Understanding Selecting Network Protocols

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Case Study: Configuring a RIP Network

This case study has been taken from the Cisco internetworking case studies book as a simple example of how to configure a RIP network. Contained within this case study is a variety of useful information concerning the proper configuration of RIP with a Cisco router.

Figure 3-5 illustrates a RIP network. Three sites are connected with serial lines. The RIP network uses a Class B address and an 8-bit subnet mask. Each site has a contiguous set of network numbers.


Figure 3-5  RIP network case study: configuring RIP.

Table 3-4 lists the network address assignments for the RIP network, including the network number, subnet range, and subnet masks. All interfaces indicate network 130.10.0.0; however, the specific address includes the subnet and subnet mask. For example, serial interface 0 on Router C has an IP address of 130.10.63.3 with a subnet mask of 255.255.255.0.

Table 3-4 RIP network address assignments
Network Number Subnets Subnet Masks

130.10.0.0 Site A: 8-15 255.255.255.0
130.10.0.0 Site B: 16-23 255.255.255.0
130.10.0.0 Site C: 24-31 255.255.255.0
130.10.0.0 Serial Backbone: 62-64 255.255.255.0

The following commands in the configuration file for Router A determine the IP address for each interface and enable RIP on those interfaces:

    interface serial 0    ip address 130.10.62.1 255.255.255.0    interface serial 1    ip address 130.10.63.1 255.255.255.0    interface ethernet 0    ip address 130.10.8.1 255.255.255.0    interface tokenring 0    ip address 130.10.9.1 255.255.255.0    router rip    network 130.10.0.0 

The following commands in the configuration file for Router B determine the IP address for each interface and enable RIP on those interfaces:

    interface serial 0    ip address 130.10.62.2 255.255.255.0    interface serial 1    ip address 130.10.64.2 255.255.255.0    interface ethernet 0    ip address 130.10.17.2 255.255.255.0    interface tokenring 0    ip address 130.10.16.2 255.255.255.0    router rip    network 130.10.0.0 

The following commands in the configuration file for Router C determine the IP address for each interface and enable RIP on those interfaces:

    interface serial 0    ip address 130.10.63.3 255.255.255.0    interface serial 1    ip address 130.10.64.3 255.255.255.0    interface ethernet 0    ip address 130.10.24.3 255.255.255.0    router rip    network 130.10.0.0 

Frequently Asked Questions

Q—What are the advantages and disadvantages of OSPF and EIGRP?
A—OSPF is an industry standard, and EIGRP is Cisco-specific. OSPF has had more time to evolve than EIGRP. EIGRP can be used for multiple protocols besides IP (AppleTalk and Novell, for example).
Q—Does Cisco support RIP to OSPF redistribution over X.25 IP unnumbered?
A—Yes.
Q—Why does RIP only maintain a single table entry for any specific destination?
A—Because it uses hop count as its sole metric, and for any specific destination it maintains information on the optimal route only.
Q—Why must the RIP route invalid timer be less than the route flush timer?
A—This is so that the router has time to update its neighbors before the invalid routes are deleted from its routing tables.
Q—How many destinations can be listed in a single RIP update packet?
A—Five destinations, and RIP will send as many as needed to update its neighbor.
Q—How many areas can a single ISIS process support?
A—Trick question. There are two answers. A Level-1 process can support only one area. You can configure up to three areas (by configuring three NETs on the process), but by definition the areas are merged into one (the Level-1 LSPs are all merged together). A Level-2 process can support many areas, because each area at Level 2 is simply a route. I would imagine that many hundreds could be supported these days.
Q—How does a router that is running IS-IS know it is a L1 or L2 router?
A—You must configure the router despite what is documented in the manuals. The default is both L1 and L2. When two neighboring routers discover they are in different areas, they could stop sending L1 hellos. Cisco routers won’t stop sending L1 hellos, because the neighbor’s level might change. When two neighboring routers are in the same area, they know they need a L1 adjacency, but there is no way for them to tell whether they should be L2 routers as well. So a person who knows the design of the network needs to configure those interfaces to be L1 only for efficiency reasons if there is no need for L2 routing.
Q—Should I specify the type of my router? (Level-1, Level-2, or Level-1-2)
A—Yes. The manual says that you don’t need to worry about this, but that is not true. In the default situation, each router will establish L2 adjacencies with all routers that have different area IDs, but L1 AND L2 adjacencies with all routers that have the same area ID. This is because a router cannot know whether its area is a transit area or not.
Not configuring the type of the router will result in each router having two databases (L1 and L2), all LSPs from both databases being flooded, and the SPF algorithm being run twice. So, configuring the type of route will save memory, CPU power, and bandwidth. When using ISIS for IP routing, not configuring L1 routers as L1 routers is even worse because each L2 router will report all IP prefixes reachable via L1 routing.
Q—How do I configure ISIS for CLNS?
A—Talk to the network administrator to get an NSAP (prefix). If this is a new network, design an area and backbone layout. If this is a small network (less than 30 routers), use only one area, and use Level-1 routing only. This means that all routers have an NSAP with the same prefix, and only the last 7 bytes (systemID plus n-selector) are different. Configure the router isis process. Preferably, do this for L1 only. Configure ISIS for each interface that needs to forward CLNS traffic.
Q—How do I configure the IS-IS router process.
A—In the router’s configuration mode, type:
    router isis    net    <my.variable.length.areaID>.<my.6.byte.systemID>.00    is-type level-1 
Q—How do I configure IS-IS for CLNS on an interface.
A—As an interface subcommand, configure clns router isis. That’s it!
    interface ethernet 0    clns router isis 
Q—How do I configure IS-IS for IP?
A—Do the same as you do for IS-IS for CLNS. Get an NSAP and configure the router process. Now on each interface where you want to establish ISIS adjacencies, configure ip router isis:
    interface ethernet 0    ip router isis 


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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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