L3 Access Switch LAN

[Yannick Vranckx]

Hi,

I have a few questions regarding a full layer 3 LAN design.

Say i have the following topology:

The first switch acts as the core, the second where the pc is connected will be the access switch.

Say i create between them a point to point network, i assign fa0/1 of SW0 172.16.1.1 255.255.255.0 and fa0/1 of SW1 172.16.1.2 255.255.255.0

I add a vlan to the core:

//Config

Vlan 10

interface vlan 10 => 192.168.10.3 255.255.255.0

On the access switch i add vlan 10 as well 

//Config

Vlan 10

interface vlan 10 => 192.168.10.4 255.255.255.0

On the access switch i add a client computer in access vlan 10 with the IP of 192.168.10.20 255.255.255.0

To make everything work i will have to create a routing protocol between the core and access, i choose OSPF. I create the following config on both switches.

//Config

router ospf 10

network 172.16.1.0 0.0.0.255 area 0 => The PTP subnet

network 192.168.10.0 0.0.0.255 area 0 => The Vlan 10 subnet

I repeat this config to the access switch

From this point i will start having troubles, the PTP subnet will show up in the OSPF. I will also see the Vlan 10 subnet in the routing table on both switches. But i think it will not yet work due to Vlan 10 on the core switch will be up/down, therefore i cannot have the pc reach the gateway on the core switch.

Any ideas?

[Mark Malone]

Hi If its a routed network between those switches vlan 10 interface should only be on the access L3 switch where client is and routed up to the core switch , vlans are broken by broadcast L3 domains so that's why its up down because it does not see your client as part of vlan 10 as its local only on core at L2 and theres no stp for vlan 10 either , these conditions must be met for a vlan interface to come up/up , if you want vlan 10 to come up on core and have client connected to access in same subnet it needs to be trunked up to core not routed

[Yannick Vranckx]

Hi Mark,

Thanks for the answer. Say i keep it Layer 3, i only will create the SVI on the L3 access switch and route it up to the core. 

I will have to add it to the OSPF process on the L3 access switch i assume?, the core will pick up this route then?

Where would be the best spot to place the default gateway for that subnet?

[Mark Malone]

Hi was off yesterday yes you would create it there and add it to ospf , as its routed in ospf the default-gateway is only required in clients and should be the vlan interface there part of , if you don't want that subnet to be routed by ospf you can route it by static or default route instead for all other traffic , but usually your default route would be on the router with multiple connections and isp link for example

[Simrid123]

I'm afraid I am unable to see the topology, however I can give you some general advice around access switching. 

I believe it is Cisco best practice to have access switches as layer 2 links, which forward the traffic up to the distribution level and then up to the core.  

If you are running a collapsed core network topology, you could leave you access switches as layer 2 links and run a HSRP instance between L3 switches or routers further up the network.  

You should then configure the HSRP address to be the default gateway.