Routing is the process of maintaining a table of destination network addresses. A router discards packets for unknown networks.
Types of Routes
Connected
- When an address and subnet mask is configured on a router’s interface, the router calculates which subnet that interface is on and puts that information in the routing table as a connected route.
Static
- Manually configured by an administrator; each static route must be configured on each router
- No overhead in processing, sending, or receiving updates
- Saves bandwidth and router CPU resources
- Routing table maintained by administrator
Dynamic
- A process that automatically exchanges information about available routes
- Uses metrics to determine the best path to a destination network
- The routing protocol must be configured on each router
- Bandwidth is consumed as routing updates are transmitted between routers
- Router CPU is used to process, send, and receive routing information
- Routing table maintained by routing process
Default
- Can be either static or dynamic
- A router uses the default route if it does not have an explicit route that matches the destination
Administrative Distance
The administrative distance is a number between 0 and 255 that rates the trustworthiness of the source of the routing information. A lower the number is considered better. The administrative distance is used when a router learns about the same route from different routing sources. Some of the default administrative distances are as follows:
| Route Source | Default Administrative Distance |
| Connected network | 0 |
| Static route | 1 |
| Internal EIGRP | 90 |
| OSPF | 110 |
| RIP (v1 and v2) | 120 |
| External EIGRP | 170 |
| Unknown | 225 |
Metric
The metric is how a routing protocol measures the “best” path to a destination network. A lower the number is considered better. Some of the routing protocol metrics are as follows:
| Routing Protocol | Metric |
| EIGRP | Composite metric; defaults to bandwidth and delay so that the fastest, lowest delay path is best. |
| OSPF | Cost; on a Cisco router this is inversely related to bandwidth so that the fastest path is best. |
| RIP (v1 and v2) | Hop count; the path with the least number of hops is best. |
Next week we’ll look at routing protocols.
Excerpted and available for download from Global Knowledge White Paper: CCNA v1.1 Exam Review: Critical Concepts of the 640 – 802 CCNA Exam
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Related Courses:
ICND1 — Interconnecting Cisco Network Devices 1
ICND2 — Interconnecting Cisco Network Devices 2
CCNAX — CCNA Boot Camp v1.1