DHCP default-router information not being obtained by PC

[Kobra09]

Hi Guys,

Would like to ask for help if someone can enlighten me why the PC on my topology is not getting the 'default-router' information (default gateway) from my DHCP server.

I have attached router R3's (the dhcp server) config here, and a screenshot of my current topology.

Currently, I have indicated two IP addresses available as default-router for my pool CCNAPOOL10 (see attached config). When I try to obtain an IP for PC3, it gets an IP address, but not the default-router info/default gateway IP.

However, if i change the default-router statement to only have one IP address, the PC obtains both IP address and default gateway IP.

PC3 is in VLAN 10 which is in the 192.168.10.0 network.

Anybody has an idea why? Thank you!

[Nithin Eluvathingal]

go through the topic What is a default gateway

 

[Richard Burts]

I am not clear how to understand the drawing that you posted, especially where the gateway & alternate gateway are. But that may not matter much because I do not believe that Cisco IOS supports specification of an alternate default router in configuring a pool for DHCP. If you are interested in providing a second gateway to provide continuing operation if the first gateway goes out of service you should look into HSRP which does provide a way for a second default gateway device to function if the primary gateway goes out of service.

[Kobra09]

Hi Richard, 

Thanks for replying. Sorry forgot to include that the default router info I have configured in R3 are the SVIs for VLAN 10 in RS1 and RS2 L3 switches (pls refer to topology). I intend to have them as the default gateways.

According to CISCO documentation, you can provide multiple default router IPs:

default-router address [address2 ... address8]

(Optional) Specifies the IP address of the default device for a DHCP client.

-The IP address should be on the same subnet as the client.
-One IP address is required; however, you can specify up to eight IP addresses in one command line. These default devices are listed in order of preference; that is, address is the most preferred device, address2 is the next most preferred device, and so on.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/15-sy/dhcp-15-sy-book/config-dhcp-server.html

[Richard Burts · Accepted solution]

One of the reasons that I continue to be active in the community is that I learn new things (and occasionally learn that something that I believed to be true is not true - at least on some platforms). Today is one of those days. The documentation that you post is clear that multiple default routers are supported. So the question is why does it not work on your router? I have 2 suggestions:

- perhaps it is supported on some platforms and not on others? What model of router is yours? Does the document that you posted apply to that model of router?

- perhaps it is a bug in the software?

If you have a service contract I would suggest that you open a case with Cisco TAC about this. 

If you do find more information about this issue please do update us.

[edit] Another possibility occurred to me. We do not know if this is being done on real equipment or is done on some emulator. If it is on an emulator it is likely that you have found another of the situations where the emulator does not accurately implement some of the commands.

[Kobra09]

Hi Richard,
Thanks and that is really great to hear. This community is really my go-to when it comes to networking help coz I have little to no source of help on my end but the internet. LOL
I am running this on GNS3 and I have to agree that this may be 1 of the 2:

- perhaps it is supported on some platforms and not on others? What model of router is yours? Does the document that you posted apply to that model of router?
- If it is on an emulator it is likely that you have found another of the situations where the emulator does not accurately implement some of the commands.

I'd have to check the release notes on that one but for now I'll have to settle with just 1 default-router info. :)

Thank you so much!

[Richard Burts]

Thanks for the additional information. Especially if this is GNS3 I think  it is not worth any more effort. Just configure a single default router and watch it work. It was an interesting discussion and I thank you for showing me something that I did not know. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[Kobra09]

I would agree on that :) I just settled with 1 default-router IP.
Thanks and great to know I was of any help! Those are kind words especially for me who's new to this community.
I really appreciate you and everyone in here helping one another. God bless everyone!