MultipleRouting Flashcards
Running Multiple Protocols
•Different routing protocols were not designed to interoperate with one
another.
• Each protocol collects different types of information and reacts to topology
changes in its own way.
• Running muliple routing protocols increases CPU utilization and
requires more memory resources to maintain all the topology, database
and routing tables.
Routing Protocol Performance Solutions
Design changes, such as limiting the number of routing
protocols used.
Using passive interfaces to prevent routing protocol updates
from being advertised out an interface.
Route filtering techniques to block specific routes from
being advertised:
• Access control lists (ACLs, used to control “data plane traffic”)
• Route maps
• Distribute lists
• Prefix lists
Why have multiple routing protocols?
Interim during conversion
• Migrating from an older IGP to a new IGP.
Application-specific protocols
• One size does not always fit all.
Political boundaries
• Multiple departments managed by
different network administrators
• Groups that do not work well with others
Mismatch between devices
• Multivendor interoperability
• Host-based routers
Company mergers
Complex Networks
Complex networks require careful routing protocol design
and traffic optimization solutions, including the following:
• Redistribution between routing protocols
• Route filtering
• Summarization (covered in EIGRP and OSPF)
Route redistribution is defined as the capability of boundary
routers connecting different routing domains to exchange and
advertise routing information between those routing domains
(autonomous systems).
Selecting the Best Route
Routers use the following two parameters to select the best path:
Administrative distance:
• Used to rate a routing protocol’s believability (also called its trustworthiness).
• This criterion is the first thing a router uses to determine which routing
protocol to believe if more than one protocol provides route information for
the same destination.
Routing metric:
• The routing metric is a value representing the path between the local
router and the destination network, according to the routing protocol
being used.
• The metric is used to determine the routing protocol’s “best” path to the
destination.
Metric Discussion
One of the most important factors that needs to be considered while
performing route redistribution is Metric.
Each routing protocol uses a unique metric to calculate the best path to
a destination network.
RIP protocol uses the hop count metric
EIGRP protocol uses the composite (bandwidth, load, delay, and
reliability) metric.
OSPF protocol uses the cost
Redistribution Methods
One-point redistribution
Only one router is redistributing one-way or two-way (both ways).
• There could still be other boundary routers but they are not configured to redistribute.
Multipoint redistribution
• Multiple routers are used to redistribute either one-way or two-way (both ways).
• More prone to routing loop problems.
One-Point Redistribution
• One-point One-way
Redistributes networks from one routing protocol into the other routing protocol.
• Typically uses a default or static route so that devices in that other part of the network can reach the first part of the network.
• One-point Two-way RIP OSPF
• Redistributes routes between the two routing processes, in both directions.
Onepoint 2-way issue:
Although one-point one-way or two-way redistribution is usually safe
from routing loops, issues can still occur if multiple boundary routers
exist and only one router is performing one-point one-way redistribution.
Multipoint Redistribution
Multipoint redistribution has two (or more) separate routers running
Redistributing RIP into OSPF both routing protocols.
Redistribution can be configured as:
• Multipoint one-way redistribution
• Multipoint two-way redistribution
Although multipoint two-way redistribution is especially problematic, either method is likely to introduce potential routing feedback loops.
Multipoint one-way redistribution only works well if:
• The receiving routing protocol is either EIGRP, BGP and OSPF because they
support different administrative distances for internal and external routes.
• Issue: the administrative distance of protocol B’s external routes is higher
than the administrative distance of protocol A’s routes, so that R2 and R3 will
use the inappropriate routes to destinations in the protocol B side of the
network.
Preventing Routing Loops
The safest way to perform redistribution is to redistribute
routes in only one direction, on only one boundary router
within the network.
• However, that this results in a single point of failure in the network.
If redistribution must be done in both directions or on
multiple boundary routers, the redistribution should be
tuned to avoid problems such as suboptimal routing and
routing loops.
Do not overlap routing protocols.
• Do not run two different protocols in the same Internetwork.
• Instead, have distinct boundaries between networks that use
different routing protocols.
Be familiar with your network.
Key Route Redistribution Points
Routes are redistributed into a routing protocol.
• Therefore, the redistribute command is configured under the
routing process that is receiving the redistributed routes.
Routes can only be redistributed between routing protocols
that support the same protocol stack.
For example IPv4 to IPv4 and IPv6 to IPv6.
• However, IPv4 routes cannot be redistributed into IPv6.
The method used to configure redistribution varies among
combinations of routing protocols.
• For example, some routing protocols require a metric to be configured
during redistribution, but others do not.
Generic Redistribution Steps
- Identify the boundary router(s) that will perform
redistribution. - Determine which routing protocol is the core protocol.
- Determine which routing protocol is the edge protocol.
• Determine whether all routes from the edge protocol need to be
propagated into the core and consider methods that reduce the
number of routes. - Select a method for injecting the required routes into the
core.
• Summarized routes at network boundaries minimizes the number of
new entries in the routing table of the core routers. - Consider how to inject the core routing information into the
edge protocol.
