Skip to main content

OSPF Graceful Shutdown - Deep Dive

OSPF Graceful Shutdown is a feature that allows a router to gracefully withdraw from OSPF routing without causing disruptions or routing instability in the network. When an OSPF graceful shutdown is triggered, the router informs its OSPF neighbors that it is no longer participating in OSPF. This process involves the router setting its OSPF links to a state that indicates they are down and withdrawing its routes, but without causing network flapping or re-convergence issues.

Key Points:

  1. Withdrawal of Routes: The router gracefully withdraws its OSPF routes from the routing table and stops sending updates to OSPF neighbors.
  2. Minimal Disruption: OSPF gracefully informs neighbors of the change, preventing sudden route drops or instability.
  3. Network Stability: Helps maintain stability during maintenance or shutdown, avoiding the need for a full re-convergence.
  4. Manual or Automatic: Can be triggered manually for planned maintenance or implemented automatically in certain cases.

Configuration Example:

router ospf 1
 graceful-shutdown

When a router performs an OSPF Graceful Shutdown, it informs its OSPF neighbors through specific actions designed to gracefully withdraw from the OSPF network. Here's how the process works:

1. LSA Update Mechanism:

  • The router sets the link-state advertisements (LSAs) for its connected networks with the maximum age (3600 seconds), marking the LSAs as expired. This action signals to neighboring routers that the routes advertised by the router should be removed from their link-state databases (LSDB).
  • Specifically, Type 1 LSAs (Router LSAs) are withdrawn, effectively making the router "invisible" to the OSPF network.

2. OSPF Neighbor Communication:

  • The router continues to send Hello packets to its OSPF neighbors for a brief period during the shutdown process to indicate that it is still alive but is gracefully withdrawing from the network.
  • Once neighbors receive the expired LSAs, they acknowledge the router’s graceful exit and re-calculate the OSPF topology to exclude the gracefully shutting down router.

3. Immediate Route Withdrawal:

  • The router sets its OSPF interfaces to an OSPF state of "Down", indicating that they are no longer available for routing OSPF traffic.
  • Neighboring routers stop forwarding traffic through the shutting-down router without causing network instability or unnecessary reconvergence.

4. Minimal Disruption:

  • The shutdown process is smooth, avoiding OSPF flapping or unnecessary route recomputation. Traffic is re-routed seamlessly via alternate paths available in the network.

By following this process, the router ensures that it leaves the network gracefully, causing minimal disruption to the overall OSPF topology.

Labels:

Comments

Post a Comment

Popular posts from this blog

How to import Putty Saved Connections to mRemoteNG

Just started using mRemoteNG and its being very cool to connect to different remote connection with different protocols e.g Window Remote Desktop, VNC to Linux, SSH, HTTP connection etc. from a single application. As new user I configured some remote desktop connection which was quite easy to figure out. But when I wanted to add SSH connections, it came in my mind to import all of the saved connections in the putty. But I couldn't figure it out how can it be done, though it was quite easy and here are the steps. Open your mRemoteNG Create a folder if you want segregation of multiple networks Create a new connection Enter the IP address of remote server under connection in Config pane Under the config pane, select protocol " SSH version 2 ".  Once you select protocol to SSH version 2 you are given option to import putty sessions, as shown in the snap below. In the above snap, I have imported CSR-AWS session from my saved sessions in Putty.
1 comment
Read more

BGP Local Preference Controlling Outbound Traffic in BGP

In BGP, Local Preference is used to control the outbound traffic path. It helps you decide which egress point (exit point) should be used when you have multiple connections to external networks, such as ISPs. Local Preference is an attribute that is local to your AS and is shared with all iBGP peers but not with eBGP neighbors. Higher Local Preference = More preferred outbound path. Example Scenario : You have two external links: ISP1 (via CE1) and ISP2 (via CE2). You want traffic to prefer ISP1 for all outbound traffic. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP2): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Higher Local Preference) : Create a route map to set the local preference to 200 for routes learned from CE1: route-map SET_LOCAL_PREF permit 10 set local-preference 200 In the BGP configuration for CE1, apply this route map to the neighbor: router bgp 65001 ne...
Post a Comment
Read more

BGP MED: Managing Inbound Traffic with Multi-Exit Discriminator

The Multi-Exit Discriminator (MED) is used in BGP to control inbound traffic into your AS. It tells a neighboring AS which entry point into your network it should prefer when there are multiple links between your AS and the neighboring AS. The lower the MED value , the more preferred the path. MED is only honored between the same neighboring AS . Example Scenario : You are connected to ISP1 via two routers, CE1 and CE2 , and want to control which router ISP1 uses to send traffic into your AS. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP1): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Lower MED, More Preferred) : Create a route map to set the MED to 50 for CE1: route-map SET_MED permit 10 set metric 50 Apply this route map to the neighbor in the BGP configuration for CE1: router bgp 65001 neighbor 10.0.1.1 remote-as 65000 neighbor 10.0.1.1 route-map SET_MED out Configuratio...
Post a Comment
Read more