First, here is the setup:
2 6500's with dual Sup 720's connected together with dual Gigabit etherchannel 802.1Q trunks. Redundancy mode is SSO/NSF and they run in native mode
Each router runs HSRP for the VLANS shared between them. One is active for all the VLANS, the other standby for all the VLANS. Rapid spanning-tree is used to decrease spanning-tree topology change times.
Most hosts connected to theses routers are single-connected to one or the other.
NSF is enable on EIGRP which is the site routing protocol
Distribution-layer switches are dual-connected to both routers
My basic question is what is the best way to upgrade the IOS on the switches to eliminate or minimize loss of network connectivity? On 4500's, I can use ISSU to do 0 downtime updates, but the only reference to ISSU in the 6500's I can find always mentions modular IOS patches. I am not opposed to moving to modular IOS if that is required, but I am unsure if Modular IOS can do a 0 downtime IOS upgrade.
Thanks for any responses