Skip to main content

Some Hot Tips on OSPF Filtering and OSPF Area Types

Here is the list of some of the tricky concepts of Types of OSPF areas and OSPF Filtering;

OSPF routers do not advertise routes, instead they advertise LSAs. Any
filtering applied to OSPF messages would need to filter the transmission
of LSAs. However, inside one area, all routers must know all LSAs, or
the whole SPF concept fails, and routing loops could occur. As a result,
OSPF cannot and does not allow the filtering of LSAs inside and area,
specifically the type-1 and type-2 LSAs that describe the intra-area
topology.

OSPF is a link state protocol that populates the Link State Database, LSD, to give routers the same area and identical perspective of the OSPF routing domain that perspective is tempered by the type of area the routers are in.

An ABR can also be an ASBR.

When an external Route is defined as an E1, ABRs generate a type 4 (ASBR Summary) LSA into non-backbone, non-stub areas.

The type 4 LSA reflects the cost from that area’s ABR to the ASBR (itself) that redistributed that route into the OSPF domain. This cost is added to the area router’s cost to reach its ABR plus the metric of the external LSA.

Inter-area Routes
Inter-area routes are those generating in another area within the same Routing domain as the local router with the exception of the default route generated by the ABR into stub and totally stubby areas. This default route is not propagated outside of a stub or totally stubby area.

The flooding of LSAs within an area can be prevented with the “ip ospf database-filter all out” commnd which can be applied to an interface.

Several methods filter routes on the local router, whether the router is in the same or a different area than the originater of the routes. Most filtering does not remove the networks from the Link State Database, LSD. the Routes are removed from the routing table, which prevents the local router from using them to forward traffic.

Within the NSSA, when the type 7 LSA reaches the ABR, the LSA is changed to a type-5 and propagated into the backbone. The route now appears as an ordinary external route the routers in non-stub areas outside of the NSSA.

A default route is not automatically generated into an NSSA. A special statement on the NSSA ASBR “Area X default-information originates” will advertise a default route into the NSSA with a type-7 LSA. This default route is propagated into non-stub area in the rest of the OSPF routing domain, by the same rules that apply to any other external routes.

Stub Areas
Native Inter-area and Intra-area route are advertised into the area, but not external routes. In place of external routes the ABR automatically advertise a default route into the stub area as an inter-area route (IA).

Some filtering methods do not remove routes from LSD of area Routers. Routes are only removed from the routing table of the local router. Other routers in the same area that do not have filters applied will continue to advertise the routes. A possible result is a BLACK HOLE in the routing domain, that means an OSPF neighbors could forward traffic to a router that is filtering the route to which it has the lowest cost path.

When filtering routes, the most common method of selecting routes is by subnet and another method is the source of the routes and in OSPF it is the Router-ID.

To choose the best inter-area route, a router uses distance Vector logic of taking its known metric to reach the ABR and adds the metric for that subnet as advertised by the ABR.

If and area has 20 Routers, and the engineer want to filter the route so that five of the routers do not learn the route, type-3 LSA filtering cannot be used. Type-3 LSA filtering can only filter the LSA from being flooded throughout the entire area.

Comments

Popular posts from this blog

What is Cisco Supervisor Engine?

Supervisor Engine is a module that is installed in the Cisco Chassis-based Catalyst Switches or Routers. Supervisor engine contains nearly all the same components of a fixed Cisco Switches or Routers. These Supervisor engines come in a variety of different types with different functionalities and are installed in the Switches/Router Chassis as per requirements of the network types.

Benefits of Supervisor Engines
By installing Latest Supervisor Engines in your existing investments (Switches and Routers) you can scale system performance and integrate next-generation services into your Networks.
Within a single multilayer switch chassis, two supervisor modules with integrated route processors can be used to provide hardware redundancy. If an entire supervisor module fails, the other module can pick up the pieces and continue operating the switch.
The supervisor engine contains the following integrated daughter cards that perform forwarding and routing and provide the protocols supported …

GNS3 Docker Error while creating node: Docker has returned an error: Cannot connect to host docker:80

Error while creating node: Docker has returned an error: Cannot connect to host docker:80 ssl:False [No such file or directory]

After adding docker template for Alpine Linux in gns3, you get above mentioned message when you want to use alpine linux in GNS3.

To get rid of this message you have to install Docker by following below link
curl -fsSL https://get.docker.com/ | sh

If you do not have curl installed then instal curl first with below command.apt-get install curl
After installing Docker you need to add your user name in the docker group with the following command. $ sudo usermod -aG docker your_username

Verify if the docker service is started with following command$ service docker status
If docker is not started then start with following command $ sudo service docker start
Logout from GNS3 Virtual Machines and log back. Start gns3 and use alpine linux.

Telnet Client is Disabled Enable it to use it from this application

This is a Cisco Network Assistant (CNA) error when you want to telnet any Cisco Devices from within CNA.
Solution to remove this error is related to Windows 7 or Windows Vista. First of all you need to Add TELNET program from Control Panel, which by Default is not added in fresh installation of Windows 7.For adding and using Telnet Program in Windows 7, Click Start > Control Panel > Uninstall a Program > Turn Windows Features On and OFF > Scroll to TELNET and Select it > Press OK After Adding Telnet Program into the Windows 7 you need to Copy Telent.exe from C:\Windows\System32 folder and paste it to C:\Windows\SysWOW64\ Now you should not get the same error.