Module 3: EIGRP 
  
3.1 EIGRP Concepts 
   
3.1.1 Comparing EIGRP and IGRP 

EIGRP has improved convergence properties and operates more efficiently over IGRP. This allows a network to have improved architecture as well as retain the current investment in IGRP.

EIGRP 已經改進收斂特性，並且更有效地在 IGRP 上運作。這允許一個網路改進其架構，並保障了目前已對 IGRP 的投資。

The comparisons between EIGRP and IGRP fall into the following major categories:

• Compatibility mode
• Metric calculation
• Hop count
• Automatic protocol redistribution
• Route tagging

EIGRP 和 IGRP 之間的比較，可分成以下的主要的類別︰

• 相容性模式
• 度量計算
• 跳躍計數
• 自動的協定再分送
• 路由標記

IGRP and EIGRP are compatible with each other. This compatibility provides seamless interoperability with IGRP routers. This is important as users can take advantage of the benefits of both protocols. EIGRP offers multiprotocol support, but IGRP does not.

IGRP 和 EIGRP 彼此相容，這個相容性提供與 IGRP 路由器無間的互操作性。當用戶要能使用兩種協定的好處時，相容性是很重要的。EIGRP 可支援多重協定，而 IGRP 不支援。

EIGRP and IGRP use different metric calculations. EIGRP scales the metric of IGRP by a factor of 256. That is because EIGRP uses a metric that is 32 bits long, and IGRP uses a 24-bit metric. EIGRP can multiply or divide by 256 to easily exchange information with IGRP.

EIGRP 和 IGRP 使用不同的度量計算，EIGRP 以 IGRP 的256倍數為度量標準。因為 EIGRP 使用32位元的度量標準，而 IGRP 使用24位元的度量標準。EIGRP 可以乘或除256，以方便和 IGRP 交換訊息。

IGRP has a maximum hop count of 255. EIGRP has a maximum hop count limit of 224. This is more than adequate to support large, properly designed internetworks.

IGRP 的最大的跳躍計數是255，EIGRP的最大的跳躍計數限制是224。這非常足夠支援大型且適當設計的網路。

Redistribution, or route sharing, is automatic between IGRP and EIGRP as long as both processes use the same AS number. In Figure , RTB automatically redistributes routes learned from EIGRP to the IGRP AS, and vice versa.

只要使用相同的自主系統號碼，則在 IGRP 和 EIGRP 之間重分送或者分享路徑是自動的。在圖 ，RTB 自動重新分發所學習由 EIGRP 到 IGRP 自主系統的路徑，反之亦然。

EIGRP tags routes learned from IGRP or any outside source as external because they did not originate from EIGRP routers. IGRP cannot differentiate between internal and external routes.

EIGRP 標記從 IGRP 學來的路徑或者任何不是發自 EIGRP 路由器的來源為外部的。IGRP 則不能區分內部和外部路徑。

Notice that in the show ip route command output for the routers in Figure , EIGRP routes are flagged with D, and external routes are denoted by EX. RTA identifies the difference between the 172.16.0.0 network, which was learned through EIGRP, and the 192.168.1.0 network that was redistributed from IGRP. In the RTC table, the IGRP protocol makes no such distinction. RTC, which uses IGRP only, just sees IGRP routes, despite the fact that both 10.1.1.0 and 172.16.0.0 were redistributed from EIGRP.  

請注意在圖 的路由器，show ip route 命令 之輸出。EIGRP 路徑被標記為D，外部路徑則標記為EX。 RTA  指出在172.16.0.0 這個透過 EIGRP 學來的網段，和從 IGRP 重新分送的 192.168.1.0 的網段之間的差異。在 RTC 表格中， IGRP 協定並不做這樣的區別。儘管 10.1.1.0 和 172.16.0.0 是從 EIGRP 重新分送的事實，RTC 只使用 IGRP，也只能看到 IGRP 路線。

3.1.2 EIGRP concepts and terminology 

EIGRP routers keep route and topology information readily available in RAM so they can react quickly to changes. Like OSPF, EIGRP saves this information in several tables and databases.

EIGRP 路由器儲存路徑和拓撲資訊在立即可用的 RAM 中，使其能迅速回應改變。如同 OSPF 一樣，EIGRP 儲存這些資訊在好幾個表格和資料庫內。

EIGRP saves routes that are learned, in specific ways. Routes are given a particular status and can be tagged to provide additional useful information.

EIGRP 保存用特定方式學習到的路徑，路徑被給定一個特別的狀態，並且可以附加標籤以提供有用的資訊。

The following three tables are maintained by EIGRP:

• Neighbor table
• Topology table
• Routing table

EIGRP維護以下三個表格︰

• 相鄰表
• 拓撲表
• 路由表

The neighbor table is the most important table in EIGRP. Each EIGRP router maintains a neighbor table that lists adjacent routers. This table is comparable to the adjacency database used by OSPF. There is a neighbor table for each protocol that EIGRP supports.

相鄰表是EIGRP最重要的表格，每個EIGRP路由器維護一個臨接的路由表，以提供鄰接路由器名單。這表格可與OSPF所使用的相鄰資料庫相比較，EIGRP所支援的每個協定皆有一個相鄰表。

When newly discovered neighbors are learned, the address and interface of the neighbor is recorded. This information is stored in the neighbor data structure. When a neighbor sends a hello packet, it advertises a hold time. The hold time is the amount of time a router treats a neighbor as reachable and operational. If a hello packet is not received within the hold time, then the hold time expires. When the hold time expires, the Diffusing Update Algorithm (DUAL), which is the EIGRP distance vector algorithm, is informed of the topology change and must recalculate the new topology.

當最近剛發現的鄰接路由器被學習到時，鄰接路由器的位址和界面會被記錄下來，這資訊被儲存在鄰居路由器的資料結構中。當一個鄰居送一個 hello 封包時，就廣播一段保持時間。保持時間是路由器把一個鄰接路由器當作可以到達和操作的時間，如果在保持時間內沒有收到 hello 封包，則保持時間終止。當保持時間終止時，DUAL 演算法，也就是 EIGRP 距離向量演算法，被通知拓撲改變，並且必須再重計算新拓撲。

The topology table is made up of all the EIGRP routing tables in the autonomous system. DUAL takes the information supplied in the neighbor table and the topology table and calculates the lowest cost routes to each destination. EIGRP tracks this information so that EIGRP routers can identify and switch to alternate routes quickly. The information that the router learns from the DUAL is used to determine the successor route, which is the term used to identify the primary or best route. This information is also entered into the topology table.

拓撲表是由自主系統中所有 EIGRP 路由表所組成，DUAL 使用儲存在相鄰表格和拓撲表格中的資訊，來計算到達每個目的地的最低費用路徑。 EIGRP 追蹤這個資訊，以便路由器 EIGRP 可以鑑定迅速地識別並切換到備用路徑。從 DUAL 所學到的資訊會用來決定後繼路徑，這是用來識別主要路徑或最佳路徑，這個資訊也會被放入拓撲表中。

Every EIGRP router maintains a topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.

EIGRP 路由器為每一個設定過組態的網路協定維護一個拓撲表格，所有學習到目的地的路徑皆被儲存在拓撲表格。

The topology table includes the following fields:

• Feasible distance (FD) - This is the lowest calculated metric to each destination. For example, the feasible distance to 32.0.0.0 is 2195456.
• Route source - The identification number of the router that originally advertised that route. This field is populated only for routes learned externally from the EIGRP network. Route tagging can be particularly useful with policy-based routing. For example, the route source to 32.0.0.0 is 200.10.10.10 through 200.10.10.10.
• Reported distance (RD) - The distance reported by an adjacent neighbor to a specific destination. For example, the reported distance to 32.0.0.0 is /281600 as indicated by (2195456/281600).
• Interface information - The interface through which the destination can be reached.
• Route status - The status of a route. Routes are identified as being either passive, which means that the route is stable and ready for use, or active, which means that the route is in the the process of being recomputed by DUAL.

下列是在拓撲表格中的欄位︰

• 可行的距離(FD) - 這是到達每個目的地的最低計算度量標準。 例如，到 32.0.0.0 網段的可行的距離是2195456。
• 路徑來源 - 廣播該路徑來源的路由器識別號碼。這欄位只對由 EIGRP 網路外部學到的路徑有填入。路由標籤對於以策略為主的繞送特別有用。 例如，到32.0.0.0 的路徑來源是經過 200.10.10.10 的 200.10.10.10。
• 報告距離 (RD) - 由一個臨近鄰居的報告得知，到特定目的地的距離。 例如，到32.0.0.0 的報告距離是2195456，這是以 (90/2195456) 來表示。
• 介面資訊–可以到達目的地的資訊。
• 路徑狀態 - 路徑的狀態。 被識別為被動路徑，即表示該路徑是穩定並且準備好可以使用，或者被識別為主動路徑，即表示路徑正在被 DUAL 重新計算的過程中。

The EIGRP routing table holds the best routes to a destination. This information is retrieved from the topology table. EIGRP routers maintain a routing table for each network protocol.

EIGRP 路由表中保存有到達目的地的最佳路徑，這個資訊是從拓撲表中取得的。EIGRP 路由器為每個網路協定維護一個路由表。

A successor is a route selected as the primary route to reach a destination. DUAL identifies this route from the information contained in the neighbor and topology tables and places it in the routing table. There can be up to four successor routes for any particular destination. These can be of equal or unequal cost and are identified as the best loop-free paths to a given destination. A copy of the successor routes is also placed in the topology table.

後繼路徑是一條被選為到達目的地的主要路徑， DUAL 由相鄰表和拓撲表中識別出主要路徑，並且把它放置在路由表內。最多可以有4條到達任何特定目的地的後繼路徑。這些被識別為到達目的地的最佳無迴路路徑，可以有相同或不相同的費用，後繼路徑的副本也會被放置在拓撲表中。

A feasible successor (FS) is a backup route. These routes are identified at the same time as the successors, but these routes are only kept in the topology table. Multiple feasible successors for a destination can be retained in the topology table although it is not mandatory.

一個可行的後繼路徑(FS) 是一條備用路線。 這些路徑在同一時間被識別為後繼路徑，但是這些路線只會放在拓撲表中。雖然不是強制的，但是多個到達目的地的可行後繼路徑，可以被保留在拓撲表中。

A router views the feasible successors as neighbors downstream, or closer to the destination than it is. Feasible successor cost is computed by the advertised cost of the neighbor router to the destination. If a successor route goes down, the router will look for an identified feasible successor. This route will be promoted to successor status. A feasible successor must have a lower advertised cost than the current successor cost to the destination. If a feasible successor is not identified from the current information, the router places an Active status on a route and sends out query packets to all neighbors in order to recompute the current topology. The router can identify any new successor or feasible successor routes from the new data that is received from the reply packets that answer the query requests. The router will then place a Passive status on the route.

