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New Member

HSRP and helper-address

We are using the following simple scenario:

- PCs should get their IP parameters via DHCP Clients

- The local routers are running HSRP

Therefore, I have to configure a helper-address. Which ip address is inserted by the routers as dhcp relay agent address (proxy address) in the clients dhcp requests:

- the ip address of the interface at which the requests are received

- the standby ip address used for this segment?

Thanks

Edgar

9 REPLIES

Re: HSRP and helper-address

The initial client request is a broadcast and the client assumes that the DHCP server is on the same network. Using an IP helper or dhcp-relay, the router(s) forward these requests to the DHCP server. In this way the initial broadcast is forwarded as a unicast, destined for the dhcp-server.

The default-gateway for the client must be the HSRP-adress for the lan that the PC is on. This is specified in the DHCP-scope for the network.

New Member

Re: HSRP and helper-address

Thanks for your reply, but this is not the answer to my question.

The DHCP server has to select the right scope. Therefore, the ip address of the dhcp relay agent ist used. Unfortunatly the active router has two ip addresses: interface address (e. g. 172.16.1.1) and hsrp address (e. g. 172.16.1.100). Which of both is inserted in the relay agent field of the clients dhcp requests???

Thanks in advance

Edgar

New Member

Re: HSRP and helper-address

Your HSRP should have a standby IP address which is the virtual address to the number of physical addresses configured.

We have the smae nev here and here is how it work.

A standby ip of 172.16.1.100 is conifigured as the 003-Router address in your scope on your DHCP Server. The scope will probably belong to a vlan number ( if you are using this).

The ip helper-address on the vlan is given the address of the DHCP Server.

U then configure the client port for this VLAN and when the client boots up, it looks to the router, reads its helper address ( DHCP Server) and then goes to the DHCP Server, checks it scope, sees its rouer address and the picks up all configuration set up for the scope in the DHCP Server.

Hope this helps.

New Member

Re: HSRP and helper-address

Sorry, but it is me again. I know how this stuff works and how to configure it. I am only interested in the ip address which is inserted by the router in the relay agent field of the dhcp request messages: The ip address of the interface (e. g. fa0/0) or the virtual ip address of the hsrp group?

Thanks

Edgar

New Member

Re: HSRP and helper-address

My guess would be the hard coded non-HSRP address, but as long as your "real" IP addresss and the HSRP address are in the same subnet, it shouldn't matter to the DHCP server.

New Member

Re: HSRP and helper-address

... that is definitly right.

Thanks

Edgar

Re: HSRP and helper-address

Edgar,

The router forwards the request using the primary IP address of the interface. See below capture output. My IP of LAN is 192.168.1.10. HSRP Standby is 192.168.1.200.

Additionally, HSRP standby addresses do not listen to broadcasts. So if multiple standby routers have IP helper defined then they all will forward the request.

Capture:

Bootstrap Protocol

Message type: Boot Request (1)

Hardware type: Ethernet

Hardware address length: 6

Hops: 1

Transaction ID: 0x9136c75e

Seconds elapsed: 0

Bootp flags: 0x0000 (Unicast)

0... .... .... .... = Broadcast flag: Unicast

.000 0000 0000 0000 = Reserved flags: 0x0000

Client IP address: 0.0.0.0 (0.0.0.0)

Your (client) IP address: 0.0.0.0 (0.0.0.0)

Next server IP address: 0.0.0.0 (0.0.0.0)

Relay agent IP address: 192.168.1.10 (192.168.1.10)

Client hardware address: 00:40:96:35:df:a3

Server host name not given

Boot file name not given

Magic cookie: (OK)

Option 53: DHCP Message Type = DHCP Discover

Option 61: Client identifier

Hardware type: Ethernet

Client hardware address: 00:40:96:35:df:a3

Option 50: Requested IP Address = 192.168.0.163

Option 12: Host Name = "sonylap"

Option 60: Vendor class identifier = "MSFT 98"

Option 55: Parameter Request List

1 = Subnet Mask

15 = Domain Name

3 = Router

6 = Domain Name Server

44 = NetBIOS over TCP/IP Name Server

46 = NetBIOS over TCP/IP Node Type

47 = NetBIOS over TCP/IP Scope

43 = Vendor-Specific Information

77 = User Class Information

End Option

Padding

New Member

Re: HSRP and helper-address

From the HSRP point of view, you will have to use the standby ip address.

Cheers

New Member

Re: HSRP and helper-address

I have debugged DHCP requests, what i see is the ip address which is active in HSRP group is added in relay agent field.So ip address of the interface at which requests are received is added not of standby.

I hope this clears your doubt.

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