Cisco Networking AcademyChapter 7 ReviewRoute Optimization

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Preventing Propagation

Times you do not want routing information propagated:When using an on-demand WAN link

• You may want to minimize, or stop entirely, the exchange of routing update information across this type of link; otherwise, the link will remain up constantly

When you want to prevent routing loops

• When a path is learned to the same destination by two different routing protocols, you may want to filter the propagation of one of the paths

When you want to preserve bandwidth

• You can ensure maximum bandwidth availability for data traffic by reducing unnecessary routing update traffic

Controlling/Preventing Propagation

Passive interfacePrevents all routing updates from being sent through an interface

Default routesInstructs the router that if it does not have a route for a given destination, to

send the packet to the default route

Static routesA route to a destination that you configured in the router

Route update filteringUse access lists to filter route update traffic about specific networks

Controlling/Preventing Propagation

To ensure effectiveness:

know your network traffic patterns and know what the intended goal is

If you do not know the problem you want to resolve when using these capabilities, you will not know how to verify that they are being effective

Passive Interface

The passive interface feature behaves differently with different protocols

With most protocols passive interface stops the router from sending updates to a particular neighbor, but continues to listen and use routing updates from that neighbor

In OSPF the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface

In EIGRP, passive interface causes the router to stop sending hello packets. When this happens, the router can't form neighbor adjacencies on the interface or send or receive routing updates

• To achieve the common effect of passive interface with EIGRP, use the distribute-list command

Configuring a Passive Interface

To configure a passive interface, regardless of the routing protocol, perform the following steps:

Select the router that requires the passive interface

Determine which interface(s) you do not want routing update traffic to be sent through

Configure the passive interface as follows:

• router(config-router) #passive-interface type number

• type refers to the type of interface, such as serial or Ethernet

• number refers to the interface number.

Gateway of Last Resort

The gateway of last resort term applied to a routing entry in the Cisco routing table that the router

forwards packets to when it lacks a more specific route

can be learned from a route provided by another router that is tagged as candidate default by the advertising router

• The ip default-network command is used to make a router tag a route as a gateway of last resort

• A router can have multiple IP default networks entered

The ip default-gateway command is used with routers that have IP routing disabled

• gives them an address to which they can forward packets whose destination IP addresses are not in their address space

IP Classless

The ip classless command affects all routing protocols in the router and should be used with caution

causes the router to ignore all aspects of address classes when making routing decisions

A simple method for forcing a router to use a gateway of last resort for remote subnets that are part of the local domain

Null0

Significant overhead can be created by packets for routes that do not exist

the router may want to send an Internet Control Message Protocol (ICMP) host or network-unreachable message for each unroutable packet

• Cisco routers send only one ICMP host-unreachable message back to the original host for each group of packets that arrive within a short time period

One solution is to configure a route to the null0 interface

a legitimate interface that accepts the packets and then throws them away

No ICMP host or network-unreachable messages are sent for packets forwarded to null0

Routing Loops

The use of gateways of last resort must be carefully planned

Misuse can lead to loss of connectivity and routing loops

• can be detected with the debug ip packet command

If a sufficient number of packets start looping the routers, the links can be overwhelmed and a routing loop storm can be created

• A quick fix is a temporary static route pointing to null0 in one of the routers,cleaning up all the looping packets

• The problem can then be fixed by placing the default routes where they belong

Policy Routing

Policy routing is a means of controlling routes that rely on the source, or source and destination, of traffic rather than destination alone

a glorified form of static routing

can be used to control traffic inside an AS as well as between ASs

used when you want to force a routing behavior different from what the dynamic routing protocols dictate

It is important to ensure that if policy routed traffic cannot be delivered because the next hop is down, some other alternative is available

Default Route

RIP and IGRP use the special address 0.0.0.0 to describe a default route

A default route is used when it is not convenient to list every possible network in the RIP updates, and when one or more closely-connected routers in the system are prepared to handle traffic destined for networks that are not listed explicitly in the routing table

• These routers should create RIP entries for the address 0.0.0.0, just as if it were a network to which they are connected

• how routers create entries for 0.0.0.0 is left to the network administrator

• the command default-information originate is used to have a RIP router advertise a default route

Default Route (Cont.)

