IP Routing Fundamentals – Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

Internet Protocol Routing Protocol Fundamentals

• A routing protocol allows a router to dynamically learn how to reach other networks
• Routing protocols may be used to connect internal and external networks
• Different types of routing protocols use different means of determining the best path
• All routing protocols will differ in their scalability and performance

Flat and Hierarchical Routing

• Routing protocol algorithms operate using either a flat or hierarchical routing system
• A hierarchical routing system uses a layered approach
• Hierarchical routing systems reduce the information routers in the area must process
• A flat routing system has no hierarchy
• In a flat routing system, routers must typically be connected to every other router
• In a flat routing system, each router essentially has the same function
• The primary advantage afforded by hierarchical routing systems is their scalability
• Flat routing systems are not scalable and are difficult to troubleshoot

IP Addressing and Address Summarization

• An IP address is divided into two parts: the network address and the host address
• The IP addressing scheme should be hierarchical
• A hierarchical addressing scheme provides points in the network for summarization
• Summarization reduces the amount of information that routers must process
• Summarization allows for faster convergence in different areas of the internetwork
• Summarization restricts the size of the area that is affected by network changes

Administrative Distance

• The administrative distance is used to determine the reliability of different routing sources
• This default administrative distance value is in an integer between 0 and 255
• Routes with an administrative value of 255 are untrusted and are not placed into the RIB
• The administrative distance is a locally significant value that affects only the local router
• The default administrative distance values are listed in t he following table:

Routing Protocol Metrics

• All routing protocol algorithms use route metrics for network route preference selection
• The lower the route metric, the more preferred the route by the routing protocol
• The lowest cost route is placed into the routing table and is used to reach the destination
• Different routing algorithms use different variables to compute the route metric
• In most cases, the metrics used by one routing protocol are incompatible with another’s
• The different routing protocol metrics may be based on one or more of the following:

1. Bandwidth
2. Cost
3. Delay
4. Load
5. Path Length
6. Reliability

Prefix Matching

• Cisco routers use the longest prefix match rule when determining which route to use
• Longer, or more specific routing table entries are preferred over less specific entries

Building the Routing Table

• The routing table may include specific network entries or simply a single default route
• The routing table itself does not actually forward traffic
• The routing table is built using the following general steps:

1. If the route entry does not currently exist in the routing table, add it to the routing table
2. If the route entry is more specific than an existing route, add it to the routing table. It should also be noted that the less specific entry is still also retained in the routing table
3. If the route entry is the same as an existing one, but is received from a more preferred route source, replace the old entry with the new entry
4. If the route entry is the same as an existing one, and is received from the same protocol:

• Discard the new route if the metric is higher than the existing route
• Replace the existing route if the metric of the new route is lower
• If the metric for both routes is the same, use both routes for load-balancing

Routing Protocol Classes

• There are two major classes of routing protocols: Distance Vector and Link State
• A one-dimensional vector is a directed quantity
• SPF creates a shortest-path tree to all hosts in an area or within the backbone
• The three sets that are used in the SPF calculation are:

1. Unknown
2. Tentative (TENT)
3. PATH

• A Distance Vector routing protocol uses either distance or hop count as its primary metric
• Distance Vector protocols are primarily based on the Bellman-Ford algorithm
• Distance Vector protocols periodically send neighbors copies of their entire routing tables
• All Distance Vector routing protocols share the following characteristics:

1. Counting To Infinity
2. Split Horizon
3. Poison Reverse
4. Hold Down timers

• Link State routing protocols are hierarchical routing protocols that use logical network areas
• Routing running Link State routing protocols create a database of the network topology
• Link State routing protocols send either Link State Advertisements or Link State Packets
• Link State routing protocols send incremental updates when changes in the network occur
• BGP is not a Distance Vector routing protocol; instead it is a Path Vector routing protocol

The Objectives of Routing Protocols

• Routing algorithms, while different in nature, all have the same basic objectives and goals
• All routing algorithms are designed with the following objectives and goals:

1. Optimal Routing
2. Stability
3. Ease of Use
4. Flexibility
5. Rapid Convergence

On Demand Routing (ODR)

• CDP is a Cisco proprietary protocol that works on both broadcast and non-broadcast media
• ODR is an enhancement to CDP that advertises the connected prefixes of stub routers
• ODR also supports VLSM, which means that it can be used in modern-day internetworks
• On Demand Routing is not a routing protocol
• ODR is not CPU intensive and it consumes very little bandwidth
• When used in internetworks, ODR only requires an additional five bytes
• ODR can be redistributed into other routing protocols