RRAS Routing and Remote Access Service (Farsi User Guide)
Resource: Windows 2000 Server Resource kit Internetworking Guide

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Synchronizing the LSDB Through Adjacencies
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در Link-State  برای ایجاد Routing Table با الگوریتهای خاصی به تمامی LSDB های Intermediate System های موجود در شبکه اعتماد می کند و سپس با برقراری ارتباط بین آنها

این LSDB ها بین  ROUTER ها پخش می شود . در این نوع همگرایی هر ROUTER فقط همگرا می شود با ROUTER های مجاور خود در شبکه .

این مراحل قبل از ساخته شدن یک SPF Tree رخ می دهد و بسار مهم می باشد که صحیح انجام شود تا یک SPF Tree بر مبنای آن محاسبه شود سپس Routing Table ها ایجاد شود .

 
Forming an Adjacency

زمانی که یک OSPF ROUTER شروع به کار می کند .یک OSPF Message ارسال می کند  بنام Hello که این پیغام بطور مداوم در زمان های خاصی ارسال می شود بین ROUTER ها و

دارای اطلاعاتی مانند ROUTER ID و نیز لیستROUTER های مجاور آن که Hello Packet را ارسال کرده اند .تذکر برای اولین بار این لیست خالی می باشد .

 

خوب زمانی که این  Hello Packet ارسال می شود  ROUTER  دریافت کنند می تواند شکلی از طراحی زیربنایی شبکه را محاسبه کند Designated Router DR در این جا برای محاسبه آن از

Hello Packet هایی که هر Interface آن در یافت کرده شکل و مکان ROUTER های مجاور محاسبه می شود و نیز اطلاعات داخل Hello Packet در تکمیل شدن آن کمک می کند .

اتفاق دیگری که می افتد آن می باشد که ROUTER دریافت کنند پیغام می فهمد که با کدام ROUTER ها می تواند ارتباط شبکه خود را برقرار کند .

در این میان بجز DR مواردی همچون  Backup Designated Router BDR نیز مشخص می شود درباره این دو مورد در بخشهای بعدی شرح داده خواهد شد .

 

دو Intermediate system میان خود شروع می کنند به ردوبدل کردن LSDB های خود در اینجا می تواند گفت Database Exchange روی داده در این زمان یکی از این ROUTER ها

Master نام دارد و دیگری را Slave و هر دو شروع به همگرایی اطلاعات  LSDB های خود می کنند

در این زمان هر ROUTER مقایسه می کند LSAs های دریافتی را به LSDB خود و LSDB های ROUTER های مجاور تا بتواند بهترین LSDB را برای خود ایجاد کند و یا Up Date کند .

تذکر هر LSAs در شبکه زمانی که دریافت می شود توسط ROUTER دریافت کننده این مورد به ROUTER ارسال کنند  اطلاع داده می شود پس اگر یک LSAs گم شود و یا خراب شود دوباره ارسال می شود تا

همگرایی بین ROUTER ها بدرستی انجام شود .

 
Downed Router

 اگر دیگر Hello Packet از طرف ROUTER ها به ROUTER شما ارسال نشود در زمان بندی های خاص ROUTER شما متوجه می شود که آن Intermediate System خاموش شده .

اگر یک ROUTER به جمه ROUTER ها اضافه شود LSDB آن باید مشخص شود برای کدام ROUTER ID می باشد به همین دلیل سریع LSDB ها ی کلیه ROUTER دوباره Update می شوند.

 
Neighbor States

قوانینی بین دو ROUTER همجوار وجود دارد که شما می توانید آنها را ببینید تشخیص بدهید که دو Intermediate Systems چه وضعیتی دارد . در جدول زیر می توانید این قوانین و یا حالت ها را مشاهده

کنید .

Neighbor State Description
Down The initial state. No information has been received from the neighbor router.
Attempt No information has been received despite attempts to contact the neighbor (for NBMA networks only).
Init A Hello packet has been received from the neighbor, but the router does not appear in the neighbor list of the neighboring router's Hello packet.
2-Way A Hello packet has been received from the neighbor, and the router does appear in the neighbor list of the neighboring router's Hello packet.
ExStart Master and slave roles for the Database Exchange Process are being negotiated. This is the first phase of the adjacency relationship.
Exchange The router is sending Database Description packets to its neighbor.
Loading Link State Request packets are being sent to the neighbor requesting missing or more recent LSAs.
Full The neighboring routers' LSDBs are synchronized, and the two routers are fully adjacent.

Table 3.2 Neighbor States for Adjacent Routers

در شکل Example 1  می توانید مشاهده کنید که ROUTER مجاور شما در حالت Full می باشد .زمانی که دو Router مجاور هم DR و یا BDR باشند این حالت را سیستم نمایش می دهد .

 

Figure 3.15 Network Infrastructure

جدول Example 1 نمایان کننده وضعیت بین دو Intermediate Systems  در شکل  Figure 3.15  می باشد

Example 1

برای دیدن Table بالا مرا حل زیر را باید انجام داد .

To view the neighbor state of neighboring routers

  • In the Routing and Remote Access snap-in, in the IP Routing container, right-click OSPF, and then click Show Neighbors.

