STP, RSTP, MST limitations in terms of number of switches in one ring

[neelesh.gaekwad]

hi,

i have 15-20 switches at access layer and 2 numbers of core switches at core layer. for redundancy i want to setup a ring topology.

My physical connectivity is as follow :

1st core switch -- access layer switch-1 -- access layer switch 2 -- access layer switch 3 -- ... access layer switch 15 -- 2nd core switch.

in this scenario what the protocol should i use, STP, RSTP, RPVST, MST or anything else.

i have already face an issue in which i have 7 switches in ring excluding core switches, in this case if a ring goes down at only one leg of ring. the entire network goes down. The reason is that after 6 switches in ring , the switches in STP ring don't understand the BPDU change and all switches in the ring becomes ROOT BRIGDES.

[dukenuk96]

Hi

why do you use such topology? Why not 'star'?

If you can not change your topology for some reason, I would recommend you to use MSTP - it is as fast as RSTP and have ability to load-balance like (R)PVST, and not by-VLAN basis, but by-instance. If you do not need load-balance your traffic, then just configure two instances - 0 with VLAN 1 only and 1 - with all other VLANs. One more benefit - it is IEEE standard, while (R)PVST is a Cisco proprietary.

The problem you have mentioned about 6 and more switches is in default timers settings, and traditional STP operation - timer values are advertised in fields within the BPDU, so they should be changed (only after careful consideration) only on the root switch. Defaults are based on reference network model having diameter of seven switches. Network diameter can be changed on root switch, so all three timers (Hello=2, Forward Delay=15, Max Age=20) will be recalculated automatically. However I would not recommend changing this setting, better change you topology.

[neelesh.gaekwad]

Hi

thanks for your reply,]

the most important thing is that we cant able to change the topology bacause it is a plant. 

second if i play around with timers, it is risk it terms of troubleshooting if things will not work after changing it. ever it might be the person dependent.

the only solution which i believe is to use REP(resiliency Ethernet protocol)  capable switch 

[dmuinoorallo]

Hello,

The 802.1D specifications recommends that a spanning tree have no more than seven bridge hops. You have your issues for this.

http://standards.ieee.org/getieee802/download/802.1D-2004.pdf

You could check if you could stack your switches or other solution is configure different MST domains, this domain will be like a switch for the others domains. You could configure 7 switch hops like a domain in MST, and next 7 switchs hops like other domain in MST.

These are only solutions that i know.