Also make sure you fully understand what the different severity levels mean:
https://www.cisco.com/c/en/us/support/web/tac/technical-services-resource-guide.html
https://www.cisco.com/c/dam/en_us/about/doing_business/legal/service_descriptions/docs/cisco-severity-and-escalation-guidelines.pdf
The severity is mostly determined by the business impact of the fault. Generally sev 1 means whole network/business/organisation down while sev 2 means very substantial impact to network/business/organisation (large % of users/sites impacted). Anything that doesn't meet those criteria will normally be sev 3. To open a sev 1 or 2 case you should always contact TAC by phone and clearly explain the business impact of the problem and be prepared to stay on hold for some time while they locate an engineer who is available to work the problem with you.
Higher severity levels commit Cisco TAC to higher levels of engagement (eg sev 1 = 24 x 7 work on the case) but requires the same level of commitment from you. If Cisco are working 24x7 on the case (they'll pass the case around the world as teams go on and off shift) you have to be prepared to work with them on it 24x7 as well (any time of day or night 7 days a week). If you do not, then they will automatically lower the severity to a level which you are prepared to work at.
I should also add that all of the above applies for paid Cisco support customers (who have purchased Cisco support contracts). If you are accessing free small business product support then refer to the small business support terms and conditions - I don't think it includes access to sev 1 or 2 support.
