Cisco – Does Router automatically exclude default-router address from DHCP leasing

Lets assume you want to deliver IP addresses to your interface Gi0/0 (addr 192.168.0.1) network 192.168.0.0/24, also you have DNS server 192.168.0.5. Then you should define DHCP pool with options:

service dhcp
ip dhcp pool NET_192.168.0.0
 network 192.168.0.0 255.255.255.0
 default-router 192.168.0.1
 dns-server 192.168.0.5
 lease 7
 domain-name net.local

This will enable DHCP service for your 192.168.0.0/24 network with lease time for each address in 7 days.

Also, you can exclude IP addresses from distributed scope by entering command:

ip dhcp excluded-address 192.168.0.1 192.168.0.10

This will prevent issuance of addresses 192.168.0.1 thru 192.168.0.10. You can add many of that entries. Exclude records can consist of only one address:

ip dhcp excluded-address 192.168.0.200

You can find more about configuring DHCP server in Cisco IOS here - Configuring DHCP

I'm going to post this as a possible solution for people to shoot holes in, and/or suggest how to clean it up. it works in the GNS3 simulation with IOSv images.

Let me first explain what happens to me a bit more clearly, and the rationale for all the complexity.

First for clarity -- I am trying to track against a distant IP, not the next hop, because (a) on DHCP don't know the next hop, and (b) sometimes connectivity problems are after the next hop.

There are simple techniques to achieve this if you do not have DHCP -- TRACK the distant item on separate statements with each next-hop IP explicit, and everything works, and you set precedence with distance.

There is no syntax I have found to use IP ROUTE...TRACK with DHCP and distinguish next-hop's from different interfaces. All the variations I have found end up in a chicken and egg problem - the track can't operate as the SLA has no route to let it reach a distant IP. You can specify ROUTE TRACK with the interface DHCP CLIENT statement, but I cannot figure out any way to make the SLA use that particular interface's next hop. Same with the verify-reachability, at least with all syntax I have found.

If the two DHCP interfaces have unequal metrics, to give one a preference, what I find happens is this (in a straightforward setup): both TRACK statements come up, but despite the SOURCE INTERFACE on them (which is just setting an address), both are going out the preferred default route. Thus when the preferred route is broken (i.e. I stop traffic on it), the route stays as preferred, and BOTH tracks go down. Source interface is not specifying the egress interface

This can partially be fixed by giving them equal preference; then both routes to 0.0.0.0 are installed in the rib, and sometimes (but not always) the two TRACK's will use their interface instead of the opposite and operate independently. Unfortunately not always; not sure why, but by observation sometimes both tracks as written above go out the same interface even with equal preference on the then default gateway.

So this proposed solution uses a route-map to force the traffic for each SLA (using different target IP's) onto a specific egress interface. If this is done with unequal preference default gateways it also fails -- the route map itself fails on the packet for the non-preferred interface; I am unclear why, it just rejects it and says "normal forwarding". I assume it's validating against the rib? But with equal preference sla6 is forced to gi0/6, and sla7 is forced to gi0/7, and work independently.

What I do not know is whether this use of a SET INTERFACE will work generally in this specific cast, i.e. is the packet using the proper next hop, or a proxy arp. I tried looking at it with wireshark but had some issues getting it to run on the simulation. I'll confirm later. I'm hoping it uses the rib for the next hop with the selection of which path made by the route map.

But it's complicated, a bit contrived, and I really would expect there to be an easier solution. The other problem is that it only works, as written, to provide preference for the tunnel, not general internet traffic from inside (that's OK in this case as that was not the intent, but I want to experiment further with additional route map clauses, perhaps based on tracking there, to prefer general traffic also).

I would sure like someone to provide a better way. And/or shoot holes in this one if it won't work.

As above, only the route preference related items shown.

PS. All the non-private addresses are simulated, not used in reality.

track 6 ip sla 6 reachability
track 7 ip sla 7 reachability

crypto ikev2 client flexvpn FLEXVPN_IKEV2_CLIENT
  peer 1 9.9.9.2
  source 1 GigabitEthernet0/7 track 7       <<< precedence established here, only for tunnels
  source 2 GigabitEthernet0/6 track 6
  client connect Tunnel31

interface Tunnel31
 ip unnumbered Loopback192
 ip nat inside
 tunnel source dynamic
 tunnel mode ipsec ipv4
 tunnel destination dynamic
 tunnel protection ipsec profile IKEV2_IPSEC_PROFILE
!

interface GigabitEthernet0/6
 ip dhcp client default-router distance 100  << must be the same as below or omitted
 ip address dhcp
 ip nat outside
!
interface GigabitEthernet0/7
 ip dhcp client default-router distance 100     << must be the same as above or omitted
 ip address dhcp
 ip nat outside
!
ip local policy route-map CHOOSE-ISP

ip sla 6
 icmp-echo 74.74.74.74 source-interface GigabitEthernet0/6
 tag Cellular connection up test 
 frequency 10
ip sla schedule 6 life forever start-time now

ip sla 7
 icmp-echo 75.75.75.75 source-interface GigabitEthernet0/7
 tag Local ISP connection up test 
 frequency 10
ip sla schedule 7 life forever start-time now
!
route-map CHOOSE-ISP permit 10
 match ip address 106
 set interface GigabitEthernet0/6   <<< give warning 
!
route-map CHOOSE-ISP permit 20
 match ip address 107
 set interface GigabitEthernet0/7   <<< give warning 
!
access-list 6 permit 74.74.74.74
access-list 7 permit 75.75.75.75