    The OSPF Neighbors window displays all neighboring routers.

Adjacency Configuration Parameters

برای همگرا شدن دو ROUTER پارامترهای زیر باید رعایت شود تا دو ROUTER با هم ارتباط برقرار کنند .

  • If authentication is being used, the neighboring routers must be using the same authentication type.

  • If simple password authentication is enabled, the neighboring routers must be using the same password.

  • The Hello Interval (default of 10 seconds), the periodic interval at which Hello packets are sent, must be the same.

  • The Dead Interval (default of 40 seconds), the amount of time after which an adjacent router is considered down after ceasing to hear that router's Hello packets, must be the same. The RFC-recommended value is four times the Hello Interval.

  • The Area ID, which identifies the area of the AS to which the router is attached, must be the same. The Area ID is configured on each router interface. Areas are discussed in more detail later in this chapter.

  • The two neighbor routers must agree as to whether they are in a stub area or not. Stub areas are discussed in more detail later in this chapter.

The Router ID of the two neighboring routers must not match in order for an adjacency to be established. Router IDs are designed to be globally unique to the AS. Duplicate Router IDs prevent an adjacency.

 
Adding a Router to a Converged OSPF Internetwork

زمانی که یک ROUTER در شبکه شروع به کار می کند اقدام  به ارسال LSA به ROUTER همجوار خود می کند .این فرایند برای ایجادیک SPF Tree و در آخر برای ایجاد یک Routing Table رخ می دهد .

در آخر کلیه Routing Table های Router ها Update می شوند .

Figure 3.15 New Adjacency Propagation

در زیر مراحل انجام شده در شکل Figure 3.15 را مشاهده می کنید .

Figure 3.15 illustrates the convergence process for a new router and new adjacency propagation in a sample OSPF internetwork.

  1. Router R1 initializes and begins sending periodic Hello packets across the point-to-point WAN link. Router R2 also sends periodic Hello packets across the link. R1 and R2 decide to form an adjacency.

  2. R1 and R2 exchange Database Description Packets. R1's Database Description Packet contains only information about itself. R2's Database Description Packet contains the latest LSAs of all the routers in the internetwork (except R1).

  3. R1 sends a Link State Request packet to R2 requesting the LSAs of all the routers on the internetwork. R2 sends the requested LSAs to R1 as Link State Update packets.

  4. R2 sends a Link State Request packet to R1 requesting its LSA. R1 sends its LSA to R2 as a Link State Update packet. R1 and R2 now have synchronized LSDBs. Upon receipt of the LSAs, R1 and R2 calculate their respective SPF trees and routing tables.

  5. Once synchronized with R1, R2 sends a Link State Update packet to all other OSPF routers to which it is adjacent (routers R3 and R4). The Link State Update packet contains the LSA learned from R1. Upon receipt of the LSA from R2, R3 and R4 calculate their respective SPF trees and routing tables.

  6. R3 and R4 flood the information in a separate Link State Update packet to their adjacent routers (routers R5 and R6). Upon receipt of the flooded LSA for R1, R5 and R6 calculate their respective SPF trees and routing tables.

The OSPF internetwork has reconverged after adding R1 and its associated network.

 
Designated Router DR

در Point-to-Point Link هر دو  سوی این Link  باید یک  WAN-i داشته باشد .در Multi-Access Networks مانند نوع Broadcast و  NBMA  سوی دیگر ارتباط ROUTER

باید چک شود ونیز کنترل شود .چون ممکن است بیشتر از یک ارتباط  وجود داشته باشد .

در Broadcast type شما می توانید همانند مثال بالا در شکل 3.15 مشاهده کنید که چگونه هر ROUTER با Intermediate System مجاور خود برخورد می کند .

در زمان شروع کار هر OSPF ROUTER  می تواند در شبکه ها NBMA , Broadcast  جمعا می تواند  با 15 ROUTER مجاور باشد در یک زمان .

در شبکه های Broadcast جمع Router مجاور طبق الگوریتم زیر شکل می گیرد و محاسبه می شود .

 

On a point-to-point link (such as a dedicated WAN link), the adjacency must occur between the two routers on either side of the link. However, on multi-access networks (such as broadcast or NBMA networks) the adjacencies must be controlled. Consider a broadcast network with 6 OSPF routers. Without controlling the adjacency behavior, each router could establish an adjacency with each other router for a total of 15 adjacency relationships. On a broadcast network with n routers, a total of n*(n-1)/2 adjacencies would be formed. The number of adjacencies scales as O(n2). In addition, unneeded flooding traffic would occur as each router attempts to synchronize with all of its adjacent routers.

To solve this scaling problem, every multi-access network (broadcast and NBMA) elects a Designated Router (DR). The DR forms adjacencies with all other routers on the network. On a broadcast network with n routers, a total of (n-1) adjacencies need to be formed. Because the DR is adjacent with all other routers, it acts as a hub for the distribution of link state information and the LSDB synchronization process.

