Transcript Chapter 7 - YSU Computer Science & Information Systems

Otero Junior College
Cisco Networking Academy
Chapter 7 Review
Route Optimization
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 interface
Prevents all routing updates from being sent through an interface
Default routes
Instructs the router that if it does not have a route for a given destination, to
send the packet to the default route
Static routes
A route to a destination that you configured in the router
Route update filtering
Use 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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Cisco Networking Academy
End Review