1 OSPF Dynamics On a Broadcast LAN M Goyal, W Xie, N Suresh, M Godala, M Soperi, SH Hosseini, K Vairavan EECS Department, University of Wisconsin - Milwaukee, 3200 N Cramer Street, Milwaukee, WI 53201 Email: mukul@uwm.edu, weigaoxie@163.com, {nsuresh,mgodala,msm2,hosseini,kv}@uwm.edu Corresponding author: M Goyal Abstract—In this paper, we analyze the behavior of OSPF’s interface state machine in a broadcast LAN scenario. We derive several interesting properties of this behavior that help us estimate the number of DR elections performed and the time required as each router on the LAN settles on the DR/BDR iden- tity. The analytical results are verified using testbed experiments. Index Terms— OSPF, Broadcast LAN, DR Election I. I NTRODUCTION OSPF [1] is a link state routing protocol used for intra-AS routing. OSPF has been a popular choice for intra-AS routing and enjoys a large deployed base perhaps because of a robust design that provides a good trade-off between the protocol’s processing-load/control-traffic and the speed with which it converges to topology changes. Given its popularity and wide deployment, it is important to analyze and understand different aspects of the OSPF protocol. In this paper, we examine the dynamics of OSPF protocol in a broadcast LAN scenario. In OSPF, two neighbor routers are said to be fully adjacent to each other if their link state databases are synchronized. The need for database synchronization makes adjacency es- tablishment and maintenance a costly process in terms of processing overhead and time consumption. In a broadcast LAN scenario, where every other router can be considered as a neighbor, establishment/maintenance of full adjacency with each neighbor may put significant burden on a router. Hence, in order to reduce the number of full adjacencies required, the OSPF protocol requires the routers on a LAN to elect one router among themselves to function as the Designated Router (DR). The DR originates a network-LSA on behalf of the LAN and establishes full adjacency with every other router on the LAN. The routers on the LAN also elect a Backup Designated Router (BDR) among themselves, which also establishes full adjacency with every other router on the LAN. In the event of the DR’s failure, the BDR can quickly take over the responsibilities of the DR since it is already fully adjacent to all the routers on the LAN. The other routers that are neither DR nor BDR establish full adjacency only with DR and BDR and consider themselves to be DR Other. The transition of a router (actually its LAN interface) from its initial Down state to one of the DR/BDR/DROther states is determined by OSPF’s interface state machine (Figure 1). Depending upon the order in which different routers on the LAN come up, the routers may take a long time to settle on the identity of the DR and BDR for the LAN and may perform several DR elections in the process. In this paper, we analyze the behavior of OSPF’s inter- face state machine over a broadcast LAN environment (e.g. Ethernet). The broadcast networks differ from non-broadcast networks in their ability to “broadcast” a single physical mes- sages to all of the routers on the network. In such networks, a router discovers other routers on the LAN dynamically using the Hello protocol[1]. On the other hand, in non-broadcast networks (e.g. X.25), dynamic discovery may not be possible and neighbors may need to be configured. Network Model and Performance Metrics: The network model considered in this paper consists of a number of routers connected over a broadcast LAN. Initially, all routers are down. The routers come up in a random order at random times. Once up, the routers stay up. Each router sends its first Hello immediately upon coming up and subsequently after each hello interval H . We assume that no Hello packet is lost and the time required for the Hello to reach other routers as well as the time required for each router to perform a DR election is negligible. Let the DR/BDR settling time for a router be the time required by the router to settle on the final identity of the DR and BDR for the LAN (i.e. the time after which the router does not alter its view of the identity of DR and BDR routers). The DR/BDR settling time for the LAN as a whole can be defined as the latest DR/BDR settling time for a router on the LAN. In other words, the DR/BDR settling time for the LAN is the time required for all the routers on the LAN to settle on the identity of the DR and BDR for the LAN. In this process, each router may perform several DR elections. Some of these elections may be performed after the settlement of DR/BDR identity. We use the DR/BDR settling time for different routers on the LAN and the number of DR elections they perform in the process as the metrics in our analysis. Terminology: In the following discussion, we use the term “router” to mean its interface on the broadcast LAN where it is more convenient to do so. A received Hello is characterized as 2-way or 1-way based on whether it lists the receiving router as a neighbor or not. A router associates 2-way or higher state with a neighbor when it receives a 2-way Hello from the neighbor 1 A router is said to have established the bidirectional communication with a neighbor when it associates the 2- way or better state with the neighbor. A router elects a DR and a BDR from the set of routers consisting of itself and its bidirectional neighbors. Henceforth, we refer to this set as the set of eligible routers. Finally, we define an eligible 1 The neighbor state can also reach 2-way or better status when the router receives a Database Description packet from the neighbor [1].