Dr. John P. Abraham Professor University of Texas Pan American Internet Routing and Routing Protocols

Static vs. Dynamic routing static: create a forwarding table when system starts and does not change entries unless administrator manually does so. Dynamic: starts with an initial static route. But, a route propagation software runs on the system that communicates with other systems and continuously updates the forwarding table to create optimum routes.

Static routing in Hosts and default route Static routing is fast since it does not require a software nor communicates with other systems for routes. However, it is inflexible. It cannot accommodate for network failures or changes in topology. If the host only has one network connection and a single router, static works very well. The default entry for next hop is the routers local address. The router forwards all packets not intended for the local LAN to the wan side. There needs to be two entries in the table, one entry for packets intended for computers in the LAN (directly connected) and another for all other packets.

Dynamic routing and routers Each router knows about directly connected networks: interface will have the same network address as the connected one (only the host portion differ). To ensure that all routers obtain information about how to reach each possible destination, each router runs software that uses a route propagation protocol to exchange route information. Each router learns about destination other routers can reach. The routing software uses incoming information to update local forwarding table.

Routing in the Global Internet Impossible to exchange information between all routers within the global internet. To limit routing traffic, the Internet uses a routing hierarchy. Routers and networks in the Internet are divided into groups. Only routers within a group exchange routing information. One representative router summarizes routing information within that group and exchanges it with the representative from another group. A group of routers are knows as AS (autonomous system). Number of routers within an AS may vary for economic, technical or administrative reasons.

Interior and Exterior Routing Protocols. Within the AS, Interior Gateway Protocols (IGPs) are used to exchange information. To exchange information between the ASs, Exterior Gateway Protocols (EGPs) are used. Here one desginated router from on AS communicates with a peer from another AS.

Autonomous systems

Figure Internal and external BGP sessions

IGP METRICS IGP Must choose optimal paths. This requires weights assigned to edges in a graph, known metrics. The metric may be different depend on need: delay, throughput, hops, cost, etc. EGP does not use metrics, nor attempt to achieve optimum path.

Figure 14.2 Popular routing protocols

Interior Gateway Protocols 1. RIP: Routing Information Protocol  Hop count matric  RIP uses UDP to transfer messages (port 520)  Uses Broadcast or Multicast Delivery  Supports both subnetting and CIDR  Allows for default route  Uses Distance Vector algorithm. Each entry in the advertisement includes reachable destination network and distance (hops).  Number of entries in propagation will depend on how many networks are reachable.

Distance vector routing tables

Interior Gateway Protocols 2. OSPF (Open Shortest Path First protocol)  Can satisfy large organizations as it allows administrator to partion an AS into smaller partitions (OSPF areas).  Uses Dijkstra’s shortest path first algorithm. CIDR support. Authenticated message exchange. OSPF allows a router to introduce routes learned from another means (example from exterior gateway protocols- BGP). Administrator may assign any metric cost. OSPF uses graph theory to compute shortest route.

Areas in an autonomous system

Interior Gateway Protocols 3. Intermediate System TO Intermediate system  Designed by DECNET V.  It is a proprietary version of OSPF

Exterior Gateway Protocols Border Gateway Protocol (BGP) is an interdomain routing protocol using path vector routing. It first appeared in 1989 and has gone through four versions. It provides the glue that holds Internet routing together. At the center of the Internet, Tier-1 ISPs use BGP to exchange routing information and learn about each other’s customers. BGP supports classless addressing and CIDR. BGP uses the services of TCP on port 179. Function is to route among Ass Provides for administrator to set policies. For example, it may be configured to restrict which routes BGP advertises to outsiders.

Types of BGP messages

Muticast Routing In multicasting, the router may forward the received packet through several of its interfaces. Dynamic group membership. Application can choose to participate in a group at any time and remain a participant for an arbitrary duration. To join a group a host informs a nearby router. Neither the sender or the reciever knows the identity or the number of group members.

IGMP-Internet Group Multicast Protocol This protocol defines the host, not the application. Multicast protocols use three different approcahes for datagram forwarding: Flood-and-Prune, Configuration-AND- Tunneling, and Core-based discovery

Multicast Protocols Distance Vector Multicast Routing Protocol Internet Multicast BackBone (MBONE)