Ch10 Open Shortest Path First(OSPF)
===

* Link-state Routing Protocol
* Open Shortest Path First(OSPF)
* Intermediate System to Intermediate System(IS-IS)
* 流程
1. 所有 node 都有每個 node 的鄰居的資訊
2. 計算最短路徑
3. 轉成 routing table
* OSPF
* Defined in RFC 2328 (RFC, Request for comments)
* A link-state routing protocol
* Dijkstra’s (Shortest Path) Algorithm
* An Interior Gateway Protocol
* Fast convergence and excellent scalability
* Every router computes its own routing table
* Commonly used in large enterprise networks
* Link-state Operation
1. 找相鄰鄰居
2. Exchange Link-State Advertisements (LSAs)
3. Build the Topology Table
* Topology table 包含所有 routing table
4. 執行 SPF (Shortest Path First) Algorithm
5. 修改 routing table
* 有 OSPF 的資訊
* OSPF 封包格式
* 要建立 OSPF 的廣播封包
* Data Link Ethernet Frame Header
* Destination multicast MAC address: 規定的
1. 01:00:5E:00:00:05
2. 01:00:5E:00:00:06
* IPv4 Packet Header
* destination address: 規定的
1. 224.0.0.5
2. 224.0.0.6
* protocol field: 89 for OSPF
* OSPF Packet Header
* OSPF packet type
* router ID
* area ID
* OSPF Packet Type Specific Data
* The content differs depending on the packet type
* OSPF 使用 link-state packet (LSP)
* Hello packet
* 跟鄰居保持關係
* Database Descriptor (DBD) packet
* 有 LSDB(Link-State Database) 簡略資訊
* 確認資料庫同步
* LSR(Link-State Request) packet
* 詢問更詳細的 DBD
* LSU(Link-State Update) packet
* 回覆 LSR
* LSAck(Link-State Acknowledgment) packet
* 確認接收 LSU
* OSPF operation state
* Establish Neighbor Adjacencies
1. down state
* 要開始看看有沒有其他 OSPF 的 router
* 廣播 Hello packet
* 交換 router ID
2. init state
* 接收到 Hello packet 後，如果那個 router ID 不在表裡，就跟它交朋友
3. Two-Way state
* Synchronize OSPF Database
1. ExStart state
* 要交換 database，所以在這裡決定誰先送 DBD packet
* router ID 較高的先送
2. Exchange state
* 開始換 DBD
* Link type
* address of advertising router
* sequence number
3. Loading state
* 更新需要更新的 entry
* Full state
* 同步狀態
* OSPF Designated Router
* Multiaccess network 的狀況
* 剛開始交朋友的時候，封包會造成 flooding
* 更新 database 的時候，也會 flooding
* 解決方案: DR(Designated Router)
* 選舉出 DR 來當收集、發散點
* BDR(Backup Designated Router) 要備份以免 DR 出狀況
* 其他人是 DROTHER