Use the ip ospf priority [1 - 255] command to set the priority to a value between 1 and 255. Use the ip ospf priority 0 command to configure the router to never function as the designated router or backup designated router. Use the no ip ospf priority command to set the priority at the default value of 1.
The default values are 10 seconds for the hello time, and 40 seconds for the dead time. The usual rule of thumb with OSPF is to keep the dead time value four times the hello interval. However, this is not a strict rule.
When a router has no UPDATE messages to send, it periodically sends KEEPALIVE messages, which, well, keep the BGP session alive. The default value for the hold time suggested in the BGP specification (RFC 4271) is 90 seconds, and keepalives should be sent at intervals of one third the hold time (30 seconds).
Usage Guidelines. Use the ip ospf priority command to set the router priority, which determines the designated router for this network. When two routers are attached to a network, both attempt to become the designated router. If there is a tie, the router with the higher router ID takes precedence.
These hello packets are exchanged between directly-connected neighbors to learn more about each other. You can use the show ip ospf neighbor command to observe the neighbor data structure. This command displays OSPF-related neighbor information.
OSPF hello packets are packets that an OSPF process sends to its OSPF neighbors to maintain connectivity with those neighbors. The hello packets are sent at a configurable interval (in seconds). The defaults are 10 seconds for an Ethernet link and 30 seconds for a non broadcast link.
Such a configuration reinforces the point that EIGRP neighbors do not require matching timers (although it's a best practice to have matching timers).
Understanding RIP TimersRIP uses several timers to regulate its operation. The update interval is the interval at which routes that are learned by RIP are advertised to neighbors. This timer controls the interval between routing updates.
To configure the Enhanced Interior Gateway Routing Protocol (EIGRP) hello interval for an interface, use the ip hello-interval eigrp command. To restore the default, use the no form of this command.
Those states are defined in the OSPF RFC 2328. , section 10.1. The states are Down, Attempt, Init, 2-Way, Exstart, Exchange, Loading, and Full.
hello-interval—Specifies the length of time, in seconds, before the routing device sends a hello packet out of an interface. By default, the routing device sends hello packets every 10 seconds. This is an interval during which the routing device receives no hello packets from the neighbor.
There are 5 OSPF network types:
- Non-Broadcast.
- Broadcast.
- Point-to-Multipoint.
- Point-to-Multipoint Non-Broadcast.
- Point-to-Point.
Which statement is correct about multiarea OSPF? Arranging routers into areas partitions a large autonomous system in order to lighten the load on routers. A network administrator configures a loopback interface as the OSPF router ID with the IP address of 192.168. 1.1/30.
The next big reason as to why you may want to manually change the timers on an EIGRP routed network is to increase network outage detection and re-convergence time. This command is executed in interface configuration mode to manually configure an EIGRP hello interval on a per-interface per autonomous system basis.
The following conditions must be met for two routers to become neighbors:
- They must have the same IP network/subnet.
- The Hello and Dead Interval timers must be identical.
- Router interfaces connecting two routers must have the same Area ID.
- Type of area must be identical (normal or stub area)
The show ip ospf neighbor command can be used to find information about any OSPF neighborships, including the interface, the state, the neighbor's address, and the neighbor's router ID. To select a router ID for OSPF, a router goes through a process. When a router ID has been found, the process stops.
Verify that these HELLO parameters match on the neighboring interfaces:
- OSPF area number - Enter the show ip ospf interface interface-name command in order to check.
- OSPF area type, such as stub or NSSA - Enter the show ip ospf command in order to check.
To clear redistribution based on the
OSPF routing
process ID, use the
clear ip ospf command in privileged EXEC mode. Syntax Description pid (Optional)
Process ID.
Clear ip ospf.
| process | Reset OSPF process. |
|---|
| counters | OSPF counters. |
| neighbor | (Optional) Neighbor statistics per interface. |
| intf | (Optional) Neighbor interface. |
Make sure that the interfaces at both ends are configured to support OSPF. There could be a mismatch in the OSPF Network Type. Verify that Hello and Dead timers match on each end of the link. Neighboring interfaces must be in the same OSPF Area.
A number of different Cisco IOS OSPF configuration commands are available, but at its most basic, OSPF configuration uses three commands: the router ospf, network, and router-id commands. The router ospf process-id command is the beginning of any Cisco IOS OSPF configuration.
Use the show ospf routes command to display the OSPF private routing table (which contains only routes that are calculated by OSPF). If something is wrong with a route in the routing information base (RIB), then you should check the OSPF copy of the route to determine if it matches the RIB contents.
Two rules are used to elect a DR and BDR:
- router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
- if there is a tie, a router with the highest router ID wins the election. The router with the second highest OSPF priority or router ID will become a BDR.
To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:
- Use the command router ospf process ID to start OSPF.
- Use the network command to enable the interfaces.
- Identify area assignments.
- (Optional) Assign the router ID.
The cost of a link in OSPF can be verified using the “show ip ospf interface <interface name> <interface ID>” command.
The backbone area (Area 0) is the core of an OSPF network. All other areas are connected to it and all traffic between areas must traverse it. All routing between areas is distributed through the backbone area.
The common myth is both BGP peer mush have matching hold time and keep alive timer for BGP neighbor relationship. It is not true! Another myth is BGP peer sends keep alive timer in open message, it is not true either!
SPF hold time is also used to delay consecutive SPF calculations (give the router some breathing space). The default for his value is 10 seconds. As a result, the min time for the routes to converge in case of failure is always going to be more than 5 secs unless the SPF timers are tuned using OSPF throttle timers.
The OSPF (Open Shortest Path First) protocol is one of a family of IP Routing protocols, and is an Interior Gateway Protocol (IGP) for the Internet, used to distribute IP routing information throughout a single Autonomous System (AS) in an IP network.
We can see the configured Hello and Hold timer values for a router interface by issuing the show ip eigrp interfaces detail interface_id command, as shown in the example below. You can see from the output that interface Gig 0/1 on router R2 has a Hello timer of 5 seconds and a Hold timer of 15 seconds.
What does the SPF algorithm consider to be the best path to a network? The path that includes the fastest cumulative bandwidth links. An EIGRP router loses the route to a network.
OSPF Database Description packets
Explanation: OSPF supports the concept of areas to prevent larger routing tables, excessive SPF calculations, and large LSDBs. This allows OSPF to scale in a hierarchical fashion with all areas that connect to a backbone area.
