On the interaction of multiple routing algorithms

Abstract

Internet routers can obtain distinct routes to the same destination from multiple routing protocols. A mechanism called administrative distance (AD) has evolved to implement route selection in such cases. AD simply imposes a preference ranking among routing protocols. A related mechanism called route redistribution (RR) is used to increase connectivity by injecting routes from one protocol into another. Recent research has shown that the current design of RR with AD mechanisms can give rise to routing and forwarding anomalies that are difficult to debug. We present a new algebraic model that captures not only RR with AD, but also sub-protocol interactions that exist within protocols such as OSPF and IS-IS. Compared with previous work, our model provides a clearer view of the issues and trade-offs involved.

Modeling complexity of enterprise routing design

“…In several recent papers [4,16,20], researchers have revealed the common use of multiple routing protocol instances in one enterprise network. This is not surprising as the evolution of an enterprise's computer network parallels that of its business.…”

“…Another complication is that multiple routing instances may simultaneously offer routes to the same destination at a border router. The operator must implement the correct order of preference between the instances, sometimes requiring an override of some protocols' default Administrative Distance (AD) [4,18] values on a per border router basis.…”

“…There is a considerable amount of prior work on modeling individual routing protocols, particularly BGP [3,8,12,14], and also OSPF [21], to ensure correct, safe, and efficient behaviors from these protocols. There is also recent progress on safe migration of IGP protocols [25] and on modeling the interaction between multiple routing algorithms deployed in the same network [4]. In contrast, our work analyzes how specific routing protocols and primitives should be combined to meet a given set of design objectives, and the focus is on minimizing the complexity of the resulting design.…”

On the forwarding paths produced by Internet routing algorithms

“…Using the A2A method it may be easier to reveal the problem being solved by the original metric/algorithm (global optimality, left-or right-local optimality). Among other applications, this method has been used to model eBGP vs. iBGP [16], areas in OSPF and levels in ISIS, route redistribution and administrative distance [17], [18].…”