19 terms

cisco routing Protocol OSPF

If the router RT1 returns the given output and has not had its router ID set manually, what value will OSPF use as its router ID
Since the router is configured with Loopback interfaces
the router uses highest IP address on one of its loopback interfaces
Ea. router in the OSPF network needs a unique ID. The ID is used to provide a unique identity to the OSPF router.
This is included in ay OSPF messages the router generates to announce itself to other OSPF routers in the area (type 1 LSA). The ID must be unique.
The router ID is chosen according to one of the two criteria
1. The highest IP address on its loopback interfaces (Loopback interface is a logical interface on a router) is chosen as router ID
2. If no loopback interface is configured than the highest IP address on one of its active physical interfaces is chosen as Router ID.
3. If there is no active interface, the OSPF process will not start and therefore you will not have any OSPF routes in your routing table.
It is highly recommended that you use a loopback interface because
it is always up and thus the router can obtain a router ID
If the interface associated with this IP address is ever brought down, or if the address is removed
the OSPF process must recalculate a new router ID and resend all its routing information out its interfaces.
Virtual interfaces (made up)
are loopback
*What is the default administrative distance of the OSPF routing protocol? (trustworthiness)
110 OSPF
What does 128 refer to in the output "0 [110/128] via, 00:35:36, Serial 0"?
OSPF cost
Letter O at the beginning of a route entry shows that this route was learned through OSPF.
OSPF uses COST as the metric.
(cost relates to bandwidth)
OSPF has a default Administrative Distance of
On point-to-point networks, OSPF hello packets are addressed to which address
OSPF Hello packets use the multicast address to detect and keep track of neighbors
All OSPF routers should be able to transmit and listen to this address.
Which parameter or parameters are used to calculate OSPF cost in Cisco routers
by default the cost of an interface is calculated based on the bandwidth, you can force the cost of an interface with the ip OSPF cost value interface subconfiguration mode command.
OSPF uses Cost as the metric
OSPF Cost is inversely proportional to the bandwidth of an interface
Inversely=Raise speed (faster speed link)=Low cost
OSPF Cost =
10000 0000/bandwidth in bps
What is the default maximum number of equal cost paths that cam be placed into the routing table of a Cisco OSPF router
4 - by default
by default, Open Shortest Path First (OSPF) installs up to four equal cost paths to a destination, in the routing table.
However, , OSPF can include up to 16 equal cost routes in the routing table and perform load balancing amongst them. In order to configure this feature, you need to use the OSPF subcommand maximum paths, ie maximum path 16
Which command is used to display (show) the collection of OSPF link states?
show ip OSPF database
we use the show ip ospf database command with no arguments to display all link state advertisements.