increasing AD of Connected networks ?

[gnijs]

This might be a strange question, but is there any way of increasing the Administrative Distance of a direcly connected subnet to above the AD of a routing protocol (for example above 110 of OSPF) ??

Suppose i have a switch A with lots of SVIs. I also have one VLAN lets say VLAN 100 which is just a L2 VLAN without any IP information. The IP and default gateway of this VLAN is configured on another switch B in the network and the subnet is advertised by OSPF.

So on the switch A, i have an OSPF route to switch B for VLAN 100 and also an extension of the VLAN 100, but with no IP information.

I now want to configure an SVI for VLAN 100, give it an IP address, but i DON'T want that this SVI to take any traffic. By default, it will be inserted in the routing table as Connected, and all other VLANs present on switch A will start using this path to the subnet. This will lead to assymetric routing and flooding in the VLAN.

Ideally, i want the directly connected SVI for VLAN 100 to have an AD of 200 for example, so that the OSPF route to that VLAN still remains in the routing table and so that the path to VLAN 100 is not changed (by simply giving it an IP address).

If switch B goes down, the OSPF route will disappear, and the directly connected route will appear in the routing table (as intended).

Is there any way to increase the AD of the directly connected route ?

regards,

GN

[MHM Cisco World]

why you not L2 both SVI in both SW ? 

can you draw topology ?

[balaji.bandi]

Not sure what is the goal here, if the switch layer2 with default gateway, you do not need OSPF here right ?

VLAN 100 only reside locally, why asymetric routing ?

[David Ruess]

Hello,

I'm pretty sure you cannot change the AD of a directly connected route. That can only be done on static routes or within a routing protocol. Its logically attached to the interface on that device so it cant be anywhere else on the network.

You might be able to do something with Policy Based Routing, where you create a policy that says for anything trying to go to this network take this path vs another one. Make sure you understand traffic flow and your network. Just because you send traffic one way doesn't mean it wont send it right back if the router you send it to says to get to that network its right back through the original device.

-David

[Jon Marshall]

Perhaps you can explain what you are trying to achieve in terms of your overall network ? 

Jon

[Giuseppe Larosa]

Hello @gnijs ,

as already noted it is not clear what you want to achieve.

As an alternative to increasing the AD for the connected subnet  I see two possible options:

a) if your addressing plan allows for this on Switch A you will place an IP address of a supernet of the actual prefix for example 192.168.0.253/23  when the actual prefix length is /24. This may be feasible or not-

If you can do this, the OSPF route will be preferred over the connected route because it is more specific longer prefix first.

b) if if you need just an host in the subnet with no routing implications you can use a VRF and you can associate the SVI of VLAN 100 to that VRF. 192.168.0.253/24 + vrf forwarding ISOLATED

Without seeing the topology it is not clear why you worry about asymmetric routing  there is a firewall on the L3 path between SWitchA and SwitchB ?

If there isn't you can enable the ip address with no issues if you want remote devices to still prefer router B just add under the SVI vlan 100 a command like

ip ospf cost 5000 on Switch A

Hope to help

Giuseppe