CS Summer 2003 CS672: MPLS Architecture, Applications and Fault-Tolerance

CS Summer 2003 Lecture 1

CS Summer 2003 Autonomous System (AS) AS is commonly defined as a set of interconnected routers that: are managed by single technical administration use one or more interior gateway protocols (IGP) to route packets within the AS (intra-AS) use exterior gateway protocols (EGPs) to route packets between AS’s (inter-AS) represent a single coherent picture of its reachable destinations. Examples of AS include: ISP, larger organization (e.g., college campus, enterprise), … To forward packets within (intra) and between (inter) ASs two different types of protocols namely IGPs and EGP. IGPs include OSPF, IS-IS, … EGPs include Border Gateway Protocol version 4 (BGP-4)

CS Summer 2003 Intra- and Inter-domain Routing AS 1 AS 2AS 3 Intra-domain routing IGP (OSPF, IS-IS, etc.) Intra-domain routing IGP (OSPF, IS-IS, etc.) Intra-domain routing IGP (OSPF, IS-IS, etc.) BGP-4 Inter-domain routing

CS Summer 2003 Stub, Multihomed and Transit AS’s If either source or destination of IP traffic resides inside an AS, that traffic is called local traffic for the AS. Otherwise, it is known as transit traffic. Based on local and transit traffic, AS can be classified into: Stub AS –has a single connection to the other AS –carries local traffic only (i.e., either source or sink) Multihomed AS –has connections more than one AS’s –carries local traffic only Transit AS –Has connections to more than one AS’s –carries local as well as transit traffic

CS Summer 2003 Stub or single home AS AS 1 (e.g.,service provider) Stub AS (e.g., enterprise) Stub AS (e.g., enterprise)

CS Summer 2003 Multihomed Non-transit AS AS 2 (e.g.,service provider B) Stub AS (e.g., enterprise) AS 1 (e.g.,service provider A)

CS Summer 2003 Transit AS AS 2 (e.g.,service provider B) Transit AS (e.g., backbone provider) AS 2 (e.g.,service provider B)

CS Summer 2003 Static Routing A router learns about routes (i.e., destination prefixes) in two ways: configurations and routing protocols Routes that are manually configured are known as static routes. Static routes continue to remain in the routing table independent of status of the network and destination reachability. Advantages of Static Routing  Simpler because does not require exchanging routing information.  Does not cause routing churn in the routers due change in network topology events (e.g., link up/down events). Disadvantages  Does not reflect actual status of the network and may result in blackholing the traffic

CS Summer 2003 Dynamic Routing Routes that are learned through routing protocols are known as dynamic routes. Dynamic routes are added or deleted based on network topology. Advantages of dynamic routing  reachability of dynamic routes depends upon the status of the network. For instance, if certain destination is not reachable due to link/node failure that route is removed from the routing table. Disadvantages of dynamic routing  requires routing protocols and exchange of routing information  False network topology change may cause generation of routing flaps, recompilations of routes through the routing domain.

CS Summer 2003 Default Routing Default routing is the term commonly used for a routing entry that a router uses to forward traffics for destinations unknown destinations. The gateway of last resort can either be configured or learned via routing protocols Simplest form of routing for a Sub AS which have a single exit point to another AS.