Tag Archives: OSPF

Routing protocols are used to discover routes to a destination. Only one routing protocol at one time determines the optimal route to a destination.

What happens if we have different routes to the same destination learnt by different routing protocols?

When multiple routing information sources coexist on the same router, the router with the highest preference is selected as the optimal route.

Below you can find the default route preferences by each routing protocol (the smaller the value, the higher the preference is):

Of course you can change these default values manually.

But what happens if different routing protocols are configured with the same preference?

Huawei NE40E OSPF basic configuration:

#ospf 1 router-id 1.1.1.1
 area 0.0.0.0
  authentication-mode simple plain labnario
  network 10.0.0.0 0.0.0.3
  network 1.1.1.1 0.0.0.0
#

How to display OSPF neighbour:

[NE40E-1]display ospf peer

         OSPF Process 1 with Router ID 1.1.1.1
                 Neighbors

 Area 0.0.0.0 interface 10.0.0.1(GigabitEthernet3/0/0)'s neighbors
 Router ID: 2.2.2.2          Address: 10.0.0.2
   State: Full  Mode:Nbr is  Master  Priority: 1
   DR: 10.0.0.2  BDR: 10.0.0.1  MTU: 0
   Dead timer due in 34  sec
   Retrans timer interval: 5
   Neighbor is up for 00:33:07
   Authentication Sequence: [ 0 ]

As you probably know there are five types of OSFP packets:

All these packets, except Hellos, are sent only between adjacent routers.

LSA types

There are 5 common LSA types:

• Router-LSA and Network-LSA calculate intra-area routes describing detailed link state information.
• Network-Summary-LSA calculates inter-area routes describing brief routing information instead of link state information
• ASBR-Summary-LSA describes how to reach ASBR
• AS-External-LSA describes how to reach destinations outside AS.

In the era of ethernet transport technology we cannot forget about older, but still widely used ATM and FR technologies. Taking OSPF into consideration, it classifies networks based on link layers protocols. That’s why we have such terms like:

• broadcast
• non-broadcast multiple access NBMA
• point-to-mulitpoint P2MP
• point-to-point P2P.

Broadcast