The 0.0.0.0 route is not as useful for IGRP as it is for RIP

IGRP does not advertise it, even if a redistribute static command and a default metric are configured

The simplest way to achieve the same effect for IGRP is to create and redistribute a static route for a fictitious network and flag it as the IP default network

If you have multiple routers with exit points to other networks that you want to use as redundant links to the outside world, you can configure the same redistributed static route in each of them

Floating Static Routes

Floating static routes are static routes that are always in the configuration of a router but are installed in a routing table only when a dynamic route to the same network is lost

The dynamic route overrides the static route because the static route is configured with a greater distance

Can be used when the alternative link is a dialup connection

Dynamic routing information is usually sent or received over a dialup link only if the link is intended to be up for a long period of time

Redistribution

Cisco routers allow internetworks using different routing protocols (referred to as ASs) to exchange routing information through a feature called route redistribution

Redistribution is the capability for boundary routers connecting different ASs to exchange and advertise routing information received from one AS to the other AS

Implementation considerations:

• You can redistribute only protocols that support the same protocol stack

• How you configure redistribution varies among protocols and among combinations of protocols

Redistribution - Key Issues

Key issues that arise when using redistribution:

Routing feedback (loops)

• Depending on how you employ redistribution, routers can send routing information received from one AS back into the AS

– similar to the split-horizon problem that occurs in distance vector technologies

Incompatible routing information • Because each routing protocol uses different metrics to determine the best path,

path selection using the redistributed route information may not be optimal

Inconsistent convergence time • Different routing protocols converge at different rates

Redistribution - Guidelines

Be familiar with your network - knowing your network will enable you to make the best decision when implementing redistribution

Do not overlap routing protocols - do not run two different protocols in the same internetwork

One-way redistribution - To avoid routing loops and having problems with varying convergence time, allow routes to be exchanged in only one direction, not both directions. In the other direction, you should consider using a default route

Two-way redistribution - If there is only one point of redistribution (one ASBR only), there are no potential problems. If you have multiple ASBRs and if you must allow two-way redistribution, enable a mechanism to reduce the chances of routing loops

Redistribution - Verifying

The best way to verify redistribution operation is:

Know your network topology, particularly where redundant routes exist

Show the routing table of the appropriate routing protocol on a variety of routers in the internetwork using the show command

Perform a trace on some of the routes that go across the ASs to verify that the shortest path is being used for routing

If you do encounter routing problems, use trace and debug commands to observe the routing update traffic on the ASBRs and internal routers

RIP and OSPF Redistribution

A common first step in converting a RIP network to an OSPF network is to add backbone routers that run both RIP and OSPF, while the remaining network devices run RIP

These backbone routers are OSPF ASBRs

• Each ASBR controls the flow of routing information between OSPF and RIP

Next, create OSPF areas using ABRs that provide route summarizations and use VLSM to conserve address space

ABRs control network information distribution between OSPF areas and the OSPF backbone

• Each router keeps a detailed record of the topology of its area and receives summarized information from the other ABRs on their respective areas

Redistribution - Keywords

To redistribute RIP routes into OSPF use the redistribute rip subnets command

The subnets keyword tells OSPF to redistribute all subnet routes

• Without the subnets keyword, only networks that are not subnetted will be redistributed by OSPF

• Redistributed routes appear as external type 2 routes in OSPF

To redistribute OSPF routes into RIP use the redistribute ospf 109 match internal external 1 external 2 command

The keyword internal indicates the OSPF intra-area and inter-area routes:

• external 1 is the external route type 1

• external 2 is the external route type 2

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End Review

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