OSPF . W.lilakiatsakun. Introduction. OSPF OSPF was designed by the IETF (Internet Engineering Task Force) OSPF Working Group, which still exists today. The development of OSPF began in 1987 and there are two current versions in use: OSPFv2: OSPF for IPv4 networks (RFC 1247 and RFC 2328)
Two routers may not form an OSPF adjacency if:
- The subnet masks do not match, causing the routers to be on separate networks.
- OSPF Hello or Dead Timers do not match.
- OSPF Network Types do not match.
- There is a missing or incorrect OSPF network command.
The show ip protocols command is a quick way to verify vital OSPF configuration
information, including the OSPF process ID, the router ID, networks the router
is advertising, the neighbors the router is receiving updates from, and the default
administrative distance, which is 110 for OSPF.
The show ip ospf command can also be used to examine the OSPF process ID and
router ID. This command displays the OSPF area information as well as the last time
the SPF algorithm was calculated. It is shown in the figure that OSPF is a very stable
routing protocol. The only OSPF-related event that R1 has participated in during the
past 11 and a half hours is to send small Hello packets to its neighbors.
The quickest way to verify Hello and Dead intervals is to use the show ip ospf interface command. These intervals are included in the OSPF Hello packets sent between neighbors.
OSPF may have different Hello and Dead intervals on various interfaces, but for OSPF
routers to become neighbors, their OSPF Hello and Dead intervals must be identical.
Each router includes the loopback interface as a directly connected network but
loopback interfaces are not advertised in OSPF.
Unlike RIPv2 and EIGRP, OSPF does not automatically summarize at major network
boundaries. OSPF is inherently classless.
The reference bandwidth defaults to 10 to the 8th power, 100,000,000 bps or 100 Mbps.
This results in interfaces with a bandwidth of 100 Mbps and higher having the same OSPF
cost of 1.
The reference bandwidth can be modified to accommodate networks with links
faster than 100,000,000 bps (100 Mbps) using the OSPF command auto-cost reference-
bandwidth. When this command is necessary, it is recommended that it is used on all
routers so the OSPF routing metric remains consistent.
OSPF Accumulates Costs
The routing table on R1 shows a cost of 65 to reach the 10.10.10.0/24 network on R2.
Because 10.10.10.0/24 is attached to a FastEthernet interface, R2 assigns the value 1 as
the cost for 10.10.10.0/24. R1 then adds the additional cost value of 64 to send data
across the default T1 link between R1 and R2.
In the figure, R1 sends LSAs to the DR.
The BDR listens as well
The DR is responsible for forwarding the LSAs from R1 to all other routers.
The DR uses the multicast address 220.127.116.11 (AllSPFRouters - All OSPF routers).
The end result is that there is only one router doing all of the flooding of all LSAs in
the multiaccess network.
The following criteria are applied:
1. DR: Router with the highest OSPF interface priority.
2. BDR: Router with the second highest OSPF interface priority.
3. If OSPF interface priorities are equal, the highest router ID is used to break the tie.
If a new router enters the network after the DR and BDR have been elected, it will not
become the DR or the BDR even if it has a higher OSPF interface priority or router ID
than the current DR or BDR. The current DR and BDR must both fail before the new router
can be elected DR or BDR.
A previous DR does not regain DR status if it returns to the network.
In the figure, RouterC has finished a reboot and becomes a DROther even though
its router ID, 192.168.31.33, is higher than the current DR and BDR.
If the BDR fails, an election is held among the DRothers to see which router will be
the new BDR. In the figure, the BDR router fails. An election is held between RouterC
and RouterD. RouterD wins the election with the higher router ID.
When RouterB fails. Because RouterD is the current BDR, it is promoted to DR.
RouterC becomes the BDR.
Modify Priority of Router ‘s interface
After doing a shutdown and a no shutdown on the FastEthernet 0/0 interfaces of all
three routers, we see the result of the change of OSPF interface priorities.
The show ip ospf neighbor command on RouterC now shows that RouterA
(Router ID 192.168.31.11) is the DR with the highest OSPF interface priority of 200
and that Router B (Router ID 192.168.31.22) is still the BDR with the next highest
OSPF interface priority of 100. Notice from RouterA's output of show ip ospf neighbor
that it does not show a DR, because RouterA is the actual DR on this network.
Let's return to the earlier topology, which now includes a new link to ISP.
The router connected to the Internet is used to propagate a default route to
other routers in the OSPF routing domain.
This router is sometimes called the edge, entrance or gateway router.
E2 denotes that this route is an OSPF External Type 2 route.