[Series of OSPF] Episode 1 OSPF summary and concept

Latest reply: Nov 28, 2018 17:21:03 305 6 1 0

In this series of posts, we will introduce you routing & switching knowledge detailedly to help you understand how they works .
We will follow the overview, detailed analysis, summary to introduce.
As the very first part of the OSPF, in this post, we will introduce the OSPF overview. 


As the well known link state routing protocol, OSPF is one of the most widely used IGP.

OSPF is a link state routing protocol, which means, unlike distance vector routing protocol, it spreads link state advertisement instead of routing information.All the routers in the same area will maintain the same LSDB(link-state data base). the LSDB is composed of LSA(link state advertisement) . Every single router will regard itself as the root, and based on LSDB  run Dijkstra's algorithm to find out the lowest cost path to the destination network segment.
From the previous paragraph, we know that before the router calculate out the lowest path, it should have the whole link state advertisement, or the caculated out path wouldn't be the appropriate choice. But the storage resources on the router are limited. So OSPF divides the routers into various areas in that way router can reduce the scale of the LSDB.To make sure the routers in a same area calculate out the paths are same, all the routers in a area should maintain their LSDB same as others'.
Then how can the routers assure their LSDB are the same as other routers'? In other words, how can the routers know the LSDB of other routers. For this purpose, routers will propagate LSAs through flooding, this will cause another problem, if the network is a multi-access network, there would be quite unnecessary traffics on the link.

To restrain these unnecessary traffics, OSPF bring in the adjacency relationship. In a multi-access network, like broadcast or NBMA network, a representative router will be elected which known as DR(designated router), and a BDR(back designated router) is elected as the backup of the DR to prevent the OSPF network failure when DR turns to down.By the way, for a P2P or P2MP network, there is no need to elect the DR or BDR because there is no third router will receive the LSA flooding.

There is a point we should pay attention to that is OSPF is based on interfaces, which means when we talk about the router belongs to an area, we mean that interfaces of that router belong to the area. DR and BDR will be elected on every multi-access network,which means a router can be a DR with one multi-access network and be a BDR with another multi-access network.

As so far, we can make a simple summary of the operation of the OSPF.
Routers are divided into a OSPF area, a DR and BDR are elected out of them for every multi-access network. the rest routers,which called DRother, spread their LSAs to DR and BDR. So the DR and BDR will have all the LSAs in this area  and will spread these LSAs to DRother through multicast.In that way, all the routers in this area have the same LSDB. A question comes out, which IP address does the router spreads the LSAs to? In OSPF, there are two multicast IP addresses, 224.0.0.5 and 224.0.0.6. DR and BDR listen to multicast address 224.0.0.6, DRother listen to multicast address 224.0.0.5, this means, DR and BDR send their OSPF packets which should be flooded to 224.0.0.5, so that DRother can receive these packets, on the contrary, DRother spreads DBD packets to 224.0.0.6, so that only the DR and BDR can receive these packets.

As we said before, when too many routers are configured with OSPF, they will be divided into different areas. There are 6 types of OSPF areas, backbone area, common area, stub area, totally stub area, NSSA(Not-So-stubby-Area) and totally NSSA.Every area should connect to backbone area directly or through a virtual-link indirectly.

When a router wants to reach another network segment, because it doesn't have the LSAs spreaded by the destination router, so it can't work out the route path.To solvef this problem, a new type LSA is introduced into OSPF, type 3 LSA, known as summary LSA.Summary LSA is used to advertise network summary of an area to another area.No LSAs are allowed to spread between two areas in which no backbone area exist.In other words, when LSAs are spreaded between two areas, one of the areas must be the backbone area.This mechanism can help OSPF to prevent routing loops.

During the OSPF running, 5 types of packets are are involved, hello packet, DBD packet, LSR packet, LSU packet, LSAck packet.

  1. hello packet is used to discover OSPF neighbor and maintain the neighbor relationship between routers.  
  2. DBD packet, database description packet, is used to describe the LSDB of the sender.Receive router can find out which LSAs it needs to request from the sender.DBD packet contains two types, one is empty content, in another type, router packages the LSAs head information into it.  
  3. LSR packet,link state request. After routers exchanged the DBD packets, they will request LSAs which doesn't exist in their own LSDB from the sender of the DBD packet by using LSR. 
  4. LSU, link state update, is used to update the information in the LSDB of router that receive them. In LSU, it contains the whole content of LSAs, not only the head information of the LSA like in DBD packet.  
  5. LSAck, link state acknowledgement, is used to acknowledge the link state update messages it receives. 


Let's summarize what we have introduced in this episode.

  1. OSPF is a link state routing protocol. 
  2. OSPF can implemented on broadcast network, NBMA network, P2P network and P2MP network. 
  3. In a multi-access network, DR and BDR should be elected, in a P2P or P2MP network, there is no need to do that.
  4. DR and BDR listen to multicast IP address 224.0.0.6, and forward multicast traffic to 224.0.0.5. DRother listen to 224.0.0.5, forward multicast traffics to 224.0.0.6. 
  5. OSPF owns 5 types of packets, hello, DBD, LSR, LSU, LSAck. 
  6. OSPF owns 6 types of areas, backbone area, common area, stub area, totally stub area, NSSA and totally NSSA. Every area should connect to backbone area directly or through a virtual-link indirectly.





This post was last edited by cWX611640 at 2018-11-22 08:59.
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Created Nov 27, 2018 14:34:21 Helpful(0) Helpful(0)

The advantages and disadvantages are compared. OSPF and rip are compared.
Rip protocol is a distance vector routing protocol. Its metric for routing selection is hop number. The maximum hop number is 15 hops. If it is more than 15 hops, it will discard the data packet.
OSPF protocol is a link-state routing protocol. Its criteria for routing are bandwidth and delay.
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Created Nov 28, 2018 17:19:08 Helpful(0) Helpful(0)

Good series
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Created Nov 28, 2018 17:19:41 Helpful(0) Helpful(0)

described it well
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Created Nov 28, 2018 17:20:02 Helpful(0) Helpful(0)

at the end, the summary points are good to have
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Created Nov 28, 2018 17:20:50 Helpful(0) Helpful(0)

good to know about the 5 types of packets involved.
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Created Nov 28, 2018 17:21:03 Helpful(0) Helpful(0)

thanks very much
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