servers need rocks back to the relay agents in your topology.

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I hope you found this video useful.

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If it's been of benefit to you, please like it and please subscribe to my YouTube channel.

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I wish you all the very best.