Short time ago I published a video on my youtube channel about Triangle vs Square Network Topology and I highly recommended triangle topology whenever it is possible.
I received couple of questions about the topologies and wanted to explain one of them in this post for everyone.
I used below topology in the video;
Left picture illustrates the triangle physical topology and right one for the square topology.
Distribution layer devices are advertising the same networks in both topology. It says router but it could be the Multilayer switch as well.
Assume we are running OSPF but using triangle instead of square applies to any other IGP protocol ( EIGRP , IS-IS , even RIP ).
The reason you want to use triangle topology is high availability.In the left topology if the link between core and distribution layer fails, will not be any routing protocol convergence since the core devices will do the ECMP ( Equal Cost Multi Path) towards distribution, and distribution will do ECMP towards core thus all the links will be in the RIB and FIB so will be used actively. ( Flow based load-balancing ).
For the square topology; if the same link fails , since the left core device to destination prefix through the other core device metric is higher than the direct (failed) path , there is no equal cost and unless you enable Unequal cost multi path with EIGRP , you can’t place two routes for that prefix in the FIB. ( You may want to check OSPF Optimized Multipath draft ).
Question : In real life deployment , would we announce the same prefix from the two different distribution switches as depicted in the picture ?.
Answer : Yes we do. If we have distribution layer as depicted in the picture, which mean we have access layer as well. If Access layer is layer 3 which mean, default gateway for the devices is the access layer switch, then access and distribution layer would be running routing protocol.And from the design point of view you would want to run OSPF since between distribution and core is also OSPF and you don’t want to have more than one IGP in your topology unless you have to.
I used layer 3 access as an example for the simplicity but, we announce the prefixes from both distribution layer devices with multilayer access design ( Access-Distribution Layer 2 ) with or without MLAG ( VSS , VPC , MLAG with ICCP ). If you are using MLAG based solution, it is a matter of the number OSPF neighbour ship counts. I would want to see your comment if you know/guess the reason.