Alternative routing Hierarchical Ranked into several tiers Non-hierarchical More flexible and efficient Accommodate sudden strong increase traffic of any od pair · Reduce cost Mutual overflow strong dependency Instability can be mitigated by trunk reservation• Alternative routing – Hierarchical • Ranked into several tiers – Non-hierarchical • More flexible and efficient • Accommodate sudden strong increase traffic of any OD pair • Reduce cost • Mutual overflow • strong dependency • Instability – can be mitigated by trunk reservation 8