ROUTING PROTOCOLS
A generic term that refers to a formula, or protocol, used by a router to determine the appropriate path over which data is transmitted. The routing protocol also specifies how routers in a network share information with each other and report changes. The routing protocol enables a network to make dynamic adjustments to its conditions, so routing decisions do not have to be predetermined and static.
Routing, Routed and Non-Routable Protocols
ROUTING | ROUTED | NON-ROUTABLE
ROUTING PROTOCOLS
ROUTING PROTOCOLS are the software that allow routers to dynamically advertise and learn routes, determine which routes are available and which are the most efficient routes to a destination. Routing protocols used by the Internet Protocol suite include:
? Routing Information Protocol (RIP and RIP II).
? Open Shortest Path First (OSPF).
? Intermediate System to Intermediate System (IS-IS).
? Interrior Gateway Routing Protocol (IGRP).
? Cisco's Enhanced Interior Gateway Routing Protocol (EIGRP).
? Border Gateway Protocol (BGP).
Routing is the process of moving data across two or more networks. Within a network, all hosts are directly accessable because they are on the same
ROUTED PROTOCOLS
ROUTED PROTOCOLS are nothing more than data being transported across the networks. Routed protocols include:
? Internet Protocol
o Telnet
o Remote Procedure Call (RPC)
o SNMP
o SMTP
? Novell IPX
? Open Standards Institute networking protocol
? DECnet
? Appletalk
? Banyan Vines
? Xerox Network System (XNS)
Outside a network, specialized devices called ROUTES are used to perform the routing process of forwarding packets between networks. Routers are connected to the edges of two or more networks to provide connectivity between them. These devices are usually dedicated machines with specialized hardware and software to speed up the routing process. These devices send and receive routing information to each other about networks that they can and cannot reach. Routers examine all routes to a destination, determine which routes have the best metric, and insert one or more routes into the IP routing table on the router. By maintaining a current list of known routes, routers can quicky and efficiently send your information on it's way when received.
There are many companies that produce routers: Cisco, Juniper, Bay, Nortel, 3Com, Cabletron, etc. Each company's product is different in how it is configured, but most will interoperate so long as they share common physical and data link layer protocols (Cisco HDLC or PPP over Serial, Ethernet etc.). Before purchasing a router for your business, always check with your Internet provider to see what equipment they use, and choose a router, which will interoperate with your Internet provider's equipment.
NON-ROUTABLE PROTOCOLS
NON-ROUTABLE PROTOCOLS cannot survive being routed. Non-routable protocols presume that all computers they will ever communicate with are on the same network (to get them working in a routed environment, you must bridge the networks). Todays modern networks are not very tolerant of protocols that do not understand the concept of a multi-segment network and most of these protocols are dying or falling out of use.
? NetBEUI
? DLC
? LAT
? DRP
? MOP
RIP (Routing Information Protocol)
RIP is a dynamic internetwork routing protocol primary used in interior routing environments. A dynamic routing protocol, as opposed to a static routing protocol, automatically discovers routes and builds routing tables. Interior environments are typically private networks (autonomous systems). In contrast, exterior routing protocols such as BGP are used to exchange route summaries between autonomous systems. BGP is used among autonomous systems on the Internet.
RIP uses the distance-vector algorithm developed by Bellman and Ford (Bellman-Ford algorithm).
Routing Information Protocol
Background
The Routing Information Protocol, or RIP, as it is more commonly called, is one of the most enduring of all routing protocols. RIP is also one of the more easily confused protocols because a variety of RIP-like routing protocols proliferated, some of which even used the same name! RIP and the myriad RIP-like protocols were based on the same set of algorithms that use distance vectors to mathematically compare routes to identify the best path to any given destination address. These algorithms emerged from academic research that dates back to 1957.
Today's open standard version of RIP, sometimes referred to as IP RIP, is formally defined in two documents: Request For Comments (RFC) 1058 and Internet Standard (STD) 56. As IP-based networks became both more numerous and greater in size, it became apparent to the Internet Engineering Task Force (IETF) that RIP needed to be updated. Consequently, the IETF released RFC 1388 in January 1993, which was then superceded in November 1994 by RFC 1723, which describes RIP 2 (the second version of RIP). These RFCs described an extension of RIP's capabilities but did not attempt to obsolete the previous version of RIP. RIP 2 enabled RIP messages to carry more information, which permitted the use of a simple authentication mechanism to secure table updates. More importantly, RIP 2 supported subnet masks, a critical feature that was not available in RIP.
This chapter summarizes the basic capabilities and features associated with RIP. Topics include the routing update process, RIP routing metrics, routing stability, and routing timers.
Routing Updates
RIP sends routing-update messages at regular intervals and when the network topology changes. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route. The metric value for the path is increased by 1, and the sender is indicated as the next hop. RIP routers maintain only the best route (the route with the lowest metric value) to a destination. After updating its routing table, the router immediately begins transmitting routing updates to inform other network routers of the change. These updates are sent independently of the regularly scheduled updates that RIP routers send.
