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Layer 2 loop-free design question


Everyone knows this standard campus Layer 2 loop free design (attached). Why the layer 3 interlink between distribution switches is needed ? Can anyone explain this.. Thanks



Yes , As per documenatation. If you have an access switch connected to a distribution pair with L2 uplinks and the distribution pair are connected by a L3 then there is no loop for STP to block .

Let me try to explain what i have taught during my studies for designing subject..:)

For example :- 

Broadcast packet looping between switches is one of the best point. Consider you have two access switches ASW1 and ASW2  connected to both DSW1 and DSW2 with L2 links.

And DSW1 and DSW2 both are connected with L3 link.

If vlans are configured same on both ASW1 and ASW2 then see what happen when a PC on ASW1 sends a BC. Packet goes to DSW1 which send to ASW2 as VLAN is also configired on that switch , As ASW2 is uplink connected with DSW2 then forwards BC to DSW2 and DSW2 forward back to ASW1 which get into loop.

Having VLAN configuration in one ASW's , you cant have this type of loop because with above explanation DSW1 and DSW2 never shared BC traffic as they are connected via L3 links.

So you can have the same vlan on multiple access switches but only one access switch can forward on both uplinks. The other access switches have to block one of their uplinks to avoid a loop.

Hope it Helps..


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It may not be required, it really depends on what you're otherwise trying to do.  Further, what's shown can create L3 issues, again depending on what has otherwise been done.

For example, suppose in the leftmost diagram, both L3 switches have a SVI for the loop free L2 topology.  Further, suppose both L3 switches have other ports in that same L2 topology.  Then suppose, one of the L3 to L2 switch links fail. You'll then have a partitioned LAN.  However, what I just described wouldn't be an issue for the middle or right most diagrams.

So, again, the diagrams are only really addressing a loop free L2 topology, don't assume what might, or might not, be needed when discussing L3.


The big advantages to doing a Layer 3 connection between the Distros is so you don't have to worry about Spanning Tree.  Spanning Tree is slow and can introduce a lot of problems.  If it's a Layer 3 connection and you have a failure, the FHRP switchover can be instantaneous, whereas STP would have to run it's whole algorithm to figure out which ports are root ports,designated, etc.

 The other big advantage is load balancing.  You can (sort of) load balance with STP by using different VLANs with different roots, etc, but really you're just routing some VLANs on one layer 2 connection, and other VLANs on the other Layer 2 connection.  It's not really load balancing, it's just making the most of the connections you have.  But say you have GLBP running on the distros, you can make use of all of the bandwidth on those Layer 2 connections for all of your VLANs.


Hey, thanks. But we can use no link instead of L3 link. So the STP calculations would be solved. And you can load balance with different primary gruops without layer 3 interlink, as i guess..

Thanks you all, but still dont see a difference if we use no link between or l3 link..

The only way it can be profit if the link between Dist1 and Core SW fails, when trafic from ASW would be forwarded to DSW1 and then through that link to DSW2. But we can do HSRP tracking uplink interface and then uplink interfaces are down HSRP could be active on DSW2 so still i dont see the use of L3 practical interlink..

Here, what i found in this guide:

The additional link between the distribution switches is required to support summarization of routing
information from the distribution layer towards the core. If the routing information is not summarized
towards the core, Enhanced Interior Gateway Protoc
ol (EIGRP) and Open Shortest Path First (OSPF)
require interaction with a potentially large number of peers to converge around a failed node, as shown.
When summarization is used, the distribution nodes interact with a bounded number of routing peers
when converging around a link or node failure. Summarizing using EIGRP or using an area boundary
for OSPF are the recommended L3 configurations fo
r the distribution-to-core layer L3 connection.
An L3 link is required between the distribution nodes. If an L3 link between the distribution nodes is not
present, return traffic (from the core to the access layer) could be dropped if an access layer link fails
and the distribution nodes are not interconnected with an L3 link, as shown in Figure 14.

The L3 link between distribution switches with L2 from the access layer is typically used where you have vlans that are only on one access switch ie. a vlan cannot be on multiple access switches.

If you have this design then GLBP is a good choice because you can use both uplinks.

In this design you need the L3 link because again typically there will be other L3 devices beyond your distribution switches and you want to advertise summary routes for all the vlans/IP subnets on the distribution switches.

So imagine that both distribution switches are connected to a router and there are two scenarios -

1) you are not summarising the vlan IP subnets. In this scenario if the access layer switch uplink fails then the SVI for that vlan should go down on the distribution switch that has the other end of the failed link.

In this case that IP subnet is no longer advertised to the router so the router only sends return traffic to the distribution switch with the active link to the access switch.

Which means you do not need a L3 link.

2) you are summarising the vlan IP subnets. In this scenario if the same failure occurs the SVI still goes down but the IP subnet for that vlan is being advertised as part of a summary route to the router so the router still sees two possible paths via both distribution switches to that IP subnet.

If it sends it to the distribution switch with the failed link without an interconnect the traffic is dropped.

However with an interconnect the distribution switch would see a L3 next hop via the other distribution switch and so traffic would be sent across the interconnect and then sent to the access switch.

So as the description you posted says it is there to allow you to summarise all the vlan/IP subnets on the distribution switches to any other L3 devices.



Thanks. So basicaly it's because of summarization. Do the summarization is mandatory ? As i rememebr it helps from lots of problems for example stuck in active and etc... if we are using eigrp.. yes ?:)

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