DHCP: Configuring a Static Route with the Next-Hop Dynamically Obtained

[jaighobahi]

Here I have an excerpt from a Cisco documentation:

The following example shows how to configure two Ethernet interfaces to obtain the next-hop router IP address from the DHCP server:

ip route 10.10.10.0 255.255.255.0 dhcp 200
ip route 10.10.20.1 255.255.255.255 ethernet 1 dhcp

The first command is quite clear, but the second command is not.  I thought host-only networks are only used with loopback interfaces.  I do not understand the prefix and prefix mask in the second command.  Please, enlighten me.

[Bilal Nawaz]

This route is also known as a "host route". A single route to a host via the next hop which is set. Please share the documentation, because there may be some context as to what the literature was trying to explain.

[jaighobahi]

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/12-4t/dhcp-12-4t-book/config-dhcp-relay-agent.html 

If you look at the complete book, the excerpt above is on page 52.

[jaighobahi]

R2#conf term
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#ip route ?
  A.B.C.D  Destination prefix
  profile  Enable IP routing table profile
  static   Allow static routes
  vrf      Configure static route for a VPN Routing/Forwarding instance

R2(config)#ip route 10.10.20.1 ?
  A.B.C.D  Destination prefix mask

R2(config)#ip route 10.10.20.1 255.255.255.255 ?
  A.B.C.D            Forwarding router's address
  Async              Async interface
  BVI                Bridge-Group Virtual Interface
  CDMA-Ix            CDMA Ix interface
  CTunnel            CTunnel interface
  DHCP               Default Gateway obtained from DHCP
  Dialer             Dialer interface
  FastEthernet       FastEthernet IEEE 802.3
  Lex                Lex interface
  Loopback           Loopback interface
  MFR                Multilink Frame Relay bundle interface
  Multilink          Multilink-group interface
  Null               Null interface
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface
  Virtual-PPP        Virtual PPP interface
  Virtual-TokenRing  Virtual TokenRing

R2(config)#ip route 10.10.20.1 255.255.255.255 fastetehrnet ?

R2(config)#ip route 10.10.20.1 255.255.255.255 fastethernet ?
  <0-0>  FastEthernet interface number

R2(config)#ip route 10.10.20.1 255.255.255.255 fastethernet 0 ?
  <1-255>    Distance metric for this route
  A.B.C.D    Forwarding router's address
  DHCP       Default Gateway obtained from DHCP
  name       Specify name of the next hop
  permanent  permanent route
  tag        Set tag for this route
  track      Install route depending on tracked item
  <cr>

R2(config)#ip route 10.10.20.1 255.255.255.255 fastethernet 0 dhcp ?
  <1-255>  Distance metric for this route
  <cr>

R2(config)#

Please, I need to be clear about this.  Does 10.10.20.1 255.255.255.255 refer to the IP address of the FastEthernet interface of R2 or does it refer to the IP address of some remote host such as that shown in the figure below?

If 10.10.20.1 is not an IP address on R2, and 10.10.20.1 is not a network address then 10.10.20.1 would not be in any of the routing tables.  So, how would the Default_Router know to reach host 10.10.20.1?

[jaighobahi]

Apology for resurrecting this thread.  I will very much appreciate it if the command

Router(config)# ip route 209.165.200.225 255.255.255.255 dhcp

can be explained in a way that a newbie can understand.  I thought a static route command needs to specify a remote network address and the mask associated with the network address.  The command above looks like a host-only network.  Does a host-only network have a next hop?  Thanks.

[Bilal Nawaz]

I'll try to explain. A static route to a remote subnet can have any suitable netmask, whether it be a host route or not, the above command, next hop is the DHCP default gateway that the router receives from the dhcp server.

It doesnt have to be host only, all static routes need a next hop or next exit interface, the router needs to know where to forward packets to.

[jaighobahi]

Thanks a lot Mr. Nawaz.  I need one more confirmation.  Is the IP address 209.165.200.255 the address of the exit interface or is it the address of an interface on a remote router?

[Bilal Nawaz]

it is the remote network. I'll break it down for you here

IP ROUTE [remote network address] [remote network subnetmask] [via next hop e.g. interface, x.x.x.x or dhcp]

So we tell the router, the remote network address, followed by the remote network mask, then we tell the router which way to go, or who to go to, to get to this network.

[jaighobahi]

Thanks once again.  My confusion lies in the fact that 209.165.200.255 255.255.255.255 specifies an IP address rather than a network address.  Does it not matter?

[Bilal Nawaz · Accepted solution]

Nope, it doesnt matter :-) i'd rather call it a prefix rather than ip address. It is a host route, with /32 netmask.

All you are doing is telling the router where the specified "prefix" is. It can be a host route or a network route. It doesn't matter, as long as you set the correct remote subnet and mask.

If I only wanted to get to a particular host for whatever reason, I would put a host route in exactly as you have stated above.

A prefix could be any length.

[jaighobahi]

I appreciate your patience!

[paul driver]

Hello

"host-only networks "

As Bilal  correctly says that is static pertaining a host iand its just stating how to reach that host... then go via this Ethernet interface.

32 bit as in the second static is  specifying a single host and not a network

Alsothe administrative distance has been specifically lowered on these statics to a value of 1 and 200 respectively  instead of the default value this router obtains from the dhcp allocation which is 254 so to make it more preferable path..

res

Paul