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Company 2
Subnet 2
OSPF2
redistribute
OSPF
Subnet1
EIGRP
Subnet1
Figure 9-1 Typical Use of Redistribution
Foundation Topics
Route Redistribution Basics
Most internetworks use a single IGP to advertise and learn IP routes. However, in some
cases, more than one routing protocol exists inside a single enterprise. Also, in some
cases, the routes learned with an IGP must then be advertised with BGP, and vice versa. In
such cases, engineers often need to take routing information learned by one routing protocol
and advertise those routes into the other routing protocol–a function provided by
the IOS route redistribution feature.
This section examines the basics of route redistribution.
The Need for Route Redistribution
The potential need for route redistribution exists when a route learned through one source
of routing information, most typically one routing protocol, needs to be distributed into a
second routing protocol domain. For example, two companies might merge, with one
company using EIGRP and the other using OSPF. The engineers could choose to immediately
migrate away from OSPF to instead use EIGRP exclusively, but that migration would
take time and potentially cause outages. Route redistribution allows those engineers to
connect a couple of routers to both routing domains, and exchange routes between the
two routing domains, with a minimal amount of configuration and with little disruption to
the existing networks.
Figure 9-1 shows just such a case, with R1 performing redistribution by using its knowledge
of subnet 1 from the EIGRP domain and advertising a route for subnet 1 into the
OSPF domain. Note that the opposite should also occur, with the OSPF domain’s subnet 2
being redistributed into the EIGRP domain.
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The main technical reason for needing redistribution is straightforward: An internetwork
uses more than one routing protocol, and the routes need to be exchanged between those
routing domains, at least temporarily. The business reasons vary widely but include the
following:
■ Mergers when different IGPs are used.
■ Mergers when the same IGP is used.
■ Momentum (The Enterprise has been using multiple routing protocols a long time.)
■ Different company divisions under separate control for business or political reasons.
■ Connections between partners.
■ To allow multivendor interoperability (OSPF on non-Cisco, EIGRP on Cisco, for
instance).
■ Between IGPs and BGP when BGP is used between large segments of a multinational
company.
■ Layer 3 WAN (MPLS).
The list begins with two entries for mergers just to make the point that even if both merging
companies use the same IGP, redistribution may still be useful. Even if both companies
use EIGRP, they probably use a different AS number in their EIGRP configuration (with
the router eigrp asn command). In such a case, to have all routers exchange routing information
with EIGRP, all the former company’s routers would need to migrate to use the
same ASN as the first company. Such a migration may be simple, but it still requires disruptive
configuration changes in a potentially large number of routers. Redistribution
could be used until a migration could be completed.
Although useful as an interim solution, many permanent designs use redistribution as
well. For example, it could be that a company has used different routing protocols (or different
instances of the same routing protocol) in different divisions of a company. The network
engineering groups may remain autonomous, and manage their own routing protocol
domains, using redistribution to exchange routes at a few key connecting points between
the divisions. Similarly, partner companies have separate engineering staffs, and want autonomy
for managing routing, but also need to exchange routes for key subnets to allow
the partnership’s key applications to function. Figure 9-2 depicts both of these cases.
The last two cases in the previous list each relate to BGP in some way. First, some large
corporations actually use BGP internal to the company’s internetwork, redistributing
routes from IGPs. Each large autonomous division of the company can design and configure
their respective routing protocol instance, redistribute into BGP, and then redistribute
out of BGP into other divisions. Also, when an Enterprise uses an MPLS VPN service, the
MPLS provider’s provider edge (PE) router typically redistributes customer routes with
BGP inside the MPLS provider’s MPLS network. Figure 9-3 shows samples of both these
cases. In each of these cases, a given prefix/length (subnet/mask) is typically distributed
into BGP at one location, advertised over a BGP domain, and redistributed back into
some IGP.
configured on the OSPF process tells RD1 to take IP routes from the IP routing table, if
learned by EIGRP process 1, and add those routes to OSPF 2’s topology table.
The process works as shown in Figure 9-5, but the figure leaves out some important details
regarding the type of routes and the metrics used. For EIGRP, the EIGRP topology
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table needs more than the integer metric value held by the IP routing table–it needs values
for the components of the EIGRP composite metric. EIGRP can use default settings that
define the metric components for all routes redistributed into EIGRP, or the engineer can
set the metric components in a variety of ways, as covered in several locations later in this
chapter.
Like EIGRP, OSPF treats the redistributed routes as external routes. OSPF creates an LSA
to represent each redistributed subnet–normally a Type 5 LSA, but when redistributed
into an NSSA area, the router instead creates a Type 7 LSA. In both cases, OSPF needs an
integer metric to assign to the external route’s LSA; the redistribution configuration
should include the OSPF cost setting, which may or may not match the metric listed for
the route in the redistributing router’s IP routing table.
The last concept before moving on to the configuration options is that the redistribute
command tells the router to take not only routes learned by the source routing protocol,
but also connected routes on interfaces enabled with that routing protocol–including passive
interfaces. Example 9-1 later in this chapter demonstrates this concept.