Static route with next-hop as dhcp-router but without default-route
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12-07-2015 09:08 AM - edited 03-05-2019 02:53 AM
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
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12-07-2015 11:08 AM
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.
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12-08-2015 12:45 AM
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
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12-08-2015 01:17 PM
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
