Tag Archives: OSPF

Instead of transmitting learned AS external routes, area border router, in a stub area, generates a default route and advertises the route to non-ABRs in the stub area. In short, stub area reduces entries in the routing table of ABR and the amount of routing information to be transmitted.

We have to remember that:

• The backbone area cannot be a stub area
• All routers in a stub area need to be configured using stub attributes
• The ASBR cannot exist in a stub area
• Virtual links cannot be configured in stub area.

Let’s try to configure a simple lab. We would like to see what happens if AREA1 becomes a stub area.

1. Based on the topology, configure IP address of each interface.
2. Enable OSPF on each router and configure basic OSPF functions
3. Configure AREA1 as stub and and check routing information on router Gdansk.
4. Check routing information on router Gdansk, previously stopping advertising type 3 LSA into the stub area.

Some time ago I wrote about local PBR and interface PBR.

It’s time to talk about routing policy, that is a different mechanism. Routing policy is applied to routing information and it is combined with routing protocols to form policies. PBR mechanism is applied to data flows and and packets are forwarded according to the configured policy.

Routing policy is a tool which can be used to filter routes and set route attributes, when importing routing information into OSPF, RIP, ISIS or BGP protocols. BGP can use routing policy to filter advertising routes as well. Routing policy defines which of the routes from the specific routing protocol are allowed to be imported into the target routing protocol. It can be also used to match routes or certain route attributes and to change these attributes when the matching rules are met.

Routing policy command syntax:

route-policy route-policy-name { permit | deny } node node

A route-policy may consists of multiple nodes, for example:

route-policy LABNARIO-POLICY permit node 10
route-policy LABNARIO-POLICY deny node 20

How to filter advertised and received routes on Huawei router?

Let’s try to check it based on the following topology:

1. Configure basic OSPF functions on all routers.
2. Configure static routes on AR1 router and import them to OSPF.
3. Use filter-policy for advertised routes on AR1.
4. Use filter-policy for received routes on AR3.

Do you remember our quiz regarding OSPF Virtual Links?

I have asked you, how many OSPF Virtual Links are required in order to avoid Area0 partitioning, while still maintaining full IP connectivity through the OSPF topology.

Look at the topology in the picture below:

OSPF virtual link is a tunnel that extends backbone area through a non-backbone area. It is interpreted by the router as unnumbered point-to-point network. Virtual links must be configured between two Area Border Routers. These ABRs are considered neighbours, by establishing the virtual link between them, although they are not linked physically. The transit area (the area through which the virtual link is configured) must have full routing information and cannot be a stub area.

Virtual links add complexity to a network and should be avoided. Use them only as a temporary fix to OSPF topology problem.

Virtual links are used for the following purposes:

• To link an area to the backbone area through a non-backbone area (most common case). On the picture below Area2 was connected to the Area0 via virtual link, which extends Area0 through transit Area1.