RIP Routing Metric
RIP uses a single routing metric (hop count) to measure the distance between the source and a destination network. Each hop in a path from source to destination is assigned a hop count value, which is typically 1. When a router receives a routing update that contains a new or changed destination network entry, the router adds 1 to the metric value indicated in the update and enters the network in the routing table. The IP address of the sender is used as the next hop.
RIP Stability Features
RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops in a path is 15. If a router receives a routing update that contains a new or changed entry, and if increasing the metric value by 1 causes the metric to be infinity (that is, 16), the network destination is considered unreachable. The downside of this stability feature is that it limits the maximum diameter of a RIP network to less than 16 hops.
RIP includes a number of other stability features that are common to many routing protocols. These features are designed to provide stability despite potentially rapid changes in a network's topology. For example, RIP implements the split horizon and holddown mechanisms to prevent incorrect routing information from being propagated.
RIP Timers
RIP uses numerous timers to regulate its performance. These include a routing-update timer, a route-timeout timer, and a route-flush timer. The routing-update timer clocks the interval between periodic routing updates. Generally, it is set to 30 seconds, with a small random amount of time added whenever the timer is reset. This is done to help prevent congestion, which could result from all routers simultaneously attempting to update their neighbors. Each routing table entry has a route-timeout timer associated with it. When the route-timeout timer expires, the route is marked invalid but is retained in the table until the route-flush timer expires.
Packet Formats
The following section focuses on the IP RIP and IP RIP 2 packet formats illustrated in Figures 44-1 and 44-2. Each illustration is followed by descriptions of the fields illustrated. RIP Packet Format
? Command—Indicates whether the packet is a request or a response. The request asks that a router send all or part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables.
? Version number—Specifies the RIP version used. This field can signal different potentially incompatible versions.
? Zero—This field is not actually used by RFC 1058 RIP; it was added solely to provide backward compatibility with prestandard varieties of RIP. Its name comes from its defaulted value: zero.
? Address-family identifier (AFI)—Specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address being specified. The AFI for IP is 2.
? Address—Specifies the IP address for the entry.
? Metric—Indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.
Note: Up to 25 occurrences of the AFI, Address, and Metric fields are permitted in a single IP RIP packet. (Up to 25 destinations can be listed in a single RIP packet.)
RIP 2 Packet Format
? Command—Indicates whether the packet is a request or a response. The request asks that a router send all or a part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables.
? Version—Specifies the RIP version used. In a RIP packet implementing any of the RIP 2 fields or using authentication, this value is set to 2.
? Unused—Has a value set to zero.
? Address-family identifier (AFI)—Specifies the address family used. RIPv2's AFI field functions identically to RFC 1058 RIP's AFI field, with one exception: If the AFI for the first entry in the message is 0xFFFF, the remainder of the entry contains authentication information. Currently, the only authentication type is simple password.
? Route tag—Provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).
? IP address—Specifies the IP address for the entry.
? Subnet mask—Contains the subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry.
?Next hop—Indicates the IP address of the next hop to which packets for the entry should be forwarded.
? Metric—Indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.
Note: Up to 25 occurrences of the AFI, Address, and Metric fields are permitted in a single IP RIP packet. That is, up to 25 routing table entries can be listed in a single RIP packet. If the AFI specifies an authenticated message, only 24 routing table entries can be specified. Given that individual table entries aren't fragmented into multiple packets, RIP does not need a mechanism to resequence datagrams bearing routing table updates from neighboring routers.
Summary
Despite RIP's age and the emergence of more sophisticated routing protocols, it is far from obsolete. RIP is mature, stable, widely supported, and easy to configure. Its simplicity is well suited for use in stub networks and in small autonomous systems that do not have enough redundant paths to warrant the overheads of a more sophisticated protocol.
Review Questions
Q—Name RIP's various stability features.
A—RIP has numerous stability features, the most obvious of which is RIP's maximum hop count. By placing a finite limit on the number of hops that a route can take, routing loops are discouraged, if not completely eliminated. Other stability features include its various timing mechanisms that help ensure that the routing table contains only valid routes, as well as split horizon and holddown mechanisms that prevent incorrect routing information from being disseminated throughout the network.
Q—What is the purpose of the timeout timer?
A—The timeout timer is used to help purge invalid routes from a RIP node. Routes that aren't refreshed for a given period of time are likely invalid because of some change in the network. Thus, RIP maintains a timeout timer for each known route. When a route's timeout timer expires, the route is marked invalid but is retained in the table until the route-flush timer expires.
Q—What two capabilities are supported by RIP 2 but not RIP?
A—RIP 2 enables the use of a simple authentication mechanism to secure table updates. More importantly, RIP 2 supports subnet masks, a critical feature that is not available in RIP.
Q—What is the maximum network diameter of a RIP network?
A—A RIP network's maximum diameter is 15 hops. RIP can count to 16, but that value is considered an error condition rather than a valid hop count.
This article is submitted by Kashif Raza <a target="_new" href="http://www.networkingtutorials.net">http://www.networkingtutorials.net</a>
