modifying subnet

[toolshed1]

Hi,

I have L3 terminating on the core. It's in use by client devices. I need to modify that from /16 to /21

Will i incur any downtime to clients when modifying that?

From:

interface vl 10

      ip addr 10.10.0.0 255.255.0.0

To:

interface vl 10

    ip addr 10.10.0.0 255.255.248.0.

[MHM Cisco World]

I think Yes 

GW is SVI of this vlan when you change mask the GW will not routing any packet anymore 

The reason that client send ask mac of GW but the GW not reply since it new IP is out old subnet.

MHM

[Reza Sharifi · Accepted solution]

Hi,

You need to do this in a maintenance window as there will be downtime specially since you are going from a larger subnet to a smaller one. I also recommend you first negate the existing IP and then add the correct one. Don't forget to change the DHCP scope if this is an end device subnet.

interface vl 10

no ip address

ip address 10.10.0.0 255.255.248.0

HTH

[M02@rt37 · Accepted solution]

Hello @toolshed1 

Potential risk of donwtime yes !

Changing the subnet mask on an interface from a /16 to a /21 involves a significant modification to the IP addressing scheme. The impact on client devices depends on the specific network design and how IP addresses are assigned to clients.

First, changing the subnet mask from /16 to /21 means you are reducing the available IP addresses in the subnet. You'll be going from 10.10.0.0 - 10.10.255.255 (65,536 addresses) to 10.10.0.0 - 10.10.7.255 (2,048 addresses). This could lead to IP address conflicts if not managed carefully.

Also, if clients are statically configured, you'll need to update the subnet mask on each device manually (if DHCP is in use, make sure to update the DHCP configuration to reflect the new subnet mask).

Changing the subnet mask itself doesn't necessarily cause downtime. However, if clients are statically configured, there might be a brief interruption when you update their settings. If DHCP is used, the impact can be minimized by adjusting the lease time to a shorter duration before making the change.

As concerned routing, ensure that the routing configuration on your network devices is updated to reflect the new subnet mask. If there are multiple routers or L3 devices involved, coordinate the changes to avoid routing issues.

[toolshed1]

Current /16 vlan only has 1400 clients and it is DHCP. I have to use 10.10.x.x IP addressing on future new vlan that will be /26.

[MHM Cisco World]

How dhcp work if GW can not detect it IP subnet.

All other problems appear because GW not reply to any packet outside it subnet include DHCP request.

So retrun to my previous point

Solve GW first then change dhcp/static route and any other features use old subnet.

MHM

[toolshed1]

Currently DHCP is advertising 255.255.248.0 and gateway 10.10.0.1 so thats the config that clients currently get. So really nothing is changing from that standpoint. I just need to reflect that on SVI on the core

[balaji.bandi]

the OP you mentioned 10.10.0.0/16, that is creating confusion here.

as long as the configuration on the interface 10.10.0.1/16 to 10.10.0.1/21 should be ok.

[balaji.bandi]

YES / NO depends on the place you are changing, this is Normal PC no critical one - change in the late off peak hours suggested so less users effected.

Make sure changing there subnet not good enough, you need to change all over where this subnet was used ( Like DHCP Scope / FW  or any static Routes so on).

[toolshed1]

clients are getting proper subnet from DHCP. Just SVI on the core was originally set to /16 a long long time ago.

[M02@rt37]

@toolshed1 

Agree with @MHM Cisco World, Addressing the gateway issue first is a prudent approach. If the gateway is not correctly configured to recognize or route traffic to the new subnet, it can lead to communication issues, including DHCP requests not being properly handled