This article defines EX-series Switch MSTP compatibility with Cisco Catalyst 2950 Switch and 2950 based blades for IBM/HP servers using pre-standard digest.

Cisco Catalyist switch computes a pre-standard digest which results in the EX-series Switch and Cisco Catalyst 2950 Switch not understanding the MSTI instances when connected on a dot1q trunk.

The Cisco Catalyist switch computes a pre-standard digest which results in the EX-series Switch and Cisco Catalyst 2950 Switch not understanding the MSTI instances when connected on a dot1q trunk.

The issue arises since the Cisco Catalyist 2950 Switch uses the pre-standard MSTP implementation. Based on this the EX-series Switch will only understand the CIST instance BPDU, or MSTI 0, and cannot understand the extended MST BPDU information since the cisco computes a pre-standard digest.

The solution is to map all instances to MIST 0 for the Cisco Switch and EX-series Switch to communicate.  We can still use LAG or etherchannels to load-balance traffic across multiple links, even though only a single topology will be in use.