Cisco Etherchannel

Unanswered Question
May 5th, 2009


Is it possible to create port channel (L3 interface bundling) between two L3 switches which ARE NOT directly connected? Both switches are connected with four interfaces to the L2 device, and those L2 devices are connected to two routers which are then connected to each other.

I presume that Po1 on one switch and Po1 on the other would have L3 connectivity, but do I get all port-channel benefits if they are not directly connected to each other?

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Overall Rating: 3 (2 ratings)
Giuseppe Larosa Tue, 05/05/2009 - 05:38

Hello Igor,

I would suggest you to attach a network diagram.

you can use a L2 etherchannel that can correctly terminates on the L2 switch.

This should be more clean and correct.

The L3 service can be achieved using an SVI (interface vlan X).

Then the two routers if I've correctly understood should extend the vlan(s) over an IP network for example using L2tpv3.

L3 switchA -- L2 channel --- L2 switchA --- RouterA --- IP WAN --- RouterB -- L2 switchB -- L2 etherchannel -- L3 SwitchB

each side can setup a L2 channel with local L2 switch.

Hope to help


lamav Tue, 05/05/2009 - 06:03


I must say your requirement is a bit odd.

Portchannels are only virtual constructs in that they bundle physical ports together to basically fool the IOS into thinking that there is only one connection. But you still need the physical ports configured.

So, to create a L3 port channel end to end, you need to have each intermediate hop configured the same, quantitatively and qualitatively. The port channel endpoints are not considered to be each extreme end, but between two directly connected neighbors.

So, the L3 switch will have, say, 4 ports configured as the port channel with L2 switch A, Switch A will have 4 ports configured with switch B, and switch B will have 4 with switch C, and so on until you reach the last L3 switch.

On each L3 switch, you would have an SVI configured for a vlan, you would place those ports in that vlan, and bundle them. The intermediate L2 switches will do the same, except for the SVI part, of course, and then the last L3 switch will be like the first.



machiavelli Tue, 05/05/2009 - 06:17

Guislar, thanks for reply.

No, I don't want to extend VLANs from one site to another, I want to use TenGigabit connection from Data Center to Disaster Site for storage replication, and I want to use 3 MDS switch gigabit interfaces for that purpose. Network diagram is as follows:

MDS_DC (3xGigEth) -- L2 -- DC_RTR <====> DS_RTR -- L2 -- MDS_DS (3xGigEth)

Now I have configured 3 IP addresses (with 30-bit mask) on MDS interfaces and 3 IP addresses (with 30-bit mask) on routers interfaces on disaster and data center sites. MDS tunnels are working, and everything is fine, but I wanted to know is it possible to use L3 port-channels in order to make this configuration more elegant. Can I configure this part of network like this:

* Bundle 3 GigEth interfaces in one logical Po1 interface on MDS switches

* Bundle 3 GigEth interfaces in one logical Po1 interface on L3 devices

* Assign IP addresses:

MDS: int po1:

L3: int po1:

* Don't bundle anything or create port-channels on L2 devices!

That's the question, more clarified...


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