一個路由器視可行後繼路徑為下游鄰居，或者是比自己更接近目的地。可行後繼路徑的費用是以鄰接路由器到目的地的廣播費用來計算的。如果一條後繼路徑當掉，路由器將尋找一條可行的後繼路徑，這條路徑將被提升為後繼路徑。一條到達目的地的可行後繼路徑的廣播費用，必須比目前後繼路徑的費用較低。如果沒有從目前的資訊指定一條可行的後繼路徑，路由器會將一條路徑設定為活躍的狀態，並且送出一個查詢封包給所有的鄰居，以便重新計算目前的拓撲結構。路由器可以從回答查詢請求的答覆封包中得到的新資料，並識別任何新的後繼路徑或可行後繼路徑。路由器將設定該路徑為被動的狀態。

The topology table can record additional information about each route. EIGRP classifies routes as either internal or external. EIGRP adds a route tag to each route to identify this classification. Internal routes originate from within the EIGRP AS.

拓撲表可以記錄每條路徑的附加資訊。EIGRP 把路徑歸類為內部或外部，EIGRP 對每條路徑附加一個路由標籤作為分類識別，內部路徑源自於 EIGRP 自主系統內部。

External routes originate outside the EIGRP AS. Routes learned or redistributed from other routing protocols, such as RIP, OSPF, and IGRP, are external. Static routes that originate outside the EIGRP AS are external. The tag can be configured to a number between 0-255 to customize the tag.

外部路徑源自於EIGRP自主系統的外部。從其他路由協定所學到的路徑或者重新分送來的路徑，例如RIP、OSPF和IGRP，都是外部路徑。源自於EIGRP自主系統外部的靜態路徑也是外部路徑。標籤號碼可以設定在0-255之間。

3.1.3 EIGRP design features 

EIGRP operates quite differently from IGRP. EIGRP is an advance distance vector routing protocol, but also acts as a link-state protocol in the way that it updates neighbors and maintains routing information. The following are advantages of EIGRP over simple distance vector protocols:

• Rapid convergence
• Efficient use of bandwidth
• Support for VLSM and CIDR.
• Multiple network layer support
• Independence from routed protocols.

EIGRP 的運作與IGRP 十分不同。EIGRP 是一個先進的距離向量路由協定，而在更新鄰居和維護路由資訊的方式上，操作如同鏈路狀態協定。下列是 EIGRP 比簡單的距離向量協定優勢的地方︰

• 收斂快速
• 頻寬使用有效率
• 支援VLSM 和CIDR
• 支援多重網路階層
• 與被繞送協定無關

Independence from routed protocols means that protocol-dependent modules (PDMs) protect EIGRP from lengthy revision. As routed protocols evolve, they may need new protocol modules, but changes to EIGRP will not be necessary.

與被繞送協定無關的意思是，協定相關模組(PDM)保護 EIGRP 免於冗長的改版。當被繞送協定逐漸改進時，它們也許需要新的協定模組，但是不必修改EIGRP。

EIGRP routers converge quickly because they rely on DUAL. DUAL guarantees loop-free operation throughout a route computation which allows all routers involved in a topology change to synchronize at the same time.

由於靠著 DUAL 之故，EIGRP 路由器的收斂非常迅速。DUAL 經由允許同步所有路由器的拓撲改變的路由計算，可以保證無迴圈運作。

EIGRP sends partial, bounded updates and makes efficient use of bandwidth. EIGRP uses minimal bandwidth when the network is stable. EIGRP routers do not send the complete tables, but instead, send partial, incremental updates. This is similar to OSPF operation, except that EIGRP routers send these partial updates only to the routers that need the information, not to all routers in an area. For this reason, they are called bounded updates. Instead of timed routing updates, EIGRP routers use small hello packets to keep in touch with each other. Though exchanged regularly, hello packets do not use up a significant amount of bandwidth.

EIGRP 只傳送部分且有限的更新，以有效率利用頻寬。當網路是穩定的時候，EIGRP 僅使用最小的頻寬。EIGRP  路由器不傳送整個表格，而只傳送部分漸增的更新。除了EIGRP路由器只傳送這些部分更新給需要訊息的路由器，而不是給在一個區域的所有路由器之外，其操作是與 OSPF  類似的。EIGRP 路由器使用小的 hello 封包與其他路由器保持聯繫，而不是用調節路由更新。經由定期的交換，hello 封包不會佔用大量的頻寬。

EIGRP supports IP, IPX, and AppleTalk through PDMs. EIGRP can redistribute IPX-RIP and IPX SAP information to improve overall performance. In effect, EIGRP can take over for these two protocols. EIGRP routers receive routing and service updates, and update other routers only when changes in the SAP or routing tables occur. In EIGRP networks, routing updates occur in partial updates.

EIGRP 透過PDMs支援 IP、IPX 和 AppleTalk。EIGRP 能重新分送IPX、RIP 和 SAP 資訊以改進總體效能。實際上， EIGRP 能接管這兩種協定。EIGRP 路由器接收路由和服務更新，並且只有當在 SAP 或路由表中有改變發生時，才會更新其他路由器。在 EIGRP 網路，路由只做部分更新。

EIGRP can also take over for the AppleTalk RTMP. As a distance vector routing protocol, RTMP relies on periodic and complete exchanges of routing information. To reduce overhead, EIGRP uses event-driven updates to redistributes AppleTalk routing information. EIGRP also uses a configurable composite metric to determine the best route to an AppleTalk network. RTMP uses hop count, which can result in suboptimal routing. AppleTalk clients expect RTMP information from local routers, so EIGRP for AppleTalk should be run only on a clientless network, such as a WAN link.

