Content 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.
Content 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.
Content 5.1 Operating a
Network Using Multiple Routing Protocols 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).
Content
5.1 Operating a Network Using Multiple
Routing Protocols 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: