Network Sales and Services Handbook (Cisco Press Networking Technology)

Each RIP routing table contains an entry for the Destination IP Address, a specific network host, and a subnet or a default route.

Technical Note: Default Routes

A default route is used when no other route is available to the specified destination. In most cases, the default route points to the Internet Service Provider (ISP) and is configured within the Cisco IOS as follows:

Router(config)# ip route 0.0.0.0 0.0.0.0 network_address network_mask

There are five fields in the RIP routing table. Table B-1 gives a brief explanation of each.

Table B-1. RIP Routing Table Fields

Field

Description

Destination IP Address

This is the destination of any data packet the RIP router receives. The RIP router looks up the destination address of the packet in its routing table to determine where to send the packet.

Metric

This is the total cost of carrying the packet from origination to destination. This field contains the sum of the costs associated with the network links comprising the end-to-end path across the network. RIP link costs are equal to 1 per link, with a total cost, or metric, of 16 being an unreachable destination.

Next Hop IP Address

This is the IP address of the next router interface in the network path to the ultimate destination. This field is populated in the router's table if the destination IP address is on a network not otherwise directly connected to the router.

Route Change

Specified in RFC 1058 this field is not always implemented by router vendors. This field is used to identify changes in routes to specific destination entries.

Route Timers

There are three timers associated with each route:

  • Update timer Initiates routing updates.

  • Route timeout Amount of time (default is 180 seconds) without a route update before the route is marked as invalid.

  • Route-flush Amount of time (default is 90 seconds) after the route timeout timer has expired before the route is flushed, or purged, from the routing table.

The following network topology, illustrated in Figure B-1, is used to demonstrate the abbreviated contents of a RIP routing table, focusing on hop count (distance-vector metric):

Figure B-1. Five-node RIP Routed Network

NOTE

The default cost for each hop is 1; however, the network administrator can manually adjust the cost so that links with less bandwidth (such as 56Kbps) could have a cost of 10, giving a weight to those links with more bandwidth (for example, T1).

Table B-2 reflects the RIP routing table for the network illustrated in Figure B-1.

Table B-2. RIP Routing Table for Five-Node Network (Figure B-1)

Router Name

Destination Host

Next Hop

Number of Hops

Via Network Path

A

Any B Network

B

1

Directly Connected

Any C Network

B

2

B-C

D

2

D-C

D

4

D-E-B-C

Any D Network

D

1

Directly Connected

Any E Network

B

2

B-E

D

2

D-E

B

4

B-C-D-E

B

Any A Network

A

1

Directly Connected

C

3

C-D-A

E

3

E-D-A

B

Any C Network

C

1

Directly Connected

A

3

A-D-C

Any D Network

A

2

A-D

C

2

C-D

E

2

E-D

Any E Network

E

1

Directly Connected

A

3

A-D-E

C

3

C-D-E

C

Any A Network

B

2

B-A

D

2

D-A

B

4

B-E-D-A

D

4

D-E-B-A

Any B Network

B

1

Directly Connected

D

3

D-A-B

Any D Network

D

1

Directly Connected

B

3

B-A-D

Any E Network

B

2

B-E

D

2

D-E

B

4

B-A-D-E

D

4

D-A-B-E

D

Any A Network

A

1

Directly Connected

C

3

C-B-A

E

3

E-B-A

Any B Network

A

2

A-B

C

2

C-B

E

2

E-B

Any C Network

C

1

Directly Connected

A

3

A-B-C

E

3

E-B-C

D

Any E Network

E

1

Directly Connected

A

3

A-B-E

C

3

C-B-E

E

Any A Network

B

2

B-A

D

2

D-A

B

4

B-C-D-A

D

4

D-C-B-A

Any B Network

B

1

Directly Connected

D

3

D-A-B

D

3

D-C-B

Any C Network

B

2

B-C

D

2

D-C

B

4

B-A-D-C

D

4

D-A-B-C

Any D Network

D

1

Directly Connected

B

3

B-A-D

B

3

B-C-D

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