Cisco IOS /32 dhcp WAN IP address support

[l1l1]

Hi All,

My new ISP offers fixed public addresses via DHCP, so far so good. But the address I get is in /32 subnet (which is obvisouly not a subnet but rather a host address) and next-hop (or default gateway) is in a different subnet.

I know that some brands will consider a next-hop valid and insert it in the routing table only if next-hop (ord efault gateway) acquired by the DHCP server is in the same subnet as the interface.

Is it supported by cisco IOS / IOS-XE routers (let say a C921-4p or C1111-4p) to have an ethernet  WAN interface (not PPPoE) configured to get IP address with a /32 subnet mask from DHCP server  ?

Regards

[Flavio Miranda]

@l1l1 

 I dont see any problem on the router reveive as DHCP one IP address /32, after all, this is what happen in any DHCP. You will never reveive, via DHCP, something different from /32.

Now, default Gateway in different subnet this is something that that I never saw.

[DanielP211]

Hello!

There is no issue if you recive a /32 over pppoe DHCP, as a matter a fact I belive this is common practice. I have a few deployments like that on ISR4K, CAT8K and ISR1K. It work completely normally and you will also recive the default route that is not part of the subnet.

BR

[Cristian Matei]

Hi,

    I guess you're now getting /32 via DHCP on a PPPoE connection, right? Plain Ethernet interfaces, being multiaccess, by design don't support a /32 mask, either static or via DHCP.

Best,

Cristian.

[l1l1]

Hi,
No that's why I precised "(not PPPoE)", I am talking about plain Ethernet interfaces. When I use a PC either running Linux or Windows 10, it works as is, I am getting the public IP address via DHCP (again without PPPoE).

I am asking because I have a Huawei AR651W router and it does not work, it cannot handle this scenario. That's why I am asking for Cisco routers because I am looking for a router that supports this feature to replace it.

It is weird because soho brands like linksys, asus or netgear supports it.

[MHM Cisco World]

Dhcp get IP and default GW is in different subnet?

I think this not work. It issue of ISP.

You can as work around config defualt route using egress interface instead of default GW (with next-hop push by isp).

MHM

[Cristian Matei]

Hi,

    Outside of the fact that Cisco routers don't support a /32 mask, for systems that do support, getting a /32 mask via DHCP and a default gateway via DHCP will work perfectly fine, as in this case DHCP server will also send you static routes for the default gateway via DHCP option 121, in which case the default gateway is valid because it's recursive over the static route received from DHCP server.

Best,

Cristian.

[MHM Cisco World]

For you @l1l1 and other

High or low end Cisco not support/32

As I mention before It issue of ISP not your router' not only /32 IP but GW in different subnet.

32-bit Subnets

A subnet mask of 255.255.255.255 (a /32 subnet) describes a subnet with only one IPv4 host address. These subnets cannot be used to assign address to network links because they always need more than one address per link. The use of /32 is strictly reserved for use on links that can have only one address. The example for Cisco routers is the loopback interface. These interfaces are internal interfaces and do not connect to other devices. As such, they can have a /32 subnet.

Example

interface Loopback0
 ip address 192.168.2.1 255.255.255.255

 https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/13788-3.html#toc-hId--2075942877

[Cristian Matei · Accepted solution]

Hi,

    Most if not all "high-end" routers will not support a /32 mask on an Ethernet link, as there was no use-case in production networks for it; "low-end" router brands like linksys, Asus, etc, have this capability as being SOHO devices and thus there is use-case for it; end systems like Windows/Linux do support this as it's a necessity in some cases.

    You're pretty stuck on using a low-end router, or change the ISP.

Best,

Cristian.

[l1l1]

Hi Cristian,

I came to the same exact conclusion, thank you!

Regards