Unit 1: Network Infrastructure
This will allow you to demonstrate your networking skills, knowledge, and abilities, with a focus on enterprise-level switching, routing, and multicast components that support cross-platform (inter)operability and integration with the most recent software-defined technologies.

OSPF Graceful Shutdown

Routing and switching devices are often maintained in a business network. On the other hand, you might want router maintenance that doesn’t interfere with users or applications on the network. For instance, IP packets (carrying voice, data, etc.) on particular segments of the network may momentarily get missed at specific nodes of the OSPF routing domain if an OSPF-enabled router is restarted or replaced with a new one. Introducing the OSPF graceful shutdown functionality, a game-changer for smoothly removing routers from networks and accelerating convergence times.

To understand this concept, we will use the following network architecture.

What does OSPF Graceful Shutdown mean?

One way to shut down an OSPF process or an OSPF-enabled device without affecting network traffic or route convergence too much is to use OSPF graceful shutdown. On top of that, traffic will not go both ways on the link that each gracefully shut down interface is linked to.

In the next few cases, we’ll show what happens to OSPF network convergence when we use the shutdown command to shut down a link instead of starting a graceful shutdown.

It is important to take note that router R4 is the next hop for R1 to reach subnet 10.10.34.0/24. This is something that can be seen in the output of the next show ip route ospf command.

R1# show ip route ospf

omitted output

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
O 10.10.23.0/24 [110/20] via 10.10.12.2, 00:00:10, Ethernet1/2
O 10.10.34.0/24 [110/11] via 10.10.14.4, 00:01:06, FastEthernet0/0

The following example shows how to disable the F0/0 interface on router R4 after instructing router R1 to send 1000 ICMP echo queries to IP address 10.10.34.3. By using the shutdown command to disable R4’s F0/0 interface, you may observe the impact on ICMP traffic between R1 and R3.

R1# ping 10.10.34.3 repeat 1000
Type escape sequence to abort.
Sending 100000, 100-byte ICMP Echos to 10.10.34.3, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!...!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

omitted output

Due to the fact that OSPF required a few seconds to locate a new path to IP address 10.10.34.3, ICMP traffic was dropped for a brief period of time, as demonstrated in the preceding example.

Following the configuration of router R1 to send 100 ICMP echo requests to IP address 10.10.34.3, which is shown below, we proceed to router R4 in order to conduct a graceful shutdown on the F0/0 interface. At this point, we re-enable the F0/0 interface of router R4.

R1# ping 10.10.34.3 repeat 100
Type escape sequence to abort.
Sending 1000, 100-byte ICMP Echos to 10.10.34.3, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Success rate is 100 percent (100/100), round-trip min/avg/max = 20/32/56 ms

As this example shows, ICMP traffic was unaffected even if the OSPF adjacency between R1 and R4 was dropped.

R4#
*Oct 15 16:41:50.859: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

What is the operation of the OSPF Graceful Shutdown?

Once you have enabled the OSPF graceful shutdown possibility on a certain interface, the following will occur:

  • In the hello packets that are transmitted to OSPF neighbors that are near to one another, the DR, BDR, and neighbor fields are all set to 0.0.0.0.

OSPF routers that are found to be adjacent over the link are asked to leave the current state and return to the INIT state.

LSAs that rely on interfaces are flooded with an age of 3600 seconds.

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