Static route with next-hop as dhcp-router but without default-route

Alexandre SEITZ · ‎12-07-2015

Hello

I am facing an architecture issue:

I am cascading a CISCO Router (892FSP, running IOS 15.2.4M8)) behind a Internet Box, this Internet Box is serving ip address/mask/default router dynamically thank to DHCP server (I have no access to configuration of this Internet Box).

My CISCO router has only 2 interfaces:

- 1 connected to the Internet box (lets call it the WAN interface)

- 1 connected to host (lets call it the LAN interface)

I would like to :

- retrieve dynamically an ip address+mask on my WAN interface: so far, it is easy with "ip address dhcp"

- prevent the CISCO router to install a default route to the DHCP router (DHCP option 3)

- install manually a static route /32 with next-hop = DHCP router

I tried to configure : "no ip dhcp client request router" on the DHCP interface, it removes DHCP option 3 from the DHCP Discover request, it prevents the router from installing the default route, but in the meanwhile, it prevent me from using commande "ip route A.B.C.D 255.255.255.255 dhcp", I really would like to keep dynamicity of DHCP, I need a generic configuration.

Any idea, remarks are welcome.

Thanks

Alex

Philip D'Ath · ‎12-07-2015

To load the /32 use a command like:

ip route a.b.c.d 255.255.255.255 DHCP

The "DHCP" keyword says to use the DHCP next-hop address address.

Alexandre SEITZ · ‎12-08-2015

Hello,

Thanks for your reply, but the main issue is the following:

What I expect :

I expect to have in my routing table following routes: (see following code)

- assuming 192.168.1.1 is the next-hop of my default route learnt with DHCP

- 192.168.1.129 is the address assigned by my DHCP server

NR-892FSP-1#sh ip route 
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set

      A.0.0.0/32 is subnetted, 1 subnets
S        A.B.C.D [1/0] via 192.168.1.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet8
L        192.168.1.129/32 is directly connected, GigabitEthernet8
.....................
...................

Test 1

!
interface GigabitEthernet8
 ip address dhcp
 load-interval 30
 duplex auto
 speed auto
end
!
ip route A.B.C.D 255.255.255.255 dhcp
!

Here is the routing table after DHCP process completes:

NR-892FSP-1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is 192.168.1.1 to network 0.0.0.0

S*    0.0.0.0/0 [254/0] via 192.168.1.1
      1.0.0.0/32 is subnetted, 1 subnets
S        1.1.1.1 [1/0] via 192.168.1.1
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet8
L        192.168.1.129/32 is directly connected, GigabitEthernet8

But I don't want the gateway of last resort to be set

Test 2

!
interface GigabitEthernet8
 no ip dhcp client request router 
 ip address dhcp
 load-interval 30
 duplex auto
 speed auto
end
!
ip route A.B.C.D 255.255.255.255 dhcp
!

the new CLI command ask the router to not request the DHCP option 3 (router).

NR-892FSP-1#sh ip route 
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is not set
      192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C        192.168.1.0/24 is directly connected, GigabitEthernet8
L        192.168.1.129/32 is directly connected, GigabitEthernet8

In this case I have neither the default route nor the host route

My expected behavior

I would like the router to:

- request for the DHCP option 3

- do not install the gateway of last resort

- allow to use DHCP keyword for manual static route

Thanks for comments.

Alex

Philip D'Ath · ‎12-08-2015

I can give you close to what you want. First change the administrative distance for DHCP routes.

ip dhcp-client default-router distance 200

Now add a default route to Null0, so traffic is biffed - or if you have any other default route it will take precendence (assuming its cost is less than 200).

ip route 0.0.0.0 0.0.0.0 Null0