8. Optimizing OSPF

What metric does OSPF use?

Cost

Equation for OSPF cost

Default reference bandwidth

10^8

What is the cost of links faster than Fast Ethernet when the default reference bandwidth is used?

1

How is the cost of a path calculated?

It is the cumulative cost of all the links on the path from the router to the network

Is low cost or high cost preferred?

Low

In what order are LSAs, SPF, the LSDB, and the routing table used in determining and recording the shortest path?

What is the command to see OSPF neighbor ID, state, priority, dead time, address and interface?

sh ip ospf neigh

What is the command to see the routing protocol, router ID, routed networks, number of paths, routing information sources, and distance?

sh ip prot

What are the two numbers in brackets in the output of sh ip route ospf?

[administrative distance / metric of the route]

What is administrative distance?

Administrative distance (AD) is an integer from 0 to 255 that rates the trustworthiness of the source of the IP routing information. The AD is only important when a router learns about a destination route from more than one source. The path with the lowest AD is the one entered in the routing table.

What is the command to see neighbor count of an interface, the IP address of neighbors, delay, priority, state, and designated router ID of an interface?

sh ip ospf int <type> <#/#>

What are the steps in configuring a passive OSPF interface?

set the routing protocol to ospf, set the interface to passive

Command to verify passive interface?

sh ip prot

What command is used to advertise a default route to a neighbor?

(config-router)# default-information originate

What will a router be treated like if it is originating type-5 AS External LSA's for the default route?

ASBR

What is the command to allow a router to advertise a default route regardless of whether it already has a default route in its routing table?

default-infromation originate always

How can you influence an interface cost?

By changing the interface bandwidth

T/F: OSPF cost is computed automatically for each interface that is assigned into an OSPF process

True

How many bits and between what range is OSPF cost?

16-bits positive number between 1 and 65,535

Does automatic cost assignment work on high-bandwidth links?

No. For links 100Mbps and more all costs are equal to 1 and therefore require manually cost setting on each interface.

T/F: The metric is only relevant on an outbound link path

True

What are the default OSPF costs of T1, Ethernet, Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet?

What command adjusts reference bandwidth to improve OSPF behavior for high-bandwidth links?

auto-cost reference-bandwidth #

What is the command to see path cost?

sh ip ospf int e0/0

What is the command to see a path's metric?

sh ip route x.x.x.x

What is the command to set an interface's OSPF cost?

T/F: Manually configuring OSPF cost on an interface does not override the calculated cost that is based on reference bandwidth and interface bandwidth

False; manually configuring OSPF cost overrides the calculated value of cost.

What is route summarization?

The process of advertising a contiguous set of addresses as a single address with a less-specific, shorter subnet mask.

What problems does route summarization solve?

large routing tables and frequent LSA flooding throughout the AS

What LSA types do ABRs and ASBRs summarize?

ABRs summarize type 3 LSAs, and ASBRs summarize type 5 LSAs.

What LSA types are generated inside each area?

1 and 2

What type of LSAs are type 1 and 2 LSAs translated into in other areas?

3

What does route summarization effect?

Bandwidth, CPU power, and memory resources that the OSPF routing process consumes

What are the key impacts of route summarization?

1. Route summarization will minimizes the number of routing table entries, 2. localizes the impact of a topology change and 3. reduce LSA flooding and 4. saves CPU resources.

T/F: With route summarization, all routes are propagated into the backbone (Area 0).

False; with summarization only the summarized routes are propagated into the backbone (Area 0).

What routing filter allows you to apply an access list to routing updates?

distribute list

What command implements a distribute list?

What factors can the distribute-list filter based on?

1. incoming interface 2. outgoing interface 3. redistribution from another routing protocol

T/F: the distribute-list command removes routes from the link state database

False; For OSPF, the distribute-list in command filters what ends up in the IP routing table, and only on the router on which the distribute-list in command is configured.

What command implements prefix lists for route filtering?

What are the drawbacks of IP access lists that are used as route filters?

1. The subnet mask cannot be easily matched.
1. Access lists are evaluated sequentially for every IP prefix in the routing update.
3. Extended access lists can be cumbersome to configure.

What are the benefits of ip prefix-list filtering over an access list?

The intended use of prefix lists is limited to route filtering, where access lists were originally intended to be used for packet filtering and were then extended to route filtering.

If none of the lines in the prefix list match the network number in the update, what happns?

implicit deny

What is the command to consolidate and summarize routes at an area boundary?

area # range x.x.x.x x.x.x.x

What are the four types of OSPF route filtering that IOS supports?

1. filter-list 2. area range not-advertise 3. distribute-list 4. summary-address not-advertise

How do you filter Type-3 LSAs with the 'filter-list' command?

How do you filter Type-3 LSAs with the 'area range' command?

How do you filter OSPF routes with the 'distribute-list' command? (3 methods)

What are the commands to use an access list with 'distribute-list'?

What are the commands to use and prefix list with 'distribute-list'?

What are the commands to use a combination of route map, prefix list, and access list with 'distribute-list'?

How do you filter Type-5 LSAs with the 'summary-address' command?

What is one major limitation of OSPFv2?

It doesn't support IPv6

What is the command to configure OSPFv2 for IPv4?

router ospf process-id

What is the command to configure OSPFv3 for IPv6 only?

ipv6 router ospf process-id

What is the command to configure OSPFv3 for IPv4 and 6?

router ospfv3 process-id

What is the command to enable ipv6 routing?

ipv6 unicast-routing

What is the command to configure the router ID?

(config-router)# router-id <#>

What is the command to enable OSPFv3 on an interface?

ipv6 ospf process-id area <area id>

How do you configure IPV6 route summarization on an ABR?

How do you configure IPV6 route summarization on an ASBR?

How do you configure IPv6 default route advertisement?

Which 2 options play key roles in route summarization?

Network numbers in areas should be assigned contiguously, VLSM

What are three common applications of route maps?

1. Redistribution route filtering 2.Policy-based routing 3. BGP policy implementation

What are the steps in creating a route map?