EIGRP 也能接管 AppleTalk RTMP。作為一個距離向量路由協定，RTMP 倚賴定期和完整交換的路由資訊。為了降低代價，EIGRP 使用事件驅動更新，來重新分送AppleTalk 路由資訊。EIGRP 也使用可設定組態的合成度量標準，來決定一個到AppleTalk網路的最好路徑。RTMP 使用跳躍計數可以找到次佳的路由。AppleTalk 客戶期望來自本地路由器的 RTMP 資訊，因而對 AppleTalk 而言，EIGRP 應該只能在一個無客戶的網路上執行，例如 WAN 的連接。

3.1.4 EIGRP technologies 

Simple distance vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors, much the same way that OSPF routers do.

簡單的距離向量路由器不會與它們的鄰居建立任何關係。RIP 和IGRP 路由器僅僅在組態的界面上，作廣播或者多重廣播的更新。相反的，EIGRP 路由器主動地與他們的鄰居建立關係，很像 OSPF 路由器所作的方式一樣。

EIGRP routers establish adjacencies as described in Figure . EIGRP routers use small hello packets to accomplish this. Hellos are sent by default every five seconds. An EIGRP router assumes that as long as it receives hello packets from known neighbors, those neighbors and their routes remain viable or passive. The following are possible when EIGRP routers form adjacencies:

• Dynamically learn of new routes that join the network
• Identify routers that become either unreachable or inoperable
• Rediscover routers that had previously been unreachable

EIGRP 路由器建立相鄰關係的方式，如圖 所示。EIGRP 路由器使用小hello封包來建立關係。 Hello 封包預設每5秒傳送一次。EIGRP 路由器假設，只要它有從已知的鄰居收到 hello 封包，那些鄰居和它們的路由就保持可行的或者被動的。在EIGRP 路由器形成相鄰關係時，下列是可能的︰

• 動態學習到新加入網路的路徑
• 識別變得無法到達或不能工作的路由器
• 重新發現以前不能到達的路由器

Reliable Transport Protocol (RTP) is a transport layer protocol that guarantees ordered delivery of EIGRP packets to all neighbors. On an IP network, hosts use TCP to sequence packets and ensure their timely delivery. However, EIGRP is protocol-independent. This means it does not rely on TCP/IP to exchange routing information the way that RIP, IGRP, and OSPF do. To stay independent of IP, EIGRP uses RTP as its own proprietary transport layer protocol to guarantee delivery of routing information.

可靠的傳送協議 (RTP) 是傳送層協定，它確保傳送 EIGRP 封包的順序抵達。在IP網路上，主機使用TCP來排列封包，並且保證他們的及時送達。不過，EIGRP 與協定無關的。這表示路由訊息並不倚賴 TCP/IP 來交換路由資訊，如同RIP、IGRP 和 OSPF 作法。為了保持與IP無關，EIGRP 使用 RTP 作為它自己專屬的傳送層協定，以保證路由資訊的送達。

EIGRP can call on RTP to provide reliable or unreliable service as the situation warrants. For example, hello packets do not require the overhead of reliable delivery because they are frequent and should be kept small. The reliable delivery of other routing information can actually speed convergence because then EIGRP routers do not wait for a timer to expire before they retransmit.

EIGRP 能要求 RTP 提供可靠或者不可靠的服務作為狀況保證。例如，hello 封包不需要可靠送達的負擔，因為他們很頻繁並且應該保持很小。其他路由資訊的可靠送達，實際上能加快收斂，因為 EIGRP 不需要等計時器終止，即可重新傳送。

With RTP, EIGRP can multicast and unicast to different peers simultaneously. This allows for maximum efficiency.

有了RTP，EIGRP 就可以同時對不同路由器作多重廣播和單一廣播，這允許獲得最大效率。

The centerpiece of EIGRP is the DUAL, which is the EIGRP route-calculation engine. The full name of this technology is DUAL finite-state machine (FSM). An FSM is an algorithm machine, not a mechanical device with parts that move. FSMs define a set of possible states that something can go through, the events that cause those states, and the events that result from those states. Designers use FSMs to describe how a device, computer program, or routing algorithm will react to a set of input events. The DUAL FSM contains all the logic used to calculate and compare routes in an EIGRP network.

DUAL是 EIGRP 的中心部份，它是 EIGRP 計算路由的引擎。這項技術的全名是 DUAL 有限狀態機 (FSM)。FSM 是演算法機器，而不是一個具有移動元件的機械設備。FSMs 定義一組可能的狀態、能引致那些狀態的事件和起因於那些狀態的事件。設計者使用 FSMs 來描述一個設備、電腦程式或者路由演算法是如何對一套輸入事件作反應的。DUAL FSM 包含有用來計算和比較在 EIGRP 網路路由的全部邏輯。

DUAL tracks all the routes advertised by neighbors. Composite metrics of each route are used to compare them. DUAL also guarantees that each path is loop free. DUAL inserts lowest cost paths into the routing table. These primary routes are known as successor routes. A copy of the successor routes is also placed in the topology table.

DUAL 追蹤從鄰接路由器所廣播來的路由。以每條路徑的合成度量作相互比較。  DUAL也保證每條路徑都是無迴圈的。DUAL 將最低費用的路徑放在路由表中，這些主要路由稱為後繼路徑，後繼路徑的副本也會被放在拓撲表中。

EIGRP keeps important route and topology information readily available in a neighbor table and a topology table. These tables supply DUAL with comprehensive route information in case of network disruption. DUAL uses the information in these tables to select alternate routes quickly. If a link goes down, DUAL looks for an alternative route path, or feasible successor, in the topology table.

