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MODULE 5 : ROUTE OPTIMIZATION

Overview

Dynamic routing, even in small internetworks, can involve much more than just enabling the default behavior of a routing protocol. Some of these behaviors may lead to inefficient use of bandwidth, security risks, or suboptimal routing.

For example, routing updates compete with user data for bandwidth and router resources. In some cases, the updates may not be required and yet are still advertised by default, contributing to bandwidth waste and increasing security risks. During route redistribution between IP routing domains, suboptimal routing can occur without manipulation.

Routing can be optimized in a network by controlling when a router exchanges routing updates and what those updates contain. To ensure that the network operates efficiently, you must control and tune routing updates. Information about networks must be sent where it is needed and filtered from where it is not needed.

This module examines the key IOS route optimization features, including route redistribution, routing update control, and policy-based routing. In addition, an overview of DHCP and how to configure support for it is covered.

5.1 Operating a Network Using Multiple Routing Protocols
5.1.1 Using Multiple Routing Protocols

Simple routing protocols work well for simple networks, but networks grow and become more complex. While running a single routing protocol throughout your entire IP internetwork is desirable, multiprotocol routing is common for a number of reasons, including company mergers, multiple departments managed by multiple network administrators, multivendor environments, or simply because the original routing protocol is no longer the best choice.

For example, Routing Information Protocol (RIP) periodically sends entire routing tables in updates. As a network grows larger, the traffic from those updates can slow the network down, indicating that a change to a more scalable routing protocol may be necessary. Another reason could be if a company is using Enhanced IGRP (EIGRP) but now needs a protocol that supports multiple vendors, or the company implements a policy that specifies a particular routing protocol.

Running a multiple protocol environment is often part of a network design, and network administrators must conduct migration from one routing protocol to another carefully and thoughtfully. Often the transition between routing protocols takes place gradually, so multiple routing protocols are operating in the network for variable lengths of time. It is important for network administrators to understand what must be changed and to create a detailed plan before making any changes. An accurate topology map of the network and an inventory of all network devices are also critical for success.

Link-state routing protocols, such as Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS), require a hierarchical network structure. Network administrators need to decide which routers will reside in the backbone area and how to divide the other routers into areas. While EIGRP does not require a hierarchical structure, it operates much more effectively within one.

Using a routing protocol to advertise routes that are learned by some other means, such as by another routing protocol, static routes, or directly connected routes, is called route redistribution.

Differences in routing protocol characteristics, such as metrics, administrative distance, classful and classless capabilities, can affect redistribution.
Hierarchical routing

5.1.2 Defining Route Redistribution

Multiple routing protocols may be necessary in the following situations:

• When you are migrating from an older interior gateway protocol (IGP) to a new IGP. Multiple redistribution boundaries may exist until the new protocol has completely
displaced the old protocol.

• When the use of another protocol is desired, but the old routing protocol is needed for host systems. This is common in environments with UNIX host-based routers running RIP.
• Some departments might not want to upgrade their routers to support a new routing protocol.

• In a mixed-router vendor environment, you can use a routing protocol specific to Cisco, such as EIGRP in the Cisco portion of the network, and a common standards-based routing protocol, like OSPF, to communicate with devices from other vendors.

When multiple routing protocols are running in different parts of the network, there may be a need for hosts in one part of the network to reach hosts in the other part. One solution is to advertise a default route into each routing protocol, but that is not always the best policy. The network design may not allow default routes.

If there is more than one way to get to a destination network, routers may need information about routes in other parts of the network to determine the best path to that destination.

Additionally, if there are multiple paths, a router must have sufficient information to determine a loop-free path to the remote networks.

Cisco routers allow internetworks using different routing protocols, referred to as routing domains or autonomous systems, to exchange routing information through a feature called route redistribution.
Redistribution is how routers connect different routing domains so that they can exchange and advertise routing information between the different autonomous systems.

Note
The term autonomous system as used here denotes internetworks using different routing protocols. These routing protocols may be IGPs or exterior gateway protocols (EGPs), which is a different use of autonomous system when referring to Border Gateway Protocol (BGP).

5.1.3 Redistributing Route Information

Within each autonomous system, the internal routers have complete knowledge about their network. The router that interconnects the autonomous systems is called a boundary router.

The boundary router must be running all the routing protocols that are exchanging routes.

When a router redistributes routes, it allows a routing protocol to advertise routes that were not learned through that routing protocol. These redistributed routes could have been learned via a different routing protocol, such as when redistributing between EIGRP and OSPF, from static routes, or by a direct connection to a network.

Routers can redistribute static and connected routes, and routes from other routing protocols.

Redistribution is always performed outbound. The router doing redistribution does not change its routing table. When redistribution between OSPF and EIGRP is configured, the OSPF process on the boundary router takes the EIGRP routes in the routing table and advertises them as OSPF routes to its OSPF neighbors.

Likewise, the EIGRP process on the boundary router takes the OSPF routes in the routing table and advertises them as EIGRP routes to its EIGRP neighbors. As a result, both autonomous systems know about the routes of the other, and each autonomous system can then make informed routing decisions for these networks.

EIGRP neighbors use the EIGRP external (D EX) listing to route traffic destined for the other autonomous system via the boundary router. The boundary router must have the OSPF routes for that destination network in its routing table to forward the traffic.

For this reason, routes must be in the routing table for them to be redistributed. This requirement may seem self-evident, but it can also be a source of confusion.

For instance, if a router learns about a network via EIGRP and OSPF, only the EIGRP route is put in the routing table because it has a lower administrative distance. Suppose RIP is also running on this router, and you want to redistribute OSPF routes into RIP. That network is not redistributed into RIP because it is in the routing table as an EIGRP route, not as an OSPF route.

