OSPF router and link status Packet Classification

OSPF router type

When an AS is divided into several OSPF regions, OSPF routers can be classified AS follows based on the role of a router in the corresponding regions:
Internal router: When all the directly connected links on an OSPF router are in the same region, we call this router an internal router. The internal router only runs the OSPF Algorithm in its region.
Regional Border Router: When a vro is connected to multiple regions, it is called a regional border router. The Region Border Router runs the OSPF algorithm defined by all the regions connected to it. It has network structure data of each region connected to it, and understands how to broadcast the link status information of the region to the backbone region, then, the backbone area is forwarded to other areas.
AS border router: The AS border router is an OSPF router that exchanges route information with the AS external router. The router broadcasts the AS external route information within the; in this way, all routers inside the AS know the routing information to the AS border router. The definition of an AS border router is independent from those of the preceding vrouters. An AS border router can be a vro within a region or a regional border router.
Specify a Router-DR: in a broadcast, multi-access network such as Ethernet, TokenRing, and FDDI environment), a specified Router Designated Router exists ), the specified router completes the following work in the OSPF Protocol: the router generates a link packet-network link used to describe the network segment, which contains all the routers in the network segment, includes the status information of the specified vro. The specified router establishes an adjacent relationship with all OSPF routers that are in the same network segment. Because the connection status database is synchronized between OSPF routers by establishing adjacent relationships and subsequent flooding, we can say that the specified router is in the center of a network segment.
It should be noted that the definition of the specified vrodr DR is different from those defined above. DR is selected through OSPF Hello data packets. During OSPF route protocol initialization, A router with the largest ID is selected as the specified router DR through the Hello packet on a broadcast network segment, and a router with the largest ID is selected as the backup of the specified router BDR, BDR can automatically replace all DR tasks when DR fails. After DR and BDR are selected for a CIDR block, all the other routers in the CIDR block are adjacent to DR and BDR only. Here, the ID of a vro is the identifier pointing to the vro. It generally refers to the loopback port of the vro or the smallest IP address on the vro.

OSPF link status broadcast packet type

With the introduction of the concept of OSPF router type, the OSPF Routing Protocol further classifies its link status broadcast packet LSA. OSPF divides link status broadcast data packets into five categories:

Type 1: Also known as Router Link information packet Router Link), all OSPF routers generate such data packets, describes the status information of a link or port connecting a vro to a region. Router link information data packets are only broadcast in a specific region and not broadcast to other regions.
In a link packet of type 1, the OSPF router sets some specific data bits in the packet to tell the other routers that they are a regional border router or an AS border router. In addition, when describing the Link to which data packets of type 1 are connected, the Link ID and Link ID are marked based on the network type connected to each Link. Table 1 lists common link types and link identifiers.
Link type description link ID
1 describes the router ID of a peer-to-peer router.
2. The port address used to describe the link DR to a broadcast network
3. It is used to describe the network number of the stub network on the link of the stub network.
4 describes the router ID of the adjacent vro of the virtual link.
Table 1: link type and link ID
Type 2: Network Link, also known as Network Link ). Network link information data packets are generated by a specified router in a broadcast, multi-point access network, such as Ethernet, licensing ring network and FDDI network environment, this link status packet is used to describe the status information of all the routers connected to the CIDR block.
The specified vrodr DR generates a network link information packet only after it establishes an adjacent relationship with at least one vro. The packet contains a description of all vrouters that have established an adjacent relationship with the DR, including the DR router itself. Link information of Type 2 is only broadcast in the area of the broadcast network that contains the DR, and is not broadcast to other OSPF routing areas.
Type 3 and type 4: Link Status broadcasting of Type 3 and type 4 is also called Summary Link in OSPF routing protocol ), this link status broadcast is generated by the regional border router or AS border router. Summary Link describes the route information outside a region. The destination address must be in the same. Summary Link will only be broadcast in a specific region. The difference between Type 3 and type 4 summative link information is that type 3 is generated by the Regional Border Router and is used to describe the link status between different regions in the same; type 4 is generated by the AS border router and used to describe the link status information of different.
It is worth mentioning that only the Summary Link of Type 3 can be broadcast into a residual domain, because AS border router is not allowed in a residual domain. A default Summary Link is generated for the Region Border Router of the residual domain to broadcast the domain, so that a default route information is generated on other routers. Using Summary Link can reduce the size of the Link status database of the vro in the residual domain, thereby reducing the utilization of vro resources and improving the operation speed of the vro.
Type 5: Type 5 link status broadcast is called AS an AS external link status information packet. The link data packet of type 5 is generated by the AS border router and used to describe the route information of the destination outside of the AS. The data packet will be broadcast in all regions except the residual domain in the. Generally, this link state information describes the routing information to a specific network outside the AS. In this case, the link ID of the link-like ingress packet of type 5 uses the IP address of the destination network. In some cases, the AS border router can broadcast the default route information to the, the default network number 0.0.0.0 is used as the link identifier of the link broadcast packet of type 5.