Resolve the differences and functions of OSPF in different regions

During the deployment of the OSPF Route Scheme, the OSPF regions are the most difficult to understand. What are the differences and functions between them? This article will answer your questions one by one.

Backbone (Backbone) Region

An OSPF network can contain multiple areas, including three common special areas: the Backbone Area and the Stub Area) the non-pure Stub area (NSSA) can also include other standard areas. The region in the OSPF network is identified by the Region ID. The region with the Region ID 0 is the backbone region.

Examples of main OSPF regions

An OSPF interconnected network always has at least one backbone area, no matter whether it is divided into regions. The backbone area has an ID 0.0.0.0, also known as area 0. In addition, the backbone area must be continuous (that is, it will not cross other areas in the middle), and the other areas must be directly connected to the backbone area (but in fact, sometimes it may not always be like this, so we have the "virtual link" technology introduced below ). The backbone Area is generally Area 0 (Area 0). The main task is to transmit route information between other regions.

The backbone area serves as the Center for Inter-region communication and distributed routing information. Inter-region communication is first routed to the backbone area, then routed to the destination area, and finally routed to the host in the destination area. The routers in the backbone area advertise the summarized routes in their areas to other routers in the backbone area. These summary notices cause a flood of routers in the region. Therefore, each router in the region has a route table that is available in the region, this route corresponds to the ABR summary announcement for other regions in.

For example, in Figure 8-1 above this chapter, R1 uses a summary notice to notify all backbone routers (R2 and R3) of all routes in Area 0.0.0.1. R1 receives the summary announcement from R2 and R3. R1 configures the summary announcement information in Area 0.0.0.0. through flooding, R1 transmits the summary route information to all routers in Area 0.0.0.1. For each vro in Area 0.0.0.1, the route table is calculated based on the summarized route information from Areas 0.0.0.0, 0.0.2, and 0.0.3.

In the actual network, the backbone area may be discontinuous, or the physical connection between a certain area and the backbone area may not exist. In this case, the system administrator can set the Virtual Link) (See Figure 8-4 ). A virtual link exists between two routers. Both routers have a port and the same non-backbone area (this area is between the backbone area and a region not directly connected to the backbone area) the virtual link establishes a logical connection point between the region and the backbone region. A virtual link is considered to be a backbone area (equivalent to an extension of the backbone area). In the OSPF routing protocol, two routers at both ends of the virtual link are connected by a point-to-point link. In addition, in the OSPF routing protocol, the routing information of virtual links is viewed as intra-domain routing. The virtual link must be established between two regional border routers, and one of the regional border routers must belong to the backbone area.

Stub (peripheral) Area

You can understand the concept of OSPF region before. After dividing the region, the routers in non-backbone areas of the OSPF network are routed to the external area, it must be forwarded through the ABR (Regional Border Router), or for a vro in the region, the ABR is a path to the external world. In this case, there is no need to know the detailed route to the external region for the vro in the region, as long as the ABR publishes a default route to the region to guide the packet sending path. In this way, only a few routes in the region and a Default-Route pointing to the ABR are required for the routers in the region to simplify the Route table in the region. And no matter how the routes outside the region change, it will not affect the simple route table of the vro in the region. This is the design concept of "Stub Area" in the OSPF routing protocol. Type 5 LSA injection is not allowed to be performed on the ABR in the Stub region (the ospf lsa type will be described later in this chapter). In these regions, the router route table scale and the number of route information transmitted will be greatly reduced, in addition, this region is not affected by external AS routing.

A Stub area can contain an ingress/egress (that is, an ABR), or multiple BRS when any ABR can be used to reach an external route destination. For the Stub region of Multi-ABR, the external route is advertised through an ASBR outside the region. AS external routers are not flooded, or pass through the Stub area. All routes destined for the external network in a Stub region are implemented through a default route (0.0.0.0 0.0.0.0. In this way, the router routing table in a Stub region has only one entry route to all AS external locations.

To create a default route, the ABR in the Stub area passes through a default route to the Stub area. The default route is routed to all routers in the Stub region, but not outside the Stub region. The default route is used for a vro In the Stub region to provide a route for any destination IP address that cannot reach the. All vrouters In the Stub region must be configured so that they are not imported in the Stub region or are routed to the wildcard AS external route. Therefore, the Stub region must be configured for all regions on all vro interfaces in the same Stub region. For example, Area 0.0.0.3 in Figure 8-5 is configured as a Stub region because all external communications must have a single ABR--R3 for it. R3 advertises a default route distribution in the internal Area 0.0.3, rather than the external network in the Area.

In the Stub region, it is specified that the external as lsa is not received, and the intra-region LSA is not sent to the external AS. That is, the Stub region does not inject the ASE (external Autonomous System) route. The Stub region must be a non-backbone region and a non-convertible region (the region where virtual connections can be configured), because it does not receive or send LSA. In addition, the Type 5 (AS external LSA) LSA is not passed in the Stub area (the LSA Type will be described later in this section ). Figure 8-4 above this section shows the relationships between the backbone areas, Stub areas, and other standard areas. Area 0 is the backbone area, area 3 is the Stub Area, and area 2 is a standard OSPF area. In this example, Area 2 is not directly connected to Area 0 in the backbone Area. In this case, the virtual link established between the vbrs in the two regions is used for direct communication. Area 1 is a forwarding Area.

Because the Stub area is usually located at the end of the OSPF network, the routers in these areas are usually composed of some low-end routers with limited processing capabilities, therefore, these low-end devices in the Stub region do not need to store large route tables or perform regular route computing. This will help reduce the size of the link status database and the usage of the memory on the internal vro In the Stub area, and increase the speed at which the router calculates the route table.

When an OSPF region has only one exit point (connected to only one other region), we can configure this area as a Stub region. At this time, the VBR in the region will advertise the default route information to the region. Note that all routers in a Stub region must know that they belong to this region, otherwise the settings in the Stub region will not work. In addition, there are two points to note for the Stub area: first, virtual links are not allowed in the Stub area, so they will not be extended as backbone areas; second, ASBR is not allowed in the Stub area, otherwise, the LSA in this AS cannot be propagated to another.

Totally Stub (full ending) region and NSSA (non-pure ending) Region

The Stub area described in the previous section is a special Type of OSPF area, which does not receive or spread Type-5 Type LSA (AS-external-LSAs ), for networks that generate a large number of Type-5 LSA (external as lsa), this processing method can effectively reduce the lsdb size of the vro In the Stub region and ease the use of vro resources by SPF operations. Normally, the Stub area is located at the edge of the autonomous system. To ensure that messages destined for the Stub region outside the autonomous system can be correctly forwarded, the ABR (Regional Border Router) in the