The DR is elected through the exchange of OSPF Hello packets. Each Hello packet contains the current DR (if elected), the sending router's Router ID, and the sending router's Router Priority. The Router Priority is an interface-specific OSPF configuration parameter that is used to elect the DR. The router with the highest Router Priority is elected the DR. The default Router Priority is 1. A Router Priority of 0 means that the router does not become a DR. If multiple routers have the same highest Router Priority, the router with the highest Router ID is elected the DR.

Caution

Router Priorities must be assigned so that at least one router on the multi-access network (broadcast or NBMA) is configured with a Router Priority of 1 or greater. If all routers on a multi-access network are configured with a Router Priority of 0, no router becomes the DR and no adjacencies are established. Without adjacencies, the LSDB cannot be synchronized and no transit traffic (traffic across that network) can be passed.

Note

If a DR is already elected for a network, an initializing router on the network does not become the DR if it has a higher Router Priority than the current DR.

در DR هر Properties یک Interface دارای یک Priority می باشد که در شکل مشاهده می کنید .

در شبکه آن OSPF ROUTER که دارای Priority بالاتری باشد در Internetwork به عنوان Router که  DR شناخته می شود.

یک DR یک ROUTER می باشد که دیگر ROUTER ها نیز همجوار آن در نظر گرفته می شود .

 
 
 

DRs on Broadcast Networks

در شکل زیر تفاوت بین DR OSPF Network , OSPF Network را مشاهده می کنید .

OSPF

 

 

DR

AND

OSPF

 

در شکل Figure 3.16 دو نوع OSPF و DR OSPF را مشاهده می کنید در اینجا شما می توانید تفاوت LSAs Traffic را

بین دو نوع را مشاهده کنید.درDRآنROUTER که دارای Priority بالایی  می باشد به عنوان یک  DR همانند یک HUB عمل می کند و

LSAs Traffic را از بین خود عبور می دهد و نیز LSDB Traffic را نیز به همین گونه.یادمان باشد که در Hello Packet همیشه

که شما باید Router Priority هرInterface را مشخص کنید را نمایش می دهد

 
 
 
 
 
 
 
 
 
 

Figure 3.16 Designated Routers on Broadcast Networks

 
 

 

DRs on NBMA Nets

در شبکه های NBMA یک در DR باید وجود داشته باشد چون فقط این DR می باشد که می تواند با دیگر ROUTER ها ارتباط برقرار کند

در واقع به هر سه ROUTER دیگر ارتباط دارد و نیز همانند HUB عمل می کند .برای اطمینان بیشتر در این شبکه DR دارای Priority =1

یا بالاتر از یک باید باشد ودیگر ROUTER ها دارای Priority =0 می باشند .

 
 

For NBMA networks, such as a Frame Relay network in a hub and spoke configuration, the DR must be the hub router because only the hub router can communicate with all the other routers.

 

To ensure that the hub router is the DR, set its Router Priority to 1 (or greater). To ensure that no spoke router becomes a DR, set spoke router Router Priorities to 0.

 
 

Figure 3.17 Designated Routers on a Frame Relay Network

 
 
Backup Designated Router

The DR acts as a central distribution point for topological changes on a multi-access network. If the DR becomes unavailable, all new adjacencies must be formed with a new DR. Until the adjacencies form and the internetwork converges, a temporary loss of connectivity for transit traffic might result.

To prevent the loss in connectivity associated with the loss of a DR, a Backup Designated Router (BDR) is also elected for each multi-access network. Like the DR, the BDR is adjacent to all routers on the network. When the DR fails, the BDR immediately becomes the DR by sending LSAs to all of its adjacent routers announcing its new role. There is a very short period of time where transit traffic could be impaired as the BDR takes over the role of the DR.

Like the DR, the BDR is elected by the exchange of Hello packets. Each Hello packet contains a field for the BDR of the network. If the BDR is not specified, the router with the highest Router Priority that is not already the DR becomes the BDR. If there are multiple routers with the highest Router Priority, the router with the highest Router ID is elected the BDR.

Interface States

Each OSPF interface can be in one of several states after forming adjacencies. Table 3.3 lists the possible interface states.

 

Table 3.3 Interface States for Adjacent Routers

Interface State Description
Down The initial interface state. No Hello packets have been sent or received.
Loopback The interface to the network is looped back (internally configured so that no packets are sent) through hardware or software.
Waiting The interface is sending and receiving Hello packets to determine the DR and BDR for the network.
Point-to-Point The interface is adjacent to its neighbor on a point-to-point network or through a virtual link.
Other The interface is on a multi-access network and is not the DR or BDR.
Designated Router The interface is the DR for the attached network.
Backup Designated Router The interface is the BDR for the attached network.
 

To view the interface state for an OSPF interface on a Windows 2000 OSPF router, in the Routing and Remote Access snap-in, in the IP Routing container, click OSPF. The contents pane displays the OSPF interfaces. The State column indicates the interface's current state. By viewing the interface state for each OSPF interface on a given network, you can determine the DR and the BDR for the network.

 

Example 3  Interface States for Adjacent Routers

در شکل بالا می تونید چگونگی نوع هر Interface را مشاهده کنید .

RRAS Routing and Remote Access Service (Farsi User Guide)

LastUpdate:2005/04/05

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