Chapter 21 - OSPF Network Types and Neighbors

Flip to see the two configurable settings you can use to influence the DR/BDR election

1. Highest OSPF interface priorty: The highest value wins during an election, with values ranging from 0 to 255 2. Highest OSPF Router ID: If the priority ties, the election chooses the router with the highest OSPF RID.

Flip to see what happens when an engineer misconfigure network type setting of two ospf routers such that one router uses broadcast, and the other uses point-to-point.

1. The two routers become fully adjacent neighbors (that is they reach full state) 2. They exchange their LSDBs. 3. They do not add IP routes to the IP routing table

The command, "show ip ospf neighbor" lists a neighbor but there are some problems that exists that prevents the eventual addition of ospf routes to the routing table. What could cause this ?

1. mismatched MTU 2. mismatched OSPF network type

What the default MTU size on most routers ?

1500 bytes

Which multi-cast address is used when sending out OSPF messsages to the DR or BDR?

224.0.0.6

What does the router do when it hears a Hello message from possible neighbor?

After an OSPF router hears a Hello from a new neighbor, the routing protocol examines the information in the Hello and compares that information with the local router's own settings, (Length of Hello Interval, Area Number, etc...) If the settings match, great. If not, the routers do not become neighbors.

Flip to see how to configure a router's interface priority to make the router the DR.

After configuring the priority, you have to force a free election (by failing the LAN Switch that sits between the routers or etc..). This will cause the routers to start over the process of electing a DR and leading R1's to win and becoming DR

On an ospf enabled interface, how does the router attempt to find neighbors?

By sending OSPF Hellos to the 224.0.0.5 multicast address

How would configure a router interface to use the broadcast network type?

By using the interface subcommand, ip ospf network broadcast

Given that the OSPF router interface is of broadcast network type but it is connected to a subnet with no other routers. What role does it take, DR, BDR or DROther?

DR

What happens to the BDR when the DR fails?

If the DR fails, the BDR becomes the DR, and a new BDR is elected.

What is MTU size?

It is a per-interface setting used by the router for it's 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

What does it mean when two routers are fully adjacent?

It means the two routers are neighbors that has reached the full state. Meaning LSDBs have be exchanged

What type of networks do OSPF point-to-point network type tend to do well in?

It tends to do well for data links that by their nature have just two routers on the link, eg. serial links.

What effects would a Hello or Dead interval mismatch have on a router ?

It would prevent the router from becoming neighbors with any OSPF routers in the subnet. Routers list their Hello and Dead interval settings in their Hello messages and choose to not become neighbors if the values do not match.

Can two routers who are neighbors exchange LSDB if they have different IPv4 MTU settings?

No They will fail to exchange their LSDBs. Eventually, after trying and failing to exchange their LSDBs, the neighbor relationship will also fail.

Flip to see neighbor requirements for OSPF

The column labeled "Required for OSPF" means that the item must be working correctly for the neighbor relationship to work correctly. Note that all the items in this column list a "yes," meaning that all must be correct for the neighbor relationship to work correctly. The last column heading states "Neighbor Missing if Incorrect." For items listing a "yes" in this column, if that item is configured incorrectly, the neighbor will not appear in lists of OSPF neighbors—for instance, with the show ip ospf neighbor command.

An OSPF router attempts to discover other neighbors on the ospf enabled interfaces by sending and listening for OSPF Hello messages. However, once discovered, two routers may not become neighbors. Why is that?

The routers may not have compatible neighbor requirements. They must have compatible values for several settings as listed in the Hellos exchanged between the two routers, to become neighbors

Flip over the see the output of the following command, show ip ospf interface brief

Under the column, Nbrs F/C, '2/3' means that there are two neighbors in the full state off port G0/0, with three total neighbors on that interface . The 'State' column, lists the local router's role on the interface. G0/1 is acting as DR and G0/0 is acting as DROther router.

When a better router enters the subnet, what happens to the DR and BDR?

When a better router enters the subnet, no preemption of the existing DR or BDR occurs. The better router will not become DR until the DR and BDR fails.

Can two routers become neighbors (2 way state) if they have different IPv4 MTU setting on their respective interfaces?

Yes. 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.

What type of network type does OSPF defaults to use on all types of Ethernet interfaces?

broadcast network type

Command used to configure IPv4 MTU setting

ip mtu size interface subcommand

Command used to configure IPv6 MTU setting

ipv6 mtu size interface subcommand

What is a Hello interval/timer?

it is a per-interface timer that tells a router how often to send OSPF Hello messages on an interface

What is a Dead interval/timer?

it is a per-interface timer that tells the router how long to wait without having received a Hello from a neighbor before believing that neighbor has failed. (Defaults to four times the Hello timer.)

What commands would you use to change the RID of a router to 3.3.3.3 and to restart the ospf process so that the OSPF will begin using the new RID?

router-id 3.3.3.3 clear ip ospf process

Which command will you use to check the network type of the router interface g0/0?

show ip ospf interface g0/0

Flip to see the output of the command, "show ip ospf neighbor" and "show ip ospf interface brief" on a point to point network type

show ip ospf neighbor command on R1 lists router R2 (RID 2.2.2.2) with a full state, but with no DR nor BDR designation, instead listing a -. The - acts as a reminder that the link does not use a DR/BDR. show ip ospf interface brief, shows the state (the local router's role) as P2P, which is short for point-to-point, with a counter of 1 for the number of fully adjacent neighbors and total number of neighbors.