Thursday, 4 July 2013

OSPF Virtual Links and Area Summarisation

OSPF Virtual Links and Area Summarisation



  • Configure multiple-area OSPF
  • Verify multiple-area behavior
  • Create an OSPF virtual link
  • Summarise an area
  • Generate a default route into OSPF

Configure multiple-area OSPF 

Everything has been configured I can use the following commands to check;

OSPF virtual link

I am now going to add the loopbacks 100 to 103 on UPSILON to area 100.

If we check the routing table on TAU it will not display these newly added networks. This is because area 100 is not connected to area 0 (backbone). If an area is not connected to the backbone, its routes are not advertised outside of its area.

To overcome this we can create a virtual link. This creates a logical extension of the backbone area across a regular area.

The adjacency message is a good sign, but we can further check using some show commands;

Summarising an Area

I am now going to summarise loopbacks 100 to 103, this will make the routing table on TAU a bit smaller.

Now lets take a look at TAUs routing table and database;

On UPSILON the summary address points to Null which means if UPLISON recieves a packet destined for something covered by the summary but is not in the routing table, it will be dropped.

Generate a Default Route into OSPF

Right now i'm going to advertise loopback 30 as the default route for all these routers. This will mean that any traffic the routers do not know about get sent to the default route.

For SIGMA to generate a default router apply the following command;

The always keyword is needed for generating a default route in this lab. Without this a default route is generated only to OSPF if one exists in the routing table.

If we check the other routers routing table we will see that the default route is being advertised;

So now if UPSILON pings SIGMA loopback 30 which is not being advertised by OSPF, the packets should be sent to the default route to SIGMA, where SIGMA knows where the network is.


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