HCNP the principle and configuration of OSPF protocol for learning notes 8-OSPF external routes

The generation of external routes, using the Asbr-summary-lsa Asbr-externel-las two types of LSA messages, the above two types of messages have a brief introduction, this paper describes in detail the role of these two types of messages in the generation of external routes.

ASBR uses a class fifth LSA to describe external routes, which flood the entire as (note: flooding throughout as ).

When the ABR advertises the received class fifth LSA to other regions, a class fourth LSA is generated for the zone describing how to reach ASBR ( Note that this is the region, so the four classes of LSA are flooded within a particular region ). See the illustration below.

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A As-external-lsa

Used to describe how to reach an external destination from ASBR; ASBR generation (RTC). As-external-lsa does not belong to any area.

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Here's the RTC Grab bag

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Note: The Link state ID is set to the destination network segment address.

Advertising router is set to the router ID of ASBR.

Two Asbr-summary-lsa

Used to describe how to reach asbr from an ABR. generated by ABR (RTB)

The following is the ASBR-SUMMARY-LSA generated by RTB

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The Link state ID is set to the router ID of the ASBR;

Advertising router is set to the router ID of the ABR;

The metric is set to the OSPF cost of reaching this ASBR from this ABR.

Summary:

• When the ABR floods a as-external-lsa outside the region, it also generates a ASBR-SUMMARY-LSA that describes the ASBR flooding outside the region.

• The fourth LSA can only be flooded within one region , and the associated ABR generates a new fourth class of LSA to describe how to reach the relevant ASBR, so that the class fifth LSA can have more than one ASBR to reach the same class Its advertising router and metric are different, indicating that they are generated by different ABR.

Three lsdb containing As-external-las

Is the LSDB in RTA after synchronization, pay attention to the red part

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In the LSDB, there is a Asbr-summary-lsa and a as-external-lsa.

In Lsdb, the link state information of the external route and the route inside the as is separated.

As you can see, As-external-lsa does not belong to any region.

Small question: Remember what type of LSA Sum-net was generated by?

If you don't know the answer, review the generation of interregional routes.

Four external routing configuration and summarization

The configuration commands for the OSPF protocol are simple, and the emphasis is on understanding the process of routing calculations, which is very helpful for troubleshooting.

command See

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Import-route {limit [limit-number] | protocol [cost value] [type value] [tag value] [Route-policy Route-policy-nam E]}

• When you introduce external routes, you can configure routing costs, external route types, and tag values.

• Limit: Specifies the maximum number of external routes that can be introduced in an OSPF process.

Asbr-summary:ip-address Mask [not-advertise | tag Tag-value]//external route aggregation

• Not-advertise: The route matching the specified IP address/mask is not advertised. If you do not specify this parameter, the aggregation route is advertised.

• Tag-value: Used to control the distribution of routes through Route-policy, the Tag-value value range is 0~4294967295. If you do not specify this parameter, the default value is 1.

• By default, only routes after aggregation are advertised.

Features of the five external routes 1-forwarding Address

Forwarding Address, understand, no configuration

In the current network environment, it is possible to generate sub-new routes, see the following example

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The 10.1.1.0/24 belongs to the OSPF routing domain, and the RTC does not run OSPF.

A static route to the Loopback0 interface that arrives at the RTC is configured on the RTB and is introduced into OSPF as an external route.

RTA can learn this external route through OSPF, but the next hop is RTB (under normal circumstances, access to the external network via ASBR Optimal), so in RTA, this route is suboptimal, and the optimal next hop should be RTC's e0/0 interface.

OSPF solves this problem by automatically setting the forwarding address.

If the next hop of an external route introduced into OSPF is within an OSPF routing domain, the Forwarding address should be set to the next hop in the ASBR routing table in the As-external-lsa that describes the external route.

Six OSPF multi-process

Note that there are two area 0 in the following example,

As a ASBR, two OSPF processes should be enabled on the RTB. RTA enables the OSPF process 1,RTC also enables the OSPF process 1.

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Enable two OSPF processes on RTB, specifying the router ID separately for each OSPF process.

When you configure each process, the routing information for the other process is introduced into the process.

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Verify:

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HCNP the principle and configuration of OSPF protocol for learning notes 8-OSPF external routes