Pinging from Interface VLAN 1 to another Interface VLAN 1

[brandon.reed1]

I am going to apologize ahead of time b/c I'm sure this will be an incredibly easy answer but it has me stuck. I will try my best to describe the topology. 

All switches are 3560s with ip routing enabled and running ospf. I have small subnets setup between each switch. Example:  Between Swx2 on the far right to Swx3 in the middle I am using the 10.40.23.32 /30 network. All of the links between the layer 3 switches in the middle and the 2 layer 3 switches on the outer parts are setup in this fashion and I am using the "no switchport" command on each link to allow for ip address assignment to the interface. I am advertising all applicable networks via ospf on all layer 3 devices. 

My issue occurs when I attempt to setup an ip address on "interface vlan 1" on the 2 outer layer 3 switches (Swx1 and Swx2) and attempt to have those two interfaces ping each other across the network. I am using the ip addresses 172.16.13.254 /24 for Swx1 and 172.16.13.24 /24 for Swx2. I am advertising the 172.16.13.0 network using ospf on both sides so that the layer 3 switches in the middle portion of the diagram know how to reach that network. Both Swx1 and Swx2 interface vlan 1 are in the up/up state.

[acampbell · Accepted solution]

Hi Brandon,

You are using the same /24 subnet at both L3 routers.
You cannot do that.

All the ospf routers will be "confused as to where that 172.16.13.x/24 "
actually is-- Is it SW1 or SW2 !!!!!

You will need to split the subnet
Say SW1 vlan 1 = 172.16.13.0/25
Say SW2 vlan 1 = 172.16.13.128/25

Also in a good network design  try to avoid using VLAN1 for
any production traffic at all. In Cisco VLAN 1 is special as it used for
othr protocols like CDP etc - NOT saying it wont work- just advisible
not to use it.

Regards
Alex

[brandon.reed1]

Alex,

That makes sense and I figured it would be a simple answer. I am using vlan 1 in this case so I do not have to hook up an external device (PC, laptop, etc) for the interface to come to an "up/up" state. It is amazing how many times I do find vlan 1 used as the native vlan for production networks though. 

Secondary question. Is there a way for me to assign a vlan 1 interface ip address on the same subnet to both of the mentioned outer layer 3 switches and have them ping each other? Would the solution be a trunk connection between the 2 switches?

Thank you for your assistance! 

Best regards, 

Brandon

[acampbell]

Hi Brandon,

Your L3 design is OK to me.
A lot networks nowadays use L3 from the access layer to the distibution.

If you cannot allocate different /24s to SW1 & SW2 then just split it like
I suggested.

That will still yield 126 hosts per site

SW1

!
int vlan 1
ip add 172.16.13.254 255.255.255.128
!
router ospf 1
network 172.16.13.128 0.0.0.127 area 0
!

SW2

!
int vlan 1
ip add 172.16.13.24 255.255.255.128
!
!
router ospf 1
network 172.16.13.0 0.0.0.127 area 0
!

Use your own OPSP proc number and the correct areas

Regards
Alex

[brandon.reed1]

Alex,

Thanks again!

Regards,

Brandon