How to minimise the service disruption -RSTP recalculation

[sasanka1912]

Hi ,

Currently, we have the following connectivity in one of our offices, and the CORE-1 switch acts as the RSTP root bridge. Between CORE-1 and CORE-2 switches, we have a PO1 (port-channel) trunk interlink.

Recently, one of my colleague tried to add a new vlan across multiple trunk links and when he tried to configure switchport trunk allowed vlan add xxx ,in po1 interlink from CORE-2 Switch,  switchport trunk allowed vlan "add" syntax wasn't copied correctly and did override all the vlans in that PO1 trunk link. (This was rectified and resolved immediately)

During this time, Operations teams had noticed that most of the Access points dissociated from the Wireless controller and reconnected back with in 30 seconds due to RSTP recalculation.

My question is , what are the  additional steps we can take to minimise such service impacts when a single changes goes wrong in one link impacting entire building . 

I am looking for some advice regarding design/topology/configuration changes should consider on this ?

[sasanka1912]

@Joseph W. Doherty  we are using HSRP for SVI redundancy between two core devices .

re-If the former, outbound WAN traffic just using one link or two links? Outbound wan traffic using one link oppose to active/active .

[r.heitmann]

it might be a good idea, to either

a) enforce symmetrical routing - when "redistributing connected" networks into IGP do this at the HSRP-Active with good-metric and at the HSRP-Standby-Device with bad-metric (not required for Nexus with VPC of course)

or better (IMHO)

b) increase MAC-Aging-Time to at least ARP-Aging-Time at the HSRP-Gateway.

or even better, use a much longer-MAC-Aging than ARP-Aging - with ARP-Aging of 300s an MAC-Aging of 1800s is a good choice - and the default for Nexus/NXOS-Devices - Cisco "learned" and made Nexus "better" than Catalyst/IOS in this topic, but of course the didn't change the Catalyst/IOS-Defaults...

Doing this, you avoid unnecessary flooding in your (large) VLANs.

[sasanka1912]

@r.heitmann Thanks and some really good points to consider. By the way , do you have any example configurations that i can look in to for the above scenarios?

[Joseph W. Doherty]

Search for "unicast flooding".

[sasanka1912]

Unicast Flooding in Switched Campus Networks - Cisco

[Joseph W. Doherty · Accepted solution]

Yup.  What @r.heitmann "b" case was trying to avoid is also explained here: https://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/10583-62.html#toc-hId--925083550