1.1.c Explain general network challenges

1.1.c [iii] Asymmetric routing

When communication between two hosts (or end points of any type) take different paths on their way out and another on their way in, it is called asymmetric routing. It can also cause packets to arrive out of order if packets that are part of a given flow take different paths.

Large amounts of flooded traffic might saturate low-bandwidth links causing network performance issues or complete connectivity outage to devices connected across such low-bandwidth links. An example of such situation could be a topology where there are two switches (ports in two VLANs, say A and B), two routers (doing inter-VLAN routing between A and B) and two hosts one in VLAN A and one in VLAN B. Now since the routers will proxy ARP for respective hosts as they are default gateways, switches will never be able to learn actual end hosts MAC addresses (router will rewrite them every single time to their own). Switch A and B will continue to flood traffic since they are unaware of the actual host A and host B MAC addresses.

The solution approach is normally to bring the router’s ARP timeout and the switch’s’ forwarding table-aging time close to each other. This will cause the ARP packets to be broadcast, relearning must occur before the L2 forwarding table entry ages out.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 958-964).  . Kindle Edition.