For most of the companies, the main criteria in the making of the decision are…. the network uptime.

If a person is running the 24×7, mission serious (lifesaving and 24 hours corporate) infrastructures, it paid off highly for using both the Tier 2 (for the cost savings) and the Tier 1, & even a backup network for the Tier 3. There are many networks which are using a Tier 1 provider for the leased line access and then fail over to the ISDN for the backup. That is horribly unsuccessful. As the common failure points in any of the physical networks remain the last mile & whatever affects a person’s last mile, it tends to affect all the circuits simultaneously. Assuming that they are in a same pipe and coming from a same CO.

Instead the famous rule is that – pick up the vendor which can give a person the maximum SLA for budget, & whom is eager to build the personal relationship smooth. These give the person a single point of contact when the problem arises - but, there are caveats and prevent the lot of accidents even though it does not prevent the provider’s side core routing failures.