Unicast Flooding due to Asymmetric Routing

Very informative article.

19 posts were merged into an existing topic: Unicast Flooding due to Asymmetric Routing

Hi Rene,

Thank you for the article. But I have a question. If I use SW1 multilayer switch as a gateway for all vlans, and sw2 as a L2 switch will the problem occur again? I think it will occur for the traffic from H2 to H1 because, SW2 will not learn the mac address of H1. Is it true ?If it is true, then for the design should we use just 1 multilayer switch and connect all hosts to it to avoid flooding?


Hi Umut,

The problem here is that some traffic is switched, some is routed. If you use SW1 as the default gateway for all VLANs and SW2 only for switching then there won’t be a problem. SW2 will be able to learn all MAC addresses, the MAC address of H2 and the MAC address on the VLAN interface of SW1.

Otherwise, changing the ARP timeout is another solution.


Aaahh!! That is an intense topic.
Is it possible in a network/ LAN to have two default gateways? I am not sure how much stupid is this question!!! Just came to mind if redundancy can give a better result.

Excellent explanation. I was thinking whole day and your answer helped me to realize the difference between an ARP message (inside Ethernet Frame) and Ethernet Frame.
What currently in my mind is - Why the switch doesn’t learn MAC address from the ARP table if it is not present in MAC table? Too many confusion in this topic… !!!

Hello Rosna,

Each router in your network can be used as a default gateway by your hosts but the problem is that most hosts only support a default gateway.

That’s why we use “gateway redundancy” protocols like HSRP, VRRP, and GLBP. You can read more about this here:

About the switch…A switch is a layer two device so all it “cares” about is forwarding Ethernet frames. It only cares about looking at the source MAC addresses to learn addresses and looking at the destination MAC address to figure out where to send it to.

ARP is just one protocol that you can find in an Ethernet frame…an Ethernet frame can also contain an IPv4 packet, an IPv6 packet or some other protocols.

ARP is used to bind a layer two address (MAC address) to a layer three address (IP address). We only need this on a switch if you access the switch with telnet/SSH or if you configure your switch as a router (that’s a layer three switch).


what does symmetric Routing look like ?

Hello Pipat

When referring to routing, asymmetric and symmetric are terms used to describe the path that the packets between two endpoints take. Symmetric routing takes places when two hosts are communicating with each other and all traffic sent from Host A to Host B takes the exact same path (passes through the same routers) as traffic that is sent from Host B to Host A.

Asymmetric routing takes place when traffic sent from Host A to Host B is different than the route taken for traffic sent from Host B to Host A.

These paths can and often are different because there may be multiple redundant paths or load balancing routing configurations that allow for the use of multiple routes to get from one host to another. Symmetric and Asymmetric routing are not static situations but can and do change based on the routing parameters and policies that are in place as well as on the changing state of network traffic and potential link failures.

I hope this has been helpful!


Hi Rene,

Again it is an excellent example and made me think.

When a datagram starts at a host for a specific destination on a different subnet what changes is the L2 header.
I think in your write up , ‘Unicast flooding can occur when a switch doesn’t know the destination MAC address’ can be better written as ‘Unicast flooding can occur when a switch cannot learn the mac address to port binding for a host’. As the frame has the destinaton Mac and not all Fs but it does not know which port to send it to and floods the frame to all pots of a Vlan. the one for which it is meant receives along with others who discards it but network performance degrades.

Many thanks and regards,


Hi @sutandrac1,

I agree, this sounds better. I just changed this.


Hi Lazaros,
Would you please simplify Symmetric and Asymmetric routing?
Although this question has been asked but I didn’t get the concept.

Hello Muhammad

Symmetric routing simply refers to the fact that the route taken from host A to host B is the same route, in reverse, that is taken from host B to host A. Now in Rene’s topology, it may look like the same route is taken in both cases, and yet Rene refers to one of the scenarios as asymmetric and the other as symmetric.

This is because a route is defined by where the routing takes place. So more precisely, symmetric routing is routing where the routers through which a packet is routed are the same as those used for the return journey, in reverse order.

With this in mind, in the topology in the Lab, a symmetric routing scenario is one where:

  • H1 --> SW1 --> H2 (where SW1 is used as the default gateway of H1 and the packets are routed there)
  • H2 --> SW1 --> H1 (where SW1 is again used as the default gateway of H2 and the packets are routed there)

An asymmetric routing scenario is one where:

  • H1 --> SW1 --> H2 (where SW1 is used as the default gateway of H1 and the packets are routed there)
  • H2 --> SW2 --> H1 (where SW2 is again used as the default gateway of H2 and the packets are routed there)

I hope this has been helpful in clarifying it further!


Great explaination.
Anyway I can’t understand the advantages of asymmetric routing.

For example in the case where H1 and H2 would be two routers to exit outside…why should I implement a topology like this?


Hello Giovanni

There really isn’t a reason why you would deliberately create asymmetric routing. The purpose of the lesson here is to inform you of the concept. You may set up a network like this without realizing that asymmetric routing is actually taking place.

Now having said that, the only time you would want to deliberately set up a network like this is if you want to more evenly distribute routing responsibilities between two devices. If you have a large network and all your routing takes place in one of the two L3 switches, a lot of CPU and memory would be used on that switch to route everything while the other’s resources remain idle. Although this is true, there are other more appropriate ways to distribute such routing responsibilities (such as Gateway Redundancy protocols).

In that case, you probably wouldn’t create such a scenario, but you’d look at implementing a multi-homed edge topology using BGP similar to those found in the following lesson.

I hope this has been helpful!