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to properly service the network.
Routing tables and network topology are stored in router memory. Using a route summarization
technique as described earlier reduces the memory requirement. In addition, routing protocols that use
areas or domains in a hierarchical topology requires the network design to use small areas rather than
large areas to help in reducing the memory consumption.
Calculation of the routes is a CPU intensive process. Through route summarization and the use of
link-state routing protocols the CPU utilization is greatly reduced since the number of routes needing
re-computing is reduced.
Bandwidth on the connections to each router becomes a factor in not only scaling the network but in
convergence time. Routing protocols learn of neighbor routers for the purpose of receiving and sending
routing table updates. The type of routing protocol in use will determine its affect on the bandwidth.
Distance-vector routing protocols such as RIP and IGRP send their routing tables at regular intervals.
The distance-vector routing protocol waits for the time interval before sending its update even when a
network change has occurred. In stable networks this type of updating mechanism wastes bandwidth,
however, protects the bandwidth from an excessive routing update load when a change has occurred.
However, due to the periodic update mechanism, distance vector protocols tend to have a slow
convergence time.
Link-state IP routing protocols such as OSPF and IS-IS address bandwidth wastefulness of
distance-vector routing protocols and slow time to converge. However, due to the complexity of
providing this enhancement link-state protocols are CPU intensive, require higher memory utilization
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and bandwidth during convergence. During network stability, link-state protocols use minimal network
bandwidth. After start-up and initial convergence, updates are sent to neighbors only when the network
topology changes. During a recognized topology change, the router will flood its neighbors with the
updates. This may cause excessive load on the bandwidth, CPU and memory of each router. However,
convergence time is lower than that of distance-vector protocol.
Cisco's proprietary routing protocol EIGRP is an advanced version of distance-vector protocols with
properties of link-state protocols. From distance-vector protocols, EIGRP has taken many of the metrics
for route calculation. The advantageous of link-state protocols are used for sending routing updates only
when changes occur. While EIGRP preserves CPU, memory and bandwidth during a stable network
environment, it does have high CPU, memory and bandwidth requirements during convergence.
The convergence ability of the routing protocols and their affect on CPU, memory and bandwidth has
resulted in guidelines form Cisco on the number of neighbors that can be effectively supported. Table
3.x lists the suggested neighbors for each protocol.
Routing Protocol Neighbors per Router
Distance vector (RIP, IGRP) 50
Link state (OSPF, IS-IS) 30
Advanced distance vector (EIGRP) 30
6. Security
Routing protocols can be used to provide a minimal level of security. Some of the security functions available on routing
protocols are:
Filtering route advertisements
Authentication
Using filtering, routing protocols can prohibit the advertisements of routes to neighbors thereby protecting certain parts of
the network. Some of the routing protocols authenticate their neighbor prior to engaging in routing table updates. Though
this is protocol specific and generally a weak form of security, it does protect unwanted connectivity from other networks
using the same routing protocol.
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Cisco Router Handbook
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Chapter Four
IP Routing Protocol Design
Routing is the process of moving packets from one network to another. The routing decision takes place
at the source network device. That is a router. The decision is made based on metrics used for a
particular routing protocol. Routing protocols may use some or all of the following metrics in
determining the best route to a destination network:
Path length
Reliability
Delay
Bandwidth
Load
Communication cost
Path length is measure in either a cost or a hop count. In link-state routing protocols, the cost is the sum
of the costs associated with each link in the path. Distance-vector routing protocols assign a hop count to
the path length, which measures the number of routers a packet traverses between the source and
destination.
Reliability is typically the bit-error rate of a link connecting this router to a source or destination
resource. For most of the routing protocols, the reliability of a link is assigned by the network engineer.
Since it is arbitrary it can be used to influence and create paths that are favorable over other paths.
The delay metric is an overall measurement of the time it takes for a packet to move through all the
internetworked devices, links and queues of each router. In addition, network congestion and the overall
distance traveled between the source and destination are taken into consideration in evaluating the delay
metric value. Because the delay value takes into account many different variables, it is an influential
metric on the optimal path calculation.
Using bandwidth as a metric in optimal path calculations may be misleading. Though bandwidth of a
bandwidth of 1.54 Mbps is greater than 56 Kbps, it may not be optimal due to the current utilization of
the link or the load on the device on the receiving end of the link.
The load is a metric that assigns a value to a network resource based on the resources overall utilization.
This value is a composite of CPU utilization, packets processed per second, and disassembly/reassembly
of packets among other things. The monitoring of the device resources itself is an intensive process.
In some cases, communication lines are charged based on usage versus a flat monthly fee for public
networks. For example, ISDN lines are charged based on usage time and potential the amount of data
transmitted during that time. In these instances, communication cost becomes an important factor in
determining the optimal route.
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In designing a routing protocol based network the routing algorithm should have the following
characteristics built into the design:
Optimality - using some or all of the metrics available for a routing protocol in order to calculate the
optimal route. Different routing protocols may apply one metric as having a higher weight to the optimal
route calculation than another has. An understanding of this behavior is important in choosing the
routing protocol. [ Pobierz całość w formacie PDF ]