CCNA 200-301 Volume I Chapter 21: OSPF Network Types and Neighbors

An engineer migrates from a more traditional OSPFv2 configuration that uses network commands in OPSF configuration mode to instead use OSPFv2 interface configuration. Which of the following commands configures the area number assigned to an interface in this new configuration? O A - The area command in interface configuration mode O B - The ip ospf command int interface configuration mode O C - The network command in interface configuration mode O D - The router ospf in interface configuration mode

B - The ip ospf command int interface configuration mode

What OSPF Network type and Point-to-point type have in common?

Both try to dynamically discover a neighbor.

What's not supported in data-link protocols like HDLC and PPP?

Data-link broadcasts

What happens to the OSPF process when there's a MTU mismatch?

In an odd twist, two OSPFv2 routers can actually become OSPF neighbors, be listed in the output of the show ip ospf neighbor command, and reach 2-way state, even if they happen to use different IPv4 MTU settings on their interfaces. However, they fail to exchange their LSDBs. Eventually, after trying and failing to exchange their LSDBs, the neighbor relationship also fails. So also keep a watch for MTU mismatches, although they may be unusual and obscure, by looking at the running-config and by using the show interfaces command (which lists the IP MTU).

What happens to the OSPF process when there's a Network Type mismatch?

Interestingly, if you misconfigure network type settings such that one router uses broadcast, and the other uses point-to-point, the following occurs: O The two routers become fully adjacent neighbors (that is, they reach a full state). O They exchange their LSDBs. O They do not add IP routes to the IP routing table. The reason for not adding the routes has to do with the details of LSAs and how the use of a DR (or not) changes those LSAs. Basically, the two routers expect different details in the LSAs, and the SPF algorithm notices those differences and cannot trust the LSAs because of those differences.

What happens when there's a mismatch in the MTU setting when two routers are configured to run OSPF?

It prevents the eventual addition of OSPF routes to the routing table.

What's the definition of E-Line?

It's a point-to-point Ethernet WAN service also called an Ethernet Private Wire Service

What's MTU?

MTU (Maximum Transmission Unity) defines a per-interface setting used by the router for its Layer 3 forwarding logic, defining the largest network layer packet that the router will forward out each interface. For instance, the IPv4 MTU size of an interface defines the maximum size IPv4 packet that the router can forward out an interface. Routers often use a default MTU size of 1500 bytes, with the ability to set the value as well.

What are the OSPF network types?

O Broadcast O Point-to-point

How to verify the OSPF Neighbor Requirements?

O Hello and dead timer - sh ip ospf int O They must be in the same area - sh ip ospf interface brief O RIDs must be unique - sh ip ospf O They must pass any neighbor authentication - sh ip ospf interface O OSPF process must not be shut down - sh ip ospf, sh ip ospf interface

What are the neighbor requirements for an OSPF neighbor relationship to be formed?

O Interfaces must be in an up/up state O Access control lists (ACL) must not filter routing protocol messages O Interfaces must be in the same subnet O They must pass routing protocol neighbor authentication (if configured) O Hello and hold/dead timers must match O Router IDs (RID) must be unique O They must be in the same area O OSPF process must not be shut down O Neighboring interfaces must use same MTU setting O Neighboring interfaces must use same OSPF network type

Which commands are used to verify if an OSPF process is running?

O sh ip OSPF interface brief (under State = DOWN) O sh ip OSPF (it will say: "Routing Process is shutdown" O sh ip ospf neighbor (no output) O sh ip ospf database (no output) O

Which command will list the OSPF neighbors off interface serial 0/0

O sh ip OSPF neighbor serial 0/0 O sh ip ospf neighbor

Which command will you use to verify the network type of OSPF?

O sh ip ospf int g0/0 O sh ip ospf int

Which commands will show a more detailed output of the OSPF neighbors?

O sh ip ospf neighbor O sh ip ospf neighbor detail O sh ip ospf int O sh ip ospf int brie test in real gear

What's the advantage of shutting down an OSPF process?

Shutting down the OSPF routing protocol process gives the network engineer a way to stop using the routing protocol on that router without having to remove all the configuration. Once shut down, the show ip ospf interface [brief] command should still list some output, as will the show ip ospf command, but the rest of the commands will list nothing.

What happens when a routing protocol is shutdown?

The IOS will: O Brings down all neighbor relationships and clears the OSPF neighbor table O Clears the LSDB O Clears the IP routing table of any OSPF-learned routes O Retains all OSPF configuration O Still lists all OSPF-enabled interfaces in the OSPF interface list (show ip ospf interface) but in a DOWN state.

What's the main difference in between an OSPF Network type and an OSPF Point-to-point type?

The use of a DR (Designated Router) and a BDR (Backup Designated Router)

Why do Engineers prefer to use an OSPF point-to-point network type between two devices instead of the broadcast type?

While the Ethernet data-link protocol supports broadcast frames, only two devices can exist on the link, and there is no advantage to using a DR/BDR.

How to configure the MTU size?

conf t int g0/0 ip mtu 1500 (or ipv6 mtu 1500)

How to manually configure the OSPF network type to Broadcast?

conf t int g0/0 ip ospf network broadcast

How to manually configure the OSPF priority of a router?

conf t int g0/0 ip ospf priority "x" (the higher the better)

How to configure the OSPF Network type to point-to-point?

conf t int g0/0/0 ip ospf network point-to-point

How to shutdown an OSPF process?

conf t router ospf 1 shutdown

How to change the RID of a router?

conf t router ospf 1 (at least one interface needs to be up or you'll get an error) router-id 1.1.1.1 clear ip ospf process (router won't use the new ID if you don't reset the process) yes

Which command will show a more detailed OSPF database?

sh ip ospf database router

Which commands will show the cost of all the interfaces of a router running ospf?

sh ip ospf int brief sh ip ospf

Which command will verify the Hello/Dead interval times in a router?

sh ip ospf int g0/0

How to verify the type of OSPF Network?

sh ip ospf int g0/0 sh ip ospf int brief (under State you will see P2P and under Nbrs you'll see 1/1) sh ip ospf neighbor