EIGRP 在立即可用的相鄰表和拓撲表中，保存重要的路徑和拓撲訊息。這些表格在網路當掉時，提供 DUAL 完整的路徑資訊。DUAL 使用在這些表格裡的資訊，迅速地選擇替代路徑。如果某條鏈路斷線了，DUAL 會在拓撲表中尋找替代的路徑或者可行的後繼者。

One of the best features of EIGRP is its modular design. Modular, or layered designs, prove to be the most scalable and adaptable. Support for routed protocols, such as IP, IPX, and AppleTalk, is included in EIGRP through PDMs. In theory, EIGRP can add PDMs to easily adapt to new or revised routed protocols such as IPv6.

EIGRP 最好的功能之一是模組化設計。模組化或分層設計，證明是最具可調節性和適應性的。透過內含在 EIGRP 中的PDM，可支援諸如 IP、IPX 和 AppleTalk 等 被繞送協定。理論上 EIGRP 可以附加P DM，使其容易適應新的或修正過的被繞送協定，例如 IPv6。

Each PDM is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following functions:

• Send and receive EIGRP packets that bear IP data
• Notify DUAL of new IP routing information that is received
• Maintain the results of DUAL routing decisions in the IP routing table
• Redistribute routing information that was learned by other IP-capable routing protocols

每個 PDM 負責與它特定被繞送協定有關的所有功能。 IP-EIGRP 模組負責下列功能︰

• 傳送與接收攜帶有IP 資料的 EIGRP 封包
• 通知 DUAL 它所收到新的IP路由資訊
• 維護在IP 路由表中，DUAL 判定路由的結果
• 重新分發由其他具有IP能力的路由協定所學習到的路由資訊

3.1.5 EIGRP data structure 

Like OSPF, EIGRP relies on different types of packets to maintain its tables and establish relationships with neighbor routers. This page will describe these packet types.

如同 OSPF 一樣，EIGRP 倚賴不同類型的封包來維護它的表格，並且建立與鄰接路由器的關係，這頁將描述這些封包的類型。

The following are the five types of EIGRP packets:

• Hello
• Acknowledgment
• Update
• Query
• Reply

下列是 EIGRP 的五種封包類型︰

• 你好
• 確認
• 更新
• 查詢
• 回答

EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers. Rediscovery occurs if EIGRP routers do not receive hellos from each other for a hold time interval but then re-establish communication.

EIGRP 倚賴 hello 封包來發現、驗證以及重新發現鄰居路由器。如果 EIGRP 路由器在保持時間內沒有從彼此收到hello封包，將執行重新發現，然後再建立通訊。

EIGRP routers send hellos at a fixed, but configurable interval called the hello interval. The default hello interval depends on the bandwidth of the interface. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.

路由器 EIGRP 每固定一段時間傳送 hello 封包，此時間間隔可以組態設定，稱為 hello 間隔。預設的 hello 間隔取決於界面的頻寬。 在IP 網路上，EIGRP 路由器把 hello 封包送到多重廣播IP 位址 224.0.0.10。

EIGRP routers store information about neighbors in the neighbor table. The neighbor table includes the Sequence Number (Seq No) field to record the number of the last received EIGRP packet that each neighbor sent. The neighbor table also includes a Hold Time field which records the time the last packet was received. Packets should be received within the Hold Time interval period to maintain a Passive state. The Passive state is a reachable and operational status.

EIGRP 路由器儲存關於鄰居的資訊在相鄰表中。相鄰表包含有括順序號 (Seq No) 欄位，記錄最近從每個鄰居送來的EIGRP 封包數量。相鄰表也包含有保持時間欄位，來記錄收到最近一個封包的時間。必須在保持時間內收到封包以維持在被動狀態，被動狀態是一個可到達且運作中的狀態。

If EIGRP does not receive a packet from a neighbor within the hold time, EIGRP considers that neighbor down. DUAL then steps in to re-evaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired.

如果EIGRP 在保持時間內沒有從一個鄰居那裡收到封包，EIGRP 會認為那個鄰居是當掉的。然後 DUAL 介入重新評估路由表。預設保持時間是 hello 間隔的3倍，但是管理者可以依照要求來設定這兩個時間。

OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each of the other respective timers through the exchange of hello packets. They then use that information to forge a stable relationship regardless of unlike timers.

OSPF 要求相鄰路由器有相同的 hello 間隔和死亡間隔，EIGRP 沒有這樣的限制。相鄰路由器透過交換 hello 封包了解其它各自的計時器。儘管計時器不同，他們使用計時訊息建立一個穩定的關係。

Hello packets are always sent unreliably. This means that no acknowledgment is transmitted.

Hello 封包的傳送是不可靠的，這意味著不會收到確認封包。

EIGRP routers use acknowledgment packets to indicate receipt of any EIGRP packet during a reliable exchange. RTP provides reliable communication between EIGRP hosts. A message that is received must be acknowledged by the recipient to be reliable. Acknowledgment packets, which are hello packets without data, are used for this purpose. Unlike multicast hellos, acknowledgment packets are unicast. Acknowledgments can be attached to other kinds of EIGRP packets, such as reply packets.

EIGRP 路由器使用確認封包來表明，在一次可靠的交換期間中收到了EIGRP 封包。RTP 提供了在 EIGRP 主機間的可靠通信。所送出的訊息必須被接受者確認才是可靠的。不含資料的確認封包，亦即 hello 封包，就是這個用途。確認封包是單一廣播，與多重廣播 hello不同。確認可以附在其他 EIGRP 封包上，例如答覆封包。

Update packets are used when a router discovers a new neighbor. EIGRP routers send unicast update packets to that new neighbor so that it can add to its topology table. More than one update packet may be needed to convey all the topology information to the newly discovered neighbor.

