Understanding Routing Protocols05/22/2001
In our previous discussion, Introducing the Router, I explained what a router is and a little bit about how it works. In this article, I'm going to pick up that thread and discuss routing protocols.
Routing protocols implement algorithms that tell routers the best paths through internetworks. Routing protocols include Border Gateway Protocol (BGP), Interior Gateway Routing Protocol (IGRP), Routing Information Protocol, and Open Shortest Path First (OSPF) to name a few. Routing protocols provide the layer 3 network state update. Protocols that are transported through a network, such as Internet Protocol (IP), Novell Internetwork Packet eXchange (IPX), and AppleTalk are called routed protocols.
In short, routing protocols route datagrams through a network. Routing is a layer 3 function, thus, routing and routed protocols are network-layer entities. Routing tables on the layer 3 switch (router) are populated by information from routing protocols. A routed protocol will enter an interface on a router, be placed in a memory buffer, then it will be forwarded out to an interface based on information in the routing table.
The IP protocol is the most common of the routed protocols. AppleTalk and Novell IPX still exist in some networks but are no longer as common as they were a few years ago. How does a routed protocol behave? Letís examine an IP packet in our fictitious Sprockets corporate network.
In this example, an Intel PC controlling robot, on the manufacturing floor segment of the Sprocketsí network, will make a database query to an IBM mainframe host, on the data center network segment. The manufacturing segment media is fast Ethernet and the IBM host segment is on token ring. A layer 3 IP datagram will have traverse different layer 2 encapsulations and layer 1 media.
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Both the IBM mainframe and the manufacturing floor robot controller are configured to use the layer 3 IP routed protocol. The IP datagram, originating in the controller PC, assigns the destination address of
188.8.131.52, the IP address of the IBM mainframe. The IP datagram is encapsulated in a layer 2 Ethernet frame and placed onto the wire. The beauty of the IP protocol is that a datagram doesnít know how to directly get to its destination. This is the work of a routing protocol.
Routing an IP datagram
The Sprocketís manufacturing router has network segment
184.108.40.206 directly connected to one of its fast Ethernet interfaces, FE1. The router sees the IP datagram as it enters on interface FE1. The router tears off the layer 2 Ethernet frame and reads the destination address,
The Sprocketís network is using Enhanced Interior Gateway Routing Protocol, EIGRP, for its network path algorithm. The router examines its routing table and determines that the IP datagramís next hop should be to the neighboring data center router, which advertises a route to segment
Based on the routing table, the datagram needs to be forwarded out through fast Ethernet interface FE0. Interface FE0 advertises a path to the desired
220.127.116.11 subnet, see Figure 2. The datagram is then encapsulated in a fast Ethernet frame and sent out interface FE0. Hop-by-hop, the IP datagram is handed off by routers in this manner until the datagram reaches its destination network segment. The idea of hops is to get the packet one hop closer each time through the network.
The next hop for the example IP datagram is the data center router, on the fast Ethernet backbone, interface FE0. The datagramís destination address is again examined. The Data Center router examines its routing table and learns that segment
18.104.22.168 is directly connected on its token ring interface, TR1. The datagram is then encapsulated for token ring and is sent out the token ring interface. The IBM mainframe listens on the wire and picks up the token ring frame from the router. The frame is removed and the IP datagram is processed.
You will notice that the IP addressing, is a layer 3 consistency in the Sprocketís network. But, the layer 2 topology was mixed with fast Ethernet and token ring architecture. The routers handled the crossing of media. The IP datagram wasnít involved with this process.
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