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Etherchannel

Barikuma
Level 1
Level 1

I'm reviewing a configuration I came across on an organization's network switches and would like some clarification.

I noticed that there was an EtherChannel between two switches—let's call them Switch A and Switch B. On Switch A, the EtherChannel was configured as a Layer 2 port-channel, acting as an access port to a VLAN. On Switch B, however, the EtherChannel was configured as a Layer 3 port-channel with an IP address assigned.

The VLAN on Switch A is in the same subnet as the Layer 3 port-channel on Switch B, and it appears that traffic is flowing correctly.

While this configuration seems to work, is it considered a valid or recommended design? If so, under what circumstances would such a setup be appropriate? 

6 Replies 6

M02@rt37
VIP
VIP

Hello @Barikuma 

From my point of view, this "design" coul cause unexpected behabiour (STP confusion, ARP issues). It is hard to troubleshoot or scale too...

Better way, it to make both side L3 or both side L2 ; but never mixed.

Best regards
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If we had more information we might be able to give better advice. But based on what we know so far it seems to me that you have a layer 2 device connected to a layer 3 device. On switch A is there a single vlan or are there multiple vlans? If there are multiple vlans , do the other vlans have connections to some other gateways?

From what we know so far it appears that you have a layer 2 vlan device connected to a layer 3 gateway. While it is a bit unusual to do this with EtherChannel, I do not see it as necessarily being a problem.

HTH

Rick

Oh no, switch A has a Layer2 etherchannel connected to switch B which has a Layer3 etherchannel. The port-channel on switch A is configured as an access port to a VLAN while that of switch B just has an IP address configured on it. The SVI on switch A and the Port-channel on switch B are both in the same VLAN.

 

Why "oh no"?  That's not a problem either, assuming I understand the topology correctly.  Could you expand upon the perceived potential issues, if any?

Consider a non VLAN switch that has two routers connected to it, each with an interface in the same subnet?

All that differs, logically one router is replaced by a SVI on the L3 switch also physically hosting the VLAN.

(BTW, for those that learned networking since L3 switches become common, it may be hard to believe, but when switches first introduced VLANs or L3, some found it difficult, to various degrees, to understand these "new" technologies.  [Consider on some early Cisco L3 switches, L3 was actually configured separately from L2.  Or, one of my favorites, the 4500 Catalyst might have MLS supported by an external router!])

I agree with Rick, i.e. don't see it as a problem, although perhaps a somewhat unusual configuration, as Rick also believes.

Consider you have a non VLAN capable switch connected to a router, logically isn't it the same configuration?

Or, for the forgoing, replace the router with a L3 switch.  On the L3 switch, although you now have the option of using an access port or trunk port with an SVI, wouldn't a "routed port" be the equivalent of the router?

As to a Etherchannel being used, logically it creates a single link, so I don't see where that matters.  The real question is L2<>L3, and as above, it's the only way to do it with a non VLAN switch and a router, so it definitely can work just fine.  Whether this is "best practice", when you have other options, I would say depends on other considerations.

BTW, sometimes I've used trunks to support L2 and L3 concurrently (such might, or might not, have also been using Etherchannel).  In my experience, it often freaks out less experienced engineers as it's uncommon.

I've also used SVIs (usually with just 2 access ports for the VLAN), to support a p2p across a single link, i.e. only one access port has a physical cable connection.  That too often confuses other engineers, at least initially, even when I explain its "why".

Thank you. Just to add, both ports have portfast configured on them