當路由器發現一個新鄰居時，就會使用更新封包。EIGRP 路由器傳送單一廣播的更新封包給新鄰居，以加入到它的拓撲表中。要將所有拓撲資訊傳送給最近發現的鄰居，可能需要不止一個更新封包。

Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors, which alerts them to the change. All update packets are sent reliably.

更新封包也被用於路由器偵測到拓撲改變時。在這種情況，EIGRP 路由器傳送一個群組廣播更新封包給所有的鄰居，提醒它們此一變化，所有更新封包的傳送都是可靠的。

An EIGRP router uses query packets whenever it needs specific information from one or all of its neighbors. A reply packet is used to respond to a query.

每當需要來自一個或所有鄰居的特定資訊時，EIGRP路由器使用詢問封包。答覆封包用來對一個詢問作出回應。

If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the Active state. A query is then multicasted to all neighbors in an attempt to locate a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.

如果EIGRP 路由器失去後繼者，並且也無法找到可行的後繼者，DUAL 會將該路徑設為活躍狀態，然後對全部鄰居群組廣播一個詢問封包，試圖找出一個通往目的地網路的後繼者。鄰居必須提供在後繼者上的咨訊，或者表明沒有資訊可提供。詢問可以是群組廣播或者單一廣播，然而答覆封包通常是單一廣播。這兩種封包的傳送都是可靠的。

3.1.6 EIGRP algorithm 

The sophisticated DUAL algorithm results in the exceptionally fast convergence of EIGRP. To better understand convergence with DUAL, consider the example in Figure . Each router has constructed a topology table that contains information about how to route to destination Network A.

複雜的 DUAL 演算法是導致 EIGRP 異常快速收斂的原因。請考慮圖 的例子以了解 DUAL 的收斂. 每個路由器皆會建立一個拓撲表，存放有關如何到達目的網路A 的路徑。

Each topology table identifies the following information:

• The routing protocol or EIGRP
• The lowest cost of the route, which is called feasible distance (FD)
• The cost of the route as advertised by the neighboring router, which is called reported distance (RD)

每個拓撲表會指明下列資訊：

• 路由協定或者 EIGRP
• 路徑的最低費用，稱為可行的距離 (FD)
• 由相鄰路由器所廣播的路徑費用，稱為報告距離 (RD)

The Topology column identifies the primary route called the successor route (successor), and, where identified, the backup route called the feasible successor (FS). Note that it is not necessary to have an identified feasible successor.

拓撲行指明主要路徑，稱為後繼路徑(後繼者)，和指明備用路徑，稱為可行的後繼者(FS)。注意，並不一定要有一個被 指定的可行後繼者。

The EIGRP network follows a sequence of actions to allow convergence between the routers, which currently have the following topology information:

• Router C has one successor route by way of Router B.
• Router C has one feasible successor route by way of Router D.
• Router D has one successor route by way of Router B.
• Router D has no feasible successor route.
• Router E has one successor route by way of Router D.
• Router E has no feasible successor.

EIGRP 網路伴隨一連串的動作，以允許在路由器之間的收斂，目前有下列拓撲資訊：

• 路由器C 經由路由器B 有一條後繼路徑
• 路由器C 經由路由器D 有一條可行的後繼路徑
• 路由器D 經由路由器B 有一條後繼路徑
• 路由器D 沒有可行的後繼路徑
• 路由器E 經由路由器D 有一條可行的後繼路徑
• 路由器E 沒有可行的後繼者。

The feasible successor route selection rules are specified in Figure .

可行的後繼路徑選擇規則在圖 裡有詳細指明：

The following example demonstrates how each router in the topology will carry out the feasible successor selection rules when the route from Router D to Router B goes down:

下列例子示範在拓撲圖中的每路由器，當從路由器D到路由器B的路徑當掉時，是如何實行可行後繼者的選擇規則︰

In Router D: 

• Route by way of Router B is removed from the topology table.
• This is the successor route. Router D has no feasible successor identified.
• Router D must complete a new route computation.

在路由器 D︰

• 經由路由器B的路徑，被從拓撲表中移除。
• 這是後繼路徑，路由器D沒有找出可行的後繼者。
• 路由器D 必須完成一個新的路徑計算。

In Router C:

• Route to Network A by way of Router D is down.
• Route by way of Router D is removed from the table.
• This is the feasible successor route for Router C.

在路由器 C︰

• 經由路由器D 到網路A的路徑是當掉的。
• 經由路由器D 的路徑被從表中移除。
• 這是路由器C 的可行的後繼路徑。

In Router D:

• Router D has no feasible successor. It cannot switch to an identified alternative backup route.
• Router D must recompute the topology of the network. The path to destination Network A is set to Active.
• Router D sends a query packet to all connected neighbors to request topology information.
• Router C does have a previous entry for Router D.
• Router D does not have a previous entry for Router E.

在路由器 D︰

• 路由器D 沒有可行的後繼路徑，它不能切換到一條指定的替代備用路徑。
• 路由器D 必須重算網路的拓撲。通向到達網路A的路徑被設為活耀的。
• 路由器D 傳送一個詢問封包送給所有連結的鄰居，以請求拓撲資訊。
• 路由器C 先前確實有一個可到達路由器D的記錄。
• 路由器D 先前沒有一個可到達路由器E的記錄。

In Router E:

• Route to Network A through Router D is down.
• The route by way of Router D is removed from the table.
• This is the successor route for Router E.
• Router E does not have a feasible route identified.
• Note that the RD cost of routing by way of Router C is 3. That is the same cost as the successor route by way of Router D.

