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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 220.127.116.11; however, the specific address includes the subnet and subnet mask. For example, serial interface 0 on Router C has an IP address of 18.104.22.168 with a subnet mask of 255.255.255.0.
Table 3-4 RIP network address assignments
|Network Number ||Subnets ||Subnet Masks |
|22.214.171.124 ||Site A: 8-15 ||255.255.255.0 |
|126.96.36.199 ||Site B: 16-23 ||255.255.255.0 |
|188.8.131.52 ||Site C: 24-31 ||255.255.255.0 |
|184.108.40.206 ||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 220.127.116.11 255.255.255.0 interface serial 1 ip address 18.104.22.168 255.255.255.0 interface ethernet 0 ip address 22.214.171.124 255.255.255.0 interface tokenring 0 ip address 126.96.36.199 255.255.255.0 router rip network 188.8.131.52
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 184.108.40.206 255.255.255.0 interface serial 1 ip address 220.127.116.11 255.255.255.0 interface ethernet 0 ip address 18.104.22.168 255.255.255.0 interface tokenring 0 ip address 22.214.171.124 255.255.255.0 router rip network 126.96.36.199
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 188.8.131.52 255.255.255.0 interface serial 1 ip address 184.108.40.206 255.255.255.0 interface ethernet 0 ip address 220.127.116.11 255.255.255.0 router rip network 18.104.22.168
Frequently Asked Questions
- QWhat are the advantages and disadvantages of OSPF and EIGRP?
- AOSPF 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).
- QDoes Cisco support RIP to OSPF redistribution over X.25 IP unnumbered?
- QWhy does RIP only maintain a single table entry for any specific destination?
- ABecause it uses hop count as its sole metric, and for any specific destination it maintains information on the optimal route only.
- QWhy must the RIP route invalid timer be less than the route flush timer?
- AThis is so that the router has time to update its neighbors before the invalid routes are deleted from its routing tables.
- QHow many destinations can be listed in a single RIP update packet?
- AFive destinations, and RIP will send as many as needed to update its neighbor.
- QHow many areas can a single ISIS process support?
- ATrick 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.
- QHow does a router that is running IS-IS know it is a L1 or L2 router?
- AYou 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 wont stop sending L1 hellos, because the neighbors 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.
- QShould I specify the type of my router? (Level-1, Level-2, or Level-1-2)
- AYes. The manual says that you dont 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.
- QHow do I configure ISIS for CLNS?
- ATalk 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.
- QHow do I configure the IS-IS router process.
- AIn the routers configuration mode, type:
router isis net <my.variable.length.areaID>.<my.6.byte.systemID>.00 is-type level-1
- QHow do I configure IS-IS for CLNS on an interface.
- AAs an interface subcommand, configure clns router isis. Thats it!
interface ethernet 0 clns router isis
- QHow do I configure IS-IS for IP?
- ADo 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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