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R1到达23网段的cost是如何计算的?
A的cost+C的cost
接口A的cost如何算的?
Ip ospf cost n
R1(config)#router ospf 1
R1(config-router)#auto-cost reference-bandwidth1000
两台运行ospf的路由器通过点对点链路连接,达到FULL状态后的两个小时之内,在两台路由器之间的链路上,一定会相互交换哪些报文?
简述OSPF建立邻居关系的过程。
1)down,死亡时间内没有收到hello包
2)attempt,NBMA网络特有,
3)Init,收到邻居的hello报文,该hello报文中不包含自己。
4)two-way,在收到的邻居的hello报文中,看到了自己。或者直接收到了邻居的DD报文。如果是多路访问网络,two-way中要选举DR、BDR
5)exstart,选举主从,协商DD的序列号
6)exchang,发送DD报文,(其实也开始发送LSR报文了)
7)loading,Link State Request list不为空,继续发送LSR报文
8)full,Link State Request list为空。
ospf使用那几种类型的数据包,各起什么作用?TTL分别为多少?
HELLO包:形成邻居关系,并保活。
数据库描述报文DD;描述自身的数据库条目。(只包含LSA头部)
LSA请求报文:用于向邻居请求自己缺省的LSA条目(只包含LSA头部,缺省的条目通过比较DD报文获得。)
LSA更新报文:对请求报文的应答,包含了完整的LSA条目。同时还具有确认作用。
LSA确认报文:显示的确认自己收到了LSA更新报文。
ospf所有报文TTL均为1
选举DR/BDR,master/slave分别用什么报文,在什么状态下进行?
Hello报文,two-way状态
DD报文,exstart状态
报文Link State Request List是怎么产生的以及消失的?如何查看?
自己的LSA与DD的LSA头部比对,不存在的则产生
LSR发出去之后,收到了对应的LSA则删除
show ip ospf request-list
Link State Retransmissionlist是如何产生以及消失的?如何查看?如果重传是组播吗?
每发出一个LSA则建立一个对应的重传列表
收到确认则删除对应的重传列表
show ip ospf retransmission-list
如果重传必为单播
explicit acknowledgement : Link State Acknowledgment packet
implicit acknowledgement : Update packet
OSPF通过什么方法来确认DD数据包?
slave 确认 master : 回复一个相同序列号的的DD
master确认slave :回复一个序列号+1的DD
OSPF中hello报文的作用?
It is the means by which neighbors are discovered.
Hello packets act as keepalives between neighbors.
It ensures bidirectional communication between neighbors.
It advertises several parameters on which two routers must agreebefore they can become neighbors.
It elects Designated Routers (DRs) and Backup Designated Routers(BDRs) on Broadcast and Nonbroadcast Multiaccess (NBMA)networks.
Hello报文包含哪些信息?那些信息将影响建立邻居关系?
Router ID of the originating router.(必须不匹配)
Area ID of the originating router interface.(必须匹配)
Address mask of the originating interface.(必须匹配)
Authentication type and authentication information for theoriginating interface.(必须匹配)
HelloInterval of the originating interface.(必须匹配)
RouterDeadInterval of the originating interface.(必须匹配)
Router Priority.
DR and BDR.
Five flag bits signifying optionalcapabilities.(必须匹配)
Active neighbor:Router IDs of the originating router'sneighbors. This list contains only routers from which Hellos wereheard on the originating interface within the lastRouterDeadInterval 如果邻居列表为空,则无此字段
区域类型:不会作为成为邻居关系的条件来比较,区域类型不同的时候,邻居关系可full,路由学不到。
简述OSPF的 processID。
This Cisco-specific feature is not part of the open standard.Cisco routers are capable of running multiple OSPF processes anduse the Process ID to distinguish them. The Process ID has nosignificance outside the router on which it is configured
cisco定义,非标准。只具有本地意义。
OSPF有那几种方法来确保可靠的洪泛。 简述一下age。
序列号,和校验和,以及老化时间。还有确认机制
路由器产生一个lsa
When an LSA reaches MaxAge, theLSA is reflooded and then flushed from the database. When a routerneeds to flush an LSA from all databases, it prematurely sets theage to MaxAge and refloods it. Only the router that originated theLSA can prematurely age it.
OSPF如何消灭一条LSA?
源产路由器将LSA的age置为最大,然后泛洪
一条LSA最大存活时间(MaxAge)是多少?如何保证其不过期?
The LSAs are aged as they reside in the link-state database. Ifthey reach MaxAge (1 hour), they are flushed from the OSPF domain.The implication here is that there must be a mechanism forpreventing legitimate LSAs from reaching MaxAge and being flushed.This mechanism is the link-state refresh. Every 30minutes, known as the LSRefreshTime, the routerthat originated the LSA floods a new copy of the LSA with anincremented sequence number and an age of zero. Upon receipt, theother OSPF routers replace the old copy of the LSA and begin agingthe new copy.(摘自 卷一)
虚链路有什么作用。那二种情况下不能配置成虚链路?
修补不连续的区域,
在末梢区域(STUB,完全STUB,NSSA,完全NSSA)中不能配置虚链路。
在不同的AREA间不能配置虚链路。并且必须是ABR间配置。
area 1(穿越的区域) virtual-link12.1.1.2
ospf是如何防环的?
1区域内采用SPF算法,算法自身保证无环路。
2、区域间只能和AREA0连接,保证区域间无环路。
3、OSPF无法防止AS间的环路,因为丢失了源信息。
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