Factors that have the most impact on redistribution include:

• Metrics
• Administrative distance
• Classful/classless capabilities of the protocols

These factors will be discussed in various sections of this module.
• Redistributing Route Information

5.1.4 Using Seed Metrics

Each routing protocol defines a metric for each route. The metric value determines the shortest or “best” path to an IP network. When a router redistributes routes from one routing domain to another, this information cannot be translated from one routing protocol to another. For example, a RIP hop cannot be dynamically recalculated to an OSPF cost by the router doing redistribution. Therefore, a seed metric artificially sets the distance, cost, and so on, to each external (redistributed) network from the redistribution point.

Seed Metrics Example

For example, if a boundary router receives a RIP route, the route has a hop count as a metric. To redistribute the route into OSPF, the router must translate the hop count into a cost metric that the OSPF router understands. This seed metric, also referred to as the default metric, is defined during redistribution configuration. When the seed metric for a redistributed route is established, the metric increasesin increments normally within the autonomous system.

Note
The exception to this rule is OSPF E2 routes, which hold their initial metric regardless of how far they are propagated across an autonomous system.

The default-metric command, used in routing process configuration mode, establishes the seed metric for all redistributed routes.

Cisco routers also allow the seed metric to be specified as part of the redistribution command, either with the metric option or by using a route map.

Whichever way it is done, the initial seed metric should be set to a value larger than the largest metric within the receiving autonomous system to help prevent suboptimal routing and routing loops.

Figure lists protocol names with the default seed metrics for the various protocols.


5.1.5 Seed Metrics Example

Figure illustrates a seed metric of 30 implemented by OSPF on the redistributed RIP routes.

The link cost of the Ethernet link to router D is 100. Therefore, the cost for networks 1.0.0.0, 2.0.0.0, and 3.0.0.0 on router D is the seed metric (30) plus the link cost (100) = 130. Notice that the metrics of the three networks in the RIP cloud are irrelevant in the OSPF cloud, because the objective is to have each OSPF router forward traffic for the three networks to the border (redistributing) router.

A metric of infinity tells the router that the route is unreachable, and therefore, it should not be advertised. When redistributing routes into RIP and EIGRP, you must specify a default metric.

For OSPF, the redistributed routes have a default type 2 metric of 20, except for redistributed BGP routes, which have a default type 2 metric of 1. For IS-IS, the redistributed routes have a default metric of 0. But unlike RIP or EIGRP, IS-IS does not treat a seed metric of 0 as unreachable. Configuring a seed metric for redistribution into IS-IS is recommended. For BGP, the redistributed routes maintain the IGP routing metrics.


5.1.6 Defining Administrative Distance

Most routing protocols have metric structures and algorithms that are not compatible with other protocols. It is critical for a network using multiple routing protocols to have seamless exchange of route information and the ability to select the best path across multiple protocols.

Cisco routers use a value called administrative distance to select the best path when they learn two or more routes to the same destination from different routing protocols. Administrative distance rates the believability of a routing protocol. Cisco has assigned a default administrative distance value to each routing protocol supported on its routers.

Each routing protocol is prioritized in the order of most believable to least believable. Some examples of prioritization are as follows:

• Prefer manually configured routes (static routes) to dynamically learned routes
• Prefer protocols with sophisticated metrics to protocols with more deterministic metrics
• Prefer External Border Gateway Protocol (EBGP) to most other dynamic protocols

In Figure , the table lists the default administrative distance of the protocols that Cisco supports. The administrative distance is a value between 0 and 255. The lower the administrative distance value, the higher the believability of the protocol.


Note
IGRP is no longer supported as of Cisco IOS Software Release 12.3.
For example, in Figure , if router A receives a route to network 10.0.0.0 from RIP and receives a route to the same network from OSPF, the router compares the administrative distance of RIP (120) with the administrative distance of OSPF (110). The router determines that OSPF is more reliable and adds the OSPF version of the route to the routing table.


Prefix Lengths
Varying prefix lengths of routes from different routing protocols can also affect routing decisions.
The prefix length is the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet, regardless of routing protocols.

For example, assume that a router has four routing processes running on it, and each process
has received these routes:

• EIGRP (internal): 192.168.32.0/26
• RIP: 192.168.32.0/24
• OSPF: 192.168.32.0/19

Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, you might assume that the first one is installed. However, since each of these routes has a different prefix length (subnet mask), they are considered different destinations. Therefore, they are all installed in the routing table.

If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet. In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits versus 24 or 19 bits).

Likewise, if a packet destined for 192.168.32.100 arrives on one of the router’s interfaces, it is forwarded to 10.1.1.2, because 192.168.32.100 does not fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length.


Interactive Media Activity .
Drag and Drop: Administrative Distance

I also pasted this so that you know you should go to the conmputer and get that
interactive media actitivity done.


5.1.7 Modifying Administrative Distances

In some cases, a router selects a suboptimal path if it believes a routing protocol with a better administrative distance, even though it is actually a routing protocol with a worse route.

Assigning an undesired routing protocol a larger administrative distance ensures that routers select routes from the desired routing protocol. You can use the distance command to change the default administrative distance for all protocols, except EIGRP and BGP. Figure describes the command parameters.

For EIGRP, use the distance eigrp command. EIGRP assigns different administrative distance values to routes learned natively through EIGRP and to routes redistributed in from other sources.

By default, EIGRP natively learned routes have an administrative distance of 90, but external routes have an administrative distance of 170.

For BGP, use the distance bgp command. BGP assigns different administrative distance
values to routes learned through IBGP and routes learned through EBGP.