Ricardo Martins

Archive for the ‘Switching’ Category

Cisco Expert – Intervlan with multilayer switchs

Posted by Ricardo Martins on July 16, 2008

In our studies towards CCNP, we learn about intervlan between 2950’s and a router called router on a stick. We also learn how to do intervlan with multilayer switchs. What we don’t learn is how to add a second or third multilayer switch in the “game”.

I thought it would be interesting to know more about this topic because in most production environments you find this sort of configuration with 6500’s switchs.

So lets imagine we have 6 vlans, vlan 1 to 6 and we want to do intervlan between all these vlans. If we would have only one multilayer switch, we would create 6 interface vlan (SVI’s) and we would have our hosts configured to point to the SVI’s as a default gateway.

However it is interesting to know how this would work with an extra multilayer switch.


In this case we need to have a trunk between both 3550’s switches and trunks to the 2950’s.
The 2950’s will have access ports to the hosts.

Now, because we can only have one default gateway per host, either has to be SW1 or SW2, meaning basically that one switch would be treated as a Layer 2 device, pointless… So what I have seen in the real world is using HSRP for the SVI’s.
SW1 would have for instance all odd vlans Active and all even vlans in Standby and SW2 vice versa. Simple and nice eh?

Worth to mention that if you wouldnt’ have HSRP in place and for example for vlan 1 to 3, SW1 would be the default gateway and for vlans 4 to 6, SW2 would be the default gateway, then you would need a p2p connection between both 3550’s, a Layer 3 etherchannel or just a Layer 2 trunk with one SVI in same vlan per router in order to be able to send traffic between vlans.

Here some configuration example:


interface Vlan1
ip address
standby 1 ip
standby 1 priority 150
standby 1 preempt

interface Vlan2
ip address
standby 2 ip

interface Vlan3
ip address
standby 3 ip
standby 3 priority 150
standby 3 preempt

interface Vlan4
ip address
standby 4 ip

interface Vlan5
ip address
standby 5 ip
standby 5 priority 150
standby 5 preempt

interface Vlan6
ip address
standby 6 ip

Then it would be the opposite configuration on SW2.

In conclusion, if you want to have 2 or more multilayer switches performing at layer 3, then it is best to run HSRP between the switches with some vlans active in one switch and other vlans active in the other switch, that’s because you can only have one default gateway per host which would be the virtual HSRP ip address.

In this case, hosts in Vlan 1 would have the ip address as default gateway, vlan 2, and so on.

IMPORTANT – Keep in mind that the diagram is just a model. In fact, in this case, HSRP wouldn’t really work unless you would have trunks between the 2950’s or an extra trunk between the 2950’s and the other 3550.

Hope this helps!

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