Criteria for a network to be strictly "Layer 3" vs "Layer 2"

[vv0bbLeS]

Hello all,

I have read about networks being described as "only Layer 3," and as such require no STP protocol, examples being VXLAN-based networks like Cisco SDA or ACI.

However, even these networks have VLAN's (like L2 VNID's in SDA), and they also still use Layer 2 Ethernet headers for forwarding traffic, despite being touted as "only Layer 3" networks.

So, what is the criteria for a network to be "only Layer 3" ? What does that actually mean? Could it be:

1. No broadcast messages are needed? (e.g. VXLAN tunneling, but even then you will have DHCP Discover broadcasts)
2. Traffic is only ever sent via tunnels?
3. Something else?

[MHM Cisco World]

There are two head in vxlan 

Now for dhcp broadcast 

Inner head is indeed send as broadcast to all host in that subnet 

Outer head is unicast ( ingress replication) or multicast 

So we looking on outer head it always l3 and hence no need any layer 2 protocol like stp.

MHM

[MHM Cisco World]

[julian.bendix]

Well it sounds like we should take a big step back from speaking about SDA and ACI.

Read about how Ethernet as a transport technology works, what it enables you to do and how routing comes into play.

Ethernet still is the most used underlying transport technology, even in routed networks. Between each routed hop you will still speak Ethernet!

[Jens Albrecht · Accepted solution]

Hello @vv0bbLeS,

when you look for a criterion to decide whether a network is Layer 2 or Layer 3 you need to ask the following question:

• What kind of information is used to make the forwarding decision on each device in your network?

Of course, even in a Layer 3-only network each packet has to be encapsulated into a Layer 2 frame in order to reach the next device.

However, the forwarding decision is then made based on the information in the Layer 3 header.

In a Layer 2 network the switch makes the forwarding decision e.g. based on VLAN-tags.

HTH!

[Joseph W. Doherty · Accepted solution]

Rather difficult to have L3 without L2, and L2 without any L3 would be unusual.  So, a network that's only L3 really means avoiding the usage of L2.

Without jumping into VXLAN or ACI, let's consider a network of 25 24 port non VLAN capable L2 switches in a star topology, using a /23.  Would such work well?  Maybe.

Replace the core L2 switch with a L3 switch, and configure each of its 24 ports as a routed port using a /27.  Most would now consider your L2 network is now a L3 network, "only" being debatable.

Next replace all the edge switches with L3 switches, and make all the ports routed ports using /31s.  The latter would come pretty close to an only L3 network, but L2 is still being used on the p2p /31s, although, logically, MACs aren't needed.

So, again, an only L3 network really means avoiding L2 especially between network transit devices.  I.e. transit hops using L3 rather than L2.

[Richard Burts · Accepted solution]

I think we need to be very careful when we talk about layer 3 only or strictly layer 3 networks. I like the approach suggested by Jens and Joseph which focuses on the information used to make forwarding decisions. 

I guess it would be possible to have a layer 2 only network that made forwarding decision based only on mac addresses. It would be a network with a single IP subnet and all devices just need to arp for neighbors and forward to their mac address. But that would not be very useful. Strictly speaking I do not believe that it is possible to have a layer 3 only network. It would need to ride on the foundation provided by layer 2.

[vv0bbLeS]

Thank you all for your replies! I think I understand what authors are trying to convey now when they use the term "L3 only network" (even though L2 is still used for transport). Perhaps a definition of a "L3 only network" would be: A "L3 only network" is a network where all links between the network devices in the network are routed (L3) ports.

@Joseph W. Doherty hit this on the head when he said "So, again, an only L3 network really means avoiding L2 especially between network transit devices.  I.e. transit hops using L3 rather than L2."

As @Richard Burts said, I also never liked the term "L3 only" because of course you need a lower-level protocol to help move the packets on the wire, but now I understand what they're "trying" to say. Thanks again all!

[MHM Cisco World]

Nop

STP used to control l2 loop

If I use ingress replication' the traffic is unicast from one vtep to ther' no l2 loop 

If we use multicast' multicast use rfp check so again there is no l2 loop (for underlay)

Hence no need stp.

MHM

[MHM Cisco World]

When you later advance read about vxlan you will find how vxlan prevent loop from 

1- using etherchannel by add ESI add to route type2/1/4

2- multihomed case by DF elect add to route type2/1/4