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48 Cards in this Set

  • Front
  • Back

Routers and Layer 3 switches learn about remote networks in one of two ways:



Manually - Remote networks are manually entered into the route table

using static routes.

Manual routes must be reconfigured anytime the

network topology changes.

Static routing has three primary uses:

Ease of routing table maintenance

Routing to and from stub networks.

Using a single default route.

Dynamically - Remote routes are automatically learned using a

dynamic routing protocol.

Dynamic routing protocols require less

administrative overhead.

The expense of using dynamic routing protocols is dedicating part of a router’s resources for

protocol operation.

Most networks use a combination of

dynamic routing protocols and static routes.

OSPF its features include:



Fast convergence



OSPF is classless and supports


Routing changes trigger

routing updates.

It uses the SPF algorithm to choose the

best path.

OSPF supports this type of authentication:

(MD5) authentication.

OSPF defines five network types:


Broadcast multiaccess

Nonbroadcast multiaccess


Virtual links

Multiaccess networks can create two challenges for OSPF regarding the flooding of LSAs:

Creation of multiple adjacencies

Extensive flooding of LSAs

The solution to managing the number of adjacencies and the flooding of LSAs on a multiaccess network is the


OSPF elects a DR to be the

collection and distribution point for LSAs.

A BDR is also elected in case the

DR fails.

If the DR stops producing Hello packets, the BDR

promotes itself and assumes the role of DR.

All other non-DR or BDR routers become


Instead of flooding LSAs to all routers in the network, DROTHERs only send their LSAs to the

DR and BDR using the multicast address

The router with the highest router-id is

elected DR.

The state of neighbors in multiaccess networks can be:





The normal state for an OSPF router is usually


If a router is stuck in another state, it is an indication that there are problems in

forming adjacencies.

Default DR/BDR Election Process:

highest interface priority > highest router ID > highest loopback IP address > highest active IPv4 address

The DR and BDR election process takes place as soon as the first router is

powered on or when the OSPF network command is configured.

After the DR is elected, it remains the DR until one of the following events occurs:

The DR fails.

The OSPF process fails or is stopped.

The multiaccess interface fails or is shutdown.

Instead of relying on the router ID, it is better to control the election by setting

interface priorities.

If the interface priority is configured after OSPF is enabled, the administrator must either:

Shut down and re-enablethe OSPF process on all routers.

Shutdown and re-enable the router interfaces.

The router located between an OSPF routing domain and a non-OSPF network is also called the

autonomous system boundary router (ASBR).

The (ASBR) router connected to the Internet is used to propagate a

a default route to other routers.

The default-information originate command

propagate the default static route in OSPF updates.

O*E2 signifies that a route was learned using

OSPF and that is an external route.

A type 1 route is the

externel and internal cost combined and is always prefered.

A type 2 route is the cost of the

external route.

The OSPF Hello and Dead intervals are configurable on a

per-interface basis.

The OSPF intervals must match or a

neighbor adjacency does not occur.

OSPF Attacks:

disrupting routing peers.

falsifying information.

The consequences of falsifying routing information are:

Redirecting traffic to create routing loops.

Monitoring traffic.

Discarding traffic.

To mitigate against routing protocol attacks, configure

OSPF authentication.

When neighbor authentication has been configured on a router, the router authenticates the

source of each routing update packet that it receives.

This is accomplished by the exchange of an

authenticating key.

OSPF authentication can either be:

none (or null)


Message Digest 5 (MD5)

Simple authentication

plaintext password exchanged between peers.

MD5 authentication

password is calculated using the MD5 algorithm.

The routing message and preshared key are used to calculate a


The recieving router combines the message and key to determine the