Understanding Selecting Network Protocols

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RIP Packet Format

The RIPv2 specification (described in RFC 1723) enables more information to be included in RIP packets and provides a simple authentication mechanism. Figure 3-1 shows the IP RIP 2 packet format:

The field definitions for a RIP packet are as follows:

  Type. Includes the following:
  Command. Indicates whether the packet is a request or a response.
  Request. Asks that a router send all or part of its routing table.
  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 being used. In a RIP packet implementing any of the RIP fields or using authentication, this value is set to 2.
  Zero. Unused.
  Address Family Identifier (AFI). Specifies the address family being 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 address family identifier for IP is 2.
  If the Address Family Identifier 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. 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.


Figure 3-1  RIP packet format.

Up to 25 occurrences of the Address Family Indicator, 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 address family indicator specifies an authenticated message, only 24 routing table entries can be specified.

Configuring RIP

If the router has a default network path, RIP advertises a route that links the router to the pseudo network 0.0.0.0. The network 0.0.0.0 does not exist; RIP treats 0.0.0.0 as a network to implement the default routing feature. The Cisco IOS software will advertise the default network if a default was learned by RIP, or if the router has a gateway of last resort and RIP is configured with a default metric.

RIP sends updates to the interfaces in the specified networks. If an interface’s network is not specified, it will not be advertised in any RIP update.

Cisco’s implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).

RIP Configuration Task List

To configure RIP, complete the tasks in the following sequence. You must enable RIP. The remaining tasks are optional.

  Enable RIP
  Allow unicast updates for RIP
  Specify a RIP version
  Enable RIP authentication
  Disable route summarization
  Run IGRP and RIP concurrently to ensure that traffic is flowing properly across the network
  Disable the validation of source IP addresses
  Configure interpacket delay

Link State Protocols

Link state routing protocols require routers to periodically send routing updates to all other routers in the internetwork. However, each router sends only that portion of the routing table that describes the state of its own links. Link state routing protocols are fast to converge their routing updates across the network in comparison to distance vector protocols.

Their fast convergence makes link state protocols less prone to routing loops than distance vector protocols. However, they also require more CPU power and system memory. One of the primary reasons additional CPU power and memory are needed is that link state protocols are based on the distributed map concept, which means every router has a copy of the network map that is regularly updated.

Link state protocols are based on link state algorithms, which are also called shortest path first (SPF) algorithms or Dijkstra algorithms.

A simple way to think about how link state technology operates is to picture the network as a large jigsaw puzzle; the number of pieces in your puzzle is dependent upon the size of your network. Each piece of the puzzle holds only one router or one LAN. Each router “draws” itself on that jigsaw piece, including little arrows to other routers and LANs. Those pieces are then replicated and sent throughout the network from router to router, until each router has a complete and accurate copy of each and every piece of the puzzle. Each router then assembles these pieces by using the shortest path first (SPF) algorithm. The SPF algorithm determines how the various pieces of the puzzle fit together.

The Link-State Database

As mentioned, the principle of link-state routing is that all the routers within a network maintain an identical copy of the network topology. From this map, each router will perform a series of calculations that will determine the best routes. This network topology is contained within a database, where each record represents the links to a particular node in the network.

Each record contains several pieces of information: an interface identifier, a link number, and metric information regarding the state of the link. With that information, each router can quickly compute the shortest path from itself to all other routers.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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