在路由器E︰

• 經過路由器D 到網路A的路徑是當掉的。
• 經由路由器D 到網路A的路徑，被從表中移除 。
• 這是路由器E. 的後繼路徑。
• 路由器E 沒有找出可行的路徑。
• 請注意，經由路由器C  的路由費用RD是3，這和經由路由器D的後繼路徑有相同的費用。

In Router C: 

• Router E sends a query packet to Router C.
• Router C removes Router E from the table.
• Router C replies to Router D with a new route to Network A.

在路由器C︰

• 路由器E 傳送一個詢問封包給路由器C。
• 路由器C 從表中移除路由器E。
• 路由器C 回答路由器D，有一條到網路A的新路徑。

In Router D:

• Route status to destination Network A is still marked as Active. The computation has not been completed yet.
• Router C has replied to Router D to confirm that a route to destination Network A is available with a cost of 5.
• Router D still waits for a reply from Router E.

在路由器D︰

• 到達目的網路A的路徑狀態，仍然被標示為活躍的，計算還沒被完成。
• 路由器C 已經答覆路由器D，確認有一條到達目的網路A的路徑可以用，其費用是5。
• 路由器D 仍然等待從路由器E來的答覆。

In Router E:

• Router E has no feasible successor to reach destination Network A.
• Router E, therefore, tags the status of the route to destination network as Active.
• Router E has to recompute the network topology.
• Router E removes the route by way of Router D from the table.
• Router E sends a query to Router C, to request topology information.
• Router E already has an entry by way of Router C. It is at a cost of 3, the same as the successor route.

在路由器E︰

• 路由器E 沒有可行的後繼者可到達目的網路A。
• 路由器E 因此將到達目的網路的路徑標示為活耀的。
• 路由器E 必須重算網路拓撲。
• 路由器E 從表中移除經由路由器D的路徑。
• 路由器E 傳送一個詢問給路由器C，以便請求拓撲資訊。
• 路由器E 已經有一個糾由路由器C的記錄，它的代價是3，與後繼路徑相同。

In Router E: 

• Router C replies with an RD of 3.
• Router E can now set the route by way of Router C as the new successor with an FD of 4 and an RD of 3.
• Router E replaces the Active status of the route to destination Network A with a Passive status. Note that a route will have a Passive status by default as long as hello packets are received. In this example, only Active status routes are flagged.

在路由器E︰

• 路由器C 回答一個3的RD。
• 路由器E 現在能設定經由路由器C的路徑為新的後繼者，其FD是4 和RD 是3。
• 路由器E 將到達目的網路A的路徑狀態，由活躍改為被動。請注意，只要有收到hello封包，一條路徑的預設狀態是被動的。在這個例子裡，只有處於活躍狀態的路徑被標示。

In Router E: 

• Router E sends a reply to Router D to inform it of the Router E topology information.

在路由器E：

• 路由器E 送一個答覆給路由器D，以通知它路由器E的拓撲資訊。

In Router D:

• Router D receives the reply packed from Router E.
• Router D enters this data for the route to destination Network A by way of Router E.
• This route becomes an additional successor route as the cost is the same as routing by way of Router C and the RD is less than the FD cost of 5.

在路由器D：

• 路由器D 收到從路由器E 傳回的答覆。
• 路由器D 輸入這資料，給經由路由器E到達目的網路A 的路徑。
• 這條路徑變成另外一條後繼路徑，其費用與經由路由器C 的路由相同，RD比FD為5的費用還少。

收斂會在所有使用 DUAL 演算法的 EIGRP 路由器之間發生。

 
3.2 EIGRP Configuration 
   
3.2.1 Configuring EIGRP 

Perform the following steps to configure EIGRP for IP:

執行下列步驟為 IP 協定之 EIGRP 組態設定︰

1. Use the following to enable EIGRP and define the autonomous system:

   使用如下內容啟動EIGRP，並且定義自主系統︰

   router(config)#router eigrp autonomous-system-number

   The autonomous system number is used to identify all routers that belong within the internetwork. This value must match all routers within the internetwork.

   自主系統號碼是用來識別屬於網路內的所有路由器，在網路內所有路由器的號碼值必須相匹配。

    

2. Indicate which networks belong to the EIGRP autonomous system on the local router by using the following command:

   在本地路由器上使用下列命令，指明哪個網路屬於該 EIGRP 自主系統︰

   router(config-router)#networknetwork-number

   The network-number is the network number that determines which interfaces of the router are participating in EIGRP and which networks are advertised by the router.

   網段號碼是指定路由器中的哪個界面加入 EIGRP，和哪個網段會被路由器廣播。

   The network command configures only connected networks. For example, network 3.1.0.0, which is on the far left of the main Figure, is not directly connected to Router A. Consequently, that network is not part of the configuration of Router A.

   Network 命令只組態有相連通的網段。例如，在主圖較靠左邊的網段 3.1.0.0，並不與路由器A直接連接。因此該網路並不是路由器A的組態中的一部分。

   　

3. When configuring serial links using EIGRP, it is important to configure the bandwidth setting on the interface. If the bandwidth for these interfaces is not changed, EIGRP assumes the default bandwidth on the link instead of the true bandwidth. If the link is slower, the router may not be able to converge, routing updates might become lost, or suboptimal path selection may result. To set the interface bandwidth, use the following syntax:

   當使用 EIGRP 組態序列線路時，對該介面設定頻寬是很重要的。如果沒改變這些界面的頻寬，EIGRP 會在該線路上給定預設的頻寬，而不是真實的頻寬。如果該線路比較慢，路由器可能不會收斂, 路由可能會失去更新，或者導致選擇次佳的路徑。使用下列命令來設定序列介面的頻寬：

   router(config-if)#bandwidthkbps

   The bandwidth command is only used by the routing process and should be set to match the line speed of the interface.

   bandwidth命令只用於路徑程序，並且應該被設定成與該界面的線路速度相配。

   　

4. Cisco also recommends adding the following command to all EIGRP configurations:

   思科也建議在所有EIGRP 組態中，加入下列命令：

   router(config-router)#eigrp log-neighbor-changes

   This command enables the logging of neighbor adjacency changes to monitor the stability of the routing system and to help detect problems.

   這命令啟動記錄相鄰路由的改變，以監控路由系統的穩定，並且幫助偵測問題。

Lab Exercise: Configuring EIGRP Routing

實驗活動︰組態 EIGRP 路由

This lab is to setup an IP addressing scheme for the network.

這個實驗是設定一個IP 位址系統給 EIGRP 網路。
 
3.2.2 Configuring EIGRP summarization 

EIGRP automatically summarizes routes at the classful boundary. This is the boundary where the network address ends, as defined by class-based addressing. This means that even though RTC is connected only to the subnet 2.1.1.0, it will advertise that it is connected to the entire Class A network, 2.0.0.0. In most cases auto summarization is beneficial because it keeps routing tables as compact as possible.

EIGRP 自動在類別邊界彙整路徑，這個邊界是網段位址結束處，由網段類別定址所定義的。這表示，即使 RTC 只與子網 2.1.1.0 相連結，它會對與整個A級網段 2.0.0.0 有連接者做廣播。 多數情況下，自動彙整是有利的，因為它使路由表儘可能保持簡潔。

However, automatic summarization may not be the preferred option in certain instances. For example, if there are discontiguous subnetworks auto-summarization must be disabled for routing to work properly. To turn off auto-summarization, use the following command:

不過，在某些情況下，自動彙整可能不是想要的選擇。例如，如果有不連續的子網段，就必須停用，以確保正確路由。請使用下列命令來停用自動彙整功能。

router(config-router)#no auto-summary

With EIGRP, a summary address can be manually configured by configuring a prefix network. Manual summary routes are configured on a per-interface basis, so the interface that will propagate the route summary must be selected first. Then the summary address can be defined with the ip summary-address eigrp command:

EIGRP 的位址彙整，可以透過使一個前序網路來手動設定組態。手動彙整路徑是以每一個界面為基礎，所以必須先選擇將傳播彙整路徑的界面。 然後以ip summary-address eigrp命令來定義彙整位址︰

router(config-if)#ip summary-address eigrpautonomous-system-number ip-address mask administrative-distance

EIGRP summary routes have an administrative distance of 5 by default. Optionally, they can be configured for a value between 1 and 255.

EIGRP 彙整路徑，預設的管理距離是5。它們的值可以設定在1 和255之間。

In Figure , RTC can be configured using the commands shown:

在圖 中，RTC 可以使用底下命令來設定：

RTC(config)#router eigrp 2446

RTC(config-router)#no auto-summary

RTC(config-router)#exit

RTC(config)#interface serial 0/0

RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0

Therefore, RTC will add a route to its table as follows:

因此，RTC 會加入如下的路徑到表格中：

D 2.1.0.0/16 is a summary, 00:00:22, Null0

Notice that the summary route is sourced from Null0 and not from an actual interface. This is because this route is used for advertisement purposes and does not represent a path that RTC can take to reach that network. On RTC, this route has an administrative distance of 5.

請注意，彙整路徑是從Null0 開始，而不是從一個實際界面。因為這條路徑是作為廣播之用，而不是描述RTC 所能到達那個網路的一條線路。在RTC，這條路徑的管理距離是5。

RTD is not aware of the summarization but accepts the route. The route is assigned the administrative distance of a normal EIGRP route, which is 90 by default.

RTD 只會接受路徑，而不知道有位址彙整。該路徑會被指定一個正規 EIGRP 路線的管理距離，管理距離的預設值是90。

In the configuration for RTC, auto-summarization is turned off with the no auto-summary command. If auto-summarization was not turned off, RTD would receive two routes, the manual summary address, which is 2.1.0.0 /16, and the automatic, classful summary address, which is 2.0.0.0 /8.

在RTC的組態中，以no auto-summary命令停用了自動彙整功能。 如果自動彙整功能沒被停用，RTD 將收到兩條路線：手動彙整位址2.1.0.0 / 16 以及自動類別彙整位址2.0.0.0 / 8。

In most cases when manually summarizing, the no auto-summary command should be issued.

當手動彙整時，多數情況下no auto-summary 命令應該被發佈。

3.2.3 Verifying basic EIGRP 

show commands can be used to verify EIGRP configurations. Figure lists the key EIGRP show commands and briefly discusses their functions.

使用show命令來驗證 EIGRP 組態。圖 列出重要的 EIGRP show命令，並且簡短討論它們的功能。

The debug feature also provides useful EIGRP monitoring commands.

debug功能也提供很有用的EIGRP 監控命令。

3.2.4 Building neighbor tables 

3.2.5 Discover routes 

3.2.6 Select routes 

3.2.7 Maintaining routing tables  

3.3 Troubleshooting Routing Protocols 
   
3.3.1 Routing protocol troubleshooting process 

3.3.2 Troubleshooting RIP configuration 

3.3.3 Troubleshooting IGRP configuration 

3.3.4 Troubleshooting EIGRP configuration 

3.3.5 Troubleshooting OSPF configuration