The OSPF routing protocol is an important part in the CCNP course. There are also relevant assessments in the SP CCIE and EI CCIE exams. At the same time, OSPF questions are also often encountered during the interview. So learning OSPF is very important.
Most people know that the Hub-Spoke structure is one of the most common WAN designs. Today we will discuss the points of attention for deploying OSPF in the Hub-Spoke environment.
First of all, OSPF is a link state protocol. The most important feature of the link state protocol is that it is in the same area, and the database database of all routers is the same. End, and then sent to all Spoke end by Hub end. That is to say, when a site has a routing flip, all sites will be affected, so in the Hub-Spoke environment, the main job of deploying OSPF is to reduce flooding. So how to reduce flooding? The answer is to draw areas and summarize and filter. As shown:
For small-scale networks, Hub and Spoke can be placed in the same area. For medium-sized networks, Spoke can be divided in the same area. If there are a large number of sites, each hub can be divided into an area if the hardware resources (CPU and memory) of the hub router are sufficient.
We have reached a consensus on the question of dividing the region, then the next question is the location of the ABR. What does it mean is whether the ABR is placed on the Hub or Spoke. As shown:
The ABR of the first picture is on the Hub. The Hub receives the detailed route from the Spoke end, and then can summarize or filter the detailed route from the Spoke on the Hub, or use a special area to send the summary route or the default route to other areas. That is the other Spoke. When the site where Spoke is located is flapping, the route transmission will affect the link from the problem Spoke to the Hub side, and the internal network of the Hub site and other Spoke sites will not be affected.
The ABR of the second picture is on each Spoke, between Spoke and Hub is Area 0, each Spoke’s site is a separate area, and each Spoke summarizes the internal routes of this site and sends it to Area 0. If there is a flapping phenomenon inside the site, route delivery will only affect the inside of each site, but since the summary route is sent to the Hub, there may be some unnecessary traffic that will waste bandwidth. To sum up:
· – The ABR is placed on the Hub, and the aggregation or filtering point is on the Hub. The LSA information is transmitted more on the link between the Hub and Spoke, but it wastes less bandwidth;
· – ABR is placed on Spoke. The aggregation or filtering point is on Spoke. LSA transmission is less, but the bandwidth between Hub and Spoke may be wasted.
The next issue that can be discussed is the interface network type used by Hub and Spoke to configure OSPF. We all know that DMVPN is one of the best solutions that can be deployed in the Hub-Spoke environment. There are three phases. In phase three, it does not matter whether the next hop of the Spoke route is Hub or Spoke, but we can still discuss it. The differences between various interface network types during deployment are as follows:
You can choose this according to your needs. Personally recommend using P2MP + BFD.
The above is the discussion when deploying OSPF in Hub-Spoke environment. In fact, there are considerations such as convergence. Will be further in-depth analysis in the EI CCIE series of courses in IELAB.
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