Between OSPF (four) OSPF routing domain

Foreword

In the previous article, we said the OSPF routing domain. Use the Router LSA and Network LSA. This article mainly on routes between OSPF domain.
OSPF has divided the region, why have divided the region it?

为了减少LSDB的规模， 减少网络流量降低CPU内存功耗。每个区域都维护自己独立的LSDB。
减小单区域规模，避免发生故障而引起的网络长时间瘫痪。
方便管理：大型企业有很多部门。

Area 0: backbone area
Area 1: non-backbone area
ABR (Area Border Router): ABR

Routers connected to two or more than two regions (one of which must be a backbone area)

Since ABR belong to two or more regions, it is necessary to maintain the LSDB and has two or more regions

Internal Router: routers within the region

All router interfaces are in the same area

Within the region and routers to calculate the optimal path routing through the LSDB within an area, then the inter-regional communication is how it?

Inter-area route calculation

As shown in FIG: RTD to an example, which corresponds to a type of LSA synchronization in Area 1, Area 1 a and ABR between Area 0: RTB, RTD will receive the type of LSA and converted to three LSA flooding in the Area 0; the RTA after receiving the three LSA, do the same operation. When ABR between 2 Area 0 and Area: After receiving the three RTC LSA, will regenerate a new three LSA transmits them to Area 2. At this point the whole area OSPF routing information are subject to 192.168.1.0/24. RTE routing information 192.168.2.1/24, and do the same operation.

note

1. 三类LSA只在相邻区域传递。在其他区域泛洪需要相邻ABR重新生成新的LSA（如上图在RTC收到三类LSA后，**会重新生成新的三类LSA再进行传递。**
2. 三类LSA只传递路由信息，不传递拓扑信息。
3. 区域间的路由信息在ABR上传递是双向的。

Three types of LSA: Network-Summary-LSA

Three types of LSA known as Network Summary LSA: originated by the ABR. ABR to send a summary LSA area network, to advertise the region outside the destination address. Indeed, these networks Summary LSA destination address is a method for internal router connected thereto in the region it can reach the ABR tell. A station may be aggregated for the ABR address LSA within the region to which it is connected via the backbone network advertisement. In one outer area, remains in a default route inside the OSPF autonomous system may be advertised in this type of LSA.

Through display ospf lsdb summary 192,168.1.0to see three types of LSA information

<RTB> display ospf lsdb summary 192.168.1.0

 OSPF Process 1 with Router ID 2.2.2.2
​          Area: 0.0.0.0
         Link State Database 
Type                       : Sum-Net          //三类LSA
 Ls id                     : 192.168.1.0      //目的网段地址
 Adv rtr                   : 2.2.2.2         //产生此三类LSA的Router ID
 Ls age                    : 86 
 Len                       : 28 
 Options                   : E 
 seq#                      : 80000001
 chksum                    : 0x7c6d
 Net mask                  : 255.255.255.0   //网络掩码
 Tos 0                    metric: 1         //开销值（RTB到RTA的距离）
 Priority                  : Low

LSA only three can be seen that the routing information is not transmitted topology information.
ABR LSA for generating three types of inter-area routing calculation.

根据三类LSA中的Adv rtr字段，判断出ABR。
根据Ls id、Net mask、Metric字段获得ABR到达目的网络号/掩码、开销。
如果多个ABR产生了指向相同目的网段的三类LSA，则根节点将根据本路由器到达目的网段的累计开销进行比较，最终生成最小开销路由。如果根节点到达目的网段的累计开销值相同，则产生等价负载的路由。

Anti-ring inter-area routing mechanisms

Generating inter-area routing loop

FIG:
RTB-converting a class, second class 1 LSA AREA1 in the LSA into three types, published in the region of 0.
RTC regenerate into three categories LSA related 192.168.1.0/24 network and posted to the Area 2 in.
Similarly, RTE will publish about three LSA 192.168.1.0/24 network to Area 3 in.
RTD turn three 192.168.1.0/24 network LSA posted to Area 1, thus forming a routing loop.
When the RTB received this three LSA again, mistakenly think that this is a new three LSA. Because Adv rtr three categories LSA RTD sent in, metric is changed.

Avoid inter-domain routing loops

How to avoid the loop?

1. All non-backbone areas must be connected to the backbone area (Area 0). Only a backbone area.
2. Communication between non-backbone areas to go through the backbone area.
3. Only ABR will produce three types of LSA
4. LSA three pass rule: three LSA from the backbone area no longer coming back to the backbone area (LSA three transfer between non-backbone areas: receives, but does not count, do not pass)

LSA passing rules for three types of explanation:

Virtual link configuration and the role of

Thinking:

只有一个区域时，区域号配置为非0会有什么问题？

Only one region, the other regions are not arranged; then even if the inner region of the non-backbone area region also can communicate with the router.
But when you want to increase the region, the normal configuration can not allow inter-regional communication.

Non-standard design area OSPF

Cause problems:

Area 0区域无法与Area 2区域路由器通信；Area 1和Area 2区域无法通信。

Because RTC is not ABR, it is not possible to generate three types of LSA.

So how to solve this problem?

Virtual Connection (the VLINK) Configuration

Virtual connection may be established on any two area border routers, but requires two area border router has a port connected to the same non-backbone area

FIG: between RTB, RTC, virtual link configuration. The equivalent of two routers to each other refers to a virtual neighbors. At this point the virtual neighbor relationship is part of Area 0 At this RTC becomes ABR

Configuring Content:

[RTB-ospf-1]ospf 1

[RTB-ospf-1]area 1 

[RTB-ospf-1-area-0.0.0.1]vlink-peer 3.3.3.3     //3.3.3.3为路由器的Router ID

[RTC-ospf-1]ospf 1

[RTC-ospf-1]area 1 

[RTC-ospf-1-area-0.0.0.1]vlink-peer 2.2.2.2

note:

1. Virtual connections are not conventional configuration, network design only for non-standard remedies, or emergency measures when network failure occurs.
2. As Topology: routing messages after virtual connection is configured RTC transmission confusing, there are: a different type of class of the LSA, has Area 1, Area LSDB 2, and also generates three LSA. It is not recommended to use the virtual connection.