is it bad practice to set router subnet as the whole network?

enzo80 · ‎11-10-2021

My friend told me at work they always use this kind of subnetting , he said all devices can reach the router this way, but in ccna i was never thought this way

https://imgur.com/a/qOn3Tp2

balaji.bandi · ‎11-10-2021

router subnet as the whole network?

this depends what network what subnet mask we looking.

most case /24 will be used or more.

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enzo80 · ‎11-10-2021

I mean in the picture topology if we set the router interface ip address : 192.168.1.1 /26 and core switch 192.168.1.2 /26

other subnets cant reach the router without adding routing protocol right? that how we were thought in ccna , never thought about router subnet have broader reach

Georg Pauwen · ‎11-10-2021

Hello,

it is difficult to tell from the picture how the router and the switch are configured. Is this a router-on-a-stick setup (with the router having subinterfaces for each Vlan), or is the switch a layer 3 switch that does the routing ?

paul driver · ‎11-10-2021

Hello

@enzo80 wrote:

I mean in the picture topology if we set the router interface ip address : 192.168.1.1 /26 and core switch 192.168.1.2 /26

other subnets cant reach the router without adding routing protocol right?

If you have other subnets other than a single /26 on the core then yes you are correct as the rtr needs to know about them, however if you have just the one /26 shared by all then users in this subnet will be able to reach each other, the core and rtr and for such a small network it wouldn’t be bad practice however i would personally still split it down .

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

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Paul

balaji.bandi · ‎11-11-2021

In short if they belong to /26 network they will, if they go beyond that subnet, so that need to be corrected.

take example if you have /24 configured on that network, if any device configured by mistake /26 still can reach since it is with in /24

not other way around..........make sense ?

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Joseph W. Doherty · ‎11-10-2021

"My friend told me at work they always use this kind of subnetting . . ."

Rather unusual, not something I would recommend.

". . . he said all devices can reach the router this way . . ."

As @Georg Pauwen noted, diagram is lacking some key information.

On a shared wired (broadcast domain) you could have the router using a /24 and hosts partitioned into one of the four /26s (within the scope of the /24 - that's important!); it should work. Hosts in one /26 trying to communicate to another /26 will treat such as "off the local network". If they don't have a gateway defined, they would just ARP for destination, which should see ARP and respond. Router should be setup for proxy, but it won't respond for the /26s within its /24. (Of course, for traffic between the /26s, router is actually being bypassed, much the same as if all the hosts were defined in the /24. About the only advantage of doing this, that immediately comes to mind, subnet broadcasts would be ignored if host isn't in the same /26.)

If they had a gateway defined, hosts should send to gateway IP and the router, I believe (assuming not configured to do so), should reply with a redirect (because it "sees" all the /26s in the same subnet, its /24).

Router sending to any of those /26s would ARP.

However, diagram shows VLAN too, and how they are setup can make or break this from working. (Actually, off the top of my head, cannot see how to make it work with VLANs.) From diagram, how does router have "foot" in the four VLANs (10, 20, 30, 40) using a single /24 IP? Perhaps router has four interfaces bridging across all four VLANs, but that sort of defeats having VLANs.

Years ago, was in an environment where we had to do something like this for a "backup" network. It was so long ago, don't remember details, but do recall backup device used larger subnet to encompass multiple host subnets.

enzo80 · ‎11-11-2021

sorry i forgot to include the configs:

core switch config:

CORE#show run

Building configuration...

Current configuration : 2297 bytes

!

version 16.3.2

no service timestamps log datetime msec

no service timestamps debug datetime msec

service password-encryption

!

hostname CORE

!

ip dhcp pool vlan-20

network 192.168.1.64 255.255.255.192

default-router 192.168.1.65

ip dhcp pool vlan-30

network 192.168.1.128 255.255.255.192

default-router 192.168.1.129

ip dhcp pool vlan-40

network 192.168.1.192 255.255.255.192

default-router 192.168.1.193

!

no ip cef

ip routing

!

no ipv6 cef

!

no ip domain-lookup

!

spanning-tree mode pvst

!

interface GigabitEthernet1/0/1

switchport trunk encapsulation dot1q

switchport mode trunk

!

interface GigabitEthernet1/0/2

switchport trunk encapsulation dot1q

switchport mode trunk

!

interface GigabitEthernet1/0/3

switchport trunk encapsulation dot1q

switchport mode trunk

!

interface GigabitEthernet1/0/4

switchport access vlan 10

switchport mode access

switchport nonegotiate

spanning-tree portfast

!

interface GigabitEthernet1/0/5

!

interface GigabitEthernet1/0/6

!

interface GigabitEthernet1/0/7

!

interface GigabitEthernet1/0/8

!

interface GigabitEthernet1/0/9

!

interface GigabitEthernet1/0/10

!

interface GigabitEthernet1/0/11

!

interface GigabitEthernet1/0/12

!

interface GigabitEthernet1/0/13

!

interface GigabitEthernet1/0/14

!

interface GigabitEthernet1/0/15

!

interface GigabitEthernet1/0/16

!

interface GigabitEthernet1/0/17

!

interface GigabitEthernet1/0/18

!

interface GigabitEthernet1/0/19

!

interface GigabitEthernet1/0/20

!

interface GigabitEthernet1/0/21

!

interface GigabitEthernet1/0/22

!

interface GigabitEthernet1/0/23

!

interface GigabitEthernet1/0/24

!

interface GigabitEthernet1/1/1

!

interface GigabitEthernet1/1/2

!

interface GigabitEthernet1/1/3

!

interface GigabitEthernet1/1/4

!

interface Vlan1

no ip address

shutdown

!

interface Vlan10

mac-address 0009.7c01.0301

ip address 192.168.1.3 255.255.255.192

!

interface Vlan20

mac-address 0009.7c01.0302

ip address 192.168.1.65 255.255.255.192

!

interface Vlan30

mac-address 0009.7c01.0303

ip address 192.168.1.129 255.255.255.192

!

interface Vlan40

mac-address 0009.7c01.0304

ip address 192.168.1.193 255.255.255.192

!

ip classless

!

ip flow-export version 9

!

line con 0

logging synchronous

!

line aux 0

!

line vty 0 4

login

!

end

Router config:

Router>

Router>en

Router#show run

Building configuration...

Current configuration : 704 bytes

!

version 15.1

no service timestamps log datetime msec

no service timestamps debug datetime msec

no service password-encryption

!

hostname Router

!

ip cef

no ipv6 cef

!

license udi pid CISCO2911/K9 sn FTX1524950Z-

!

spanning-tree mode pvst

!

interface GigabitEthernet0/0

ip address 192.168.1.1 255.255.255.0

duplex auto

speed auto

!

interface GigabitEthernet0/1

no ip address

duplex auto

speed auto

shutdown

!

interface GigabitEthernet0/2

no ip address

duplex auto

speed auto

shutdown

!

interface Vlan1

no ip address

shutdown

!

ip classless

!

ip flow-export version 9

!

line con 0

!

line aux 0

!

line vty 0 4

login

!

end

access switches only have vlans

Georg Pauwen · ‎11-11-2021

Hello,

that configuration indeed works.

What did they tell you during the CCNA class/course with regard to configuring that kind of topology ? I guess typically, for any L3 link, you would usually configure a separate subnet. Both would work in your case.

paul driver · ‎11-11-2021

Hello
You configuration as it is WONT work, Because the rtr doesn't have any idea how to reach the core switch vlans and has no idea how to reach non LAN traffic (internet/wan etc..) Also, the core switch doesn't have any route egress towards the rtr for non Lan traffic.

You need to either apply a IGP routing process (rip/eigrp/ospf) or apply static routing and if the rtr is internet facing then you would need to also apply Network Translation (NAT) so LAN traffic has access to the internet.

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

mlund · ‎11-11-2021

Hi

I agree with Paul, there is no routing to unknown networks here, so nothing can leave this environment. But what happens inside this network.

If host on network 20,30 and 40 wants to speak with each other, it will work OK if they have the correct gateway configured.

What happens if a host want to send packets to the router? It will send to the default-gateway, wich is the coreswitch. The core send it to the router.

When the router want to send packets to the host, the router think that the host is directly connected, and will try to arp for the ip address, and that will only work if the switch supports proxy arp. If it doesn't support proxy arp it will not work.

/Mikael