Brief Introduction to the routing mechanism of OSPF Dynamic Routing Protocol

The OSPF dynamic routing protocol is worth learning a lot. Here we mainly introduce the internal zone routers, including the area border routers and backbone routers. One reason for OSPF's scalability is its route update mechanism. The OSPF dynamic routing protocol uses LSA to share routing information among OSPF nodes. The broadcast information will be transmitted throughout the zone but will not go beyond the same zone. Therefore, every vro in the zone knows the topology of the zone. However, the topology of a zone is unknown to the zone.

Considering that there are actually four different types of OSPF dynamic routing protocols-intra-zone routers, intra-zone vbrs, autonomous system vbrs, and backbone routers-obviously each vro type has different peer-to-peer physical sets, the router exchanges LSA with these peer entities.

1. Internal zone Router

The internal zone router must directly exchange LSA with other vrouters in the Zone, including the vrouters in each zone and the vbrs that act as zone members. Figure 1 shows the situation of forwarding or flooding LSA in the OSPF example network mentioned above in area 1. Note that OSPF routers in the same zone can share the LSA information without being directly connected to each other. The OSPF router directly sends the LSA packet to each known router in the zone and uses any available link to forward those packets. Figure 1 shows the use of ASBR for autonomous system interconnection.

What is not worth noting in Figure 2 is that convergence can happen quite quickly. There are two reasons. The first reason is that the OSPF router can directly address and send the LSA to all routers in the zone (flood) at the same time ), this is totally different from the convergence method of "neighbor to neighbor" used by RIP. The result is that the routers in the zone almost converge to the new topology at the same time. Convergence is accelerated by the definition and use of zones. Topology data is not transmitted outside the zone boundary. Therefore, convergence does not occur on all routers in the autonomous system, but only in the affected zone. This feature accelerates convergence and increases Network stability, because only one subnet in the autonomous system experiences instability, which is brought about by the convergence process itself.

2. Zone border router

The Zone border router maintains the topology information for each zone connected to their interfaces in the database. Therefore, if a VBR is interconnected with two different zones, it must exchange LSA with the peer-to-peer entities in the two networks. Like the intra-zone ro, these LSA are directly addressable and transmitted to the peer entity in the zone. Figure 3 shows this. Another feature of OSPF dynamic routing protocol to enhance performance is route aggregation. The topology information of a zone is not shared with the vrouters outside the zone. On the contrary, the Zone border router summarizes the addresses in all zones connected to it. The summarized route data is shared by the LSA packet and the peer router in each zone connected to it. The OSPF dynamic routing protocol uses several different types of LSA: each has different functions. The LSA used to share the summary route data is type 3 LSA. All ospf lsa types are described in the rest of this chapter. In Figure 3, the VBR directly broadcasts the summarized data to all vrouters in Area 0. OSPF dynamic routing protocol does not allow connection between zones greater than or equal to 1. All such interconnection must pass through Zone 0. Therefore, it means that a VBR connects a non-zero zone with Zone 0.

3. Backbone Router

The backbone router is responsible for maintaining the backbone topology information and spreading the summarized topology information for each other area in the autonomous system. Figure 4 shows how the backbone router switches the LSA. Although the differences between backbone routers, vbrs, and internal vrouters seem clear, the vro can support multi-I/O port connections to other vrouters, the three will still cause confusion. Theoretically, each port can be connected to a different zone. Therefore, a router can form a boundary between different zones connected to it.