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?

Thanks,
Umut

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.

Rene

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).

Rene

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!

Laz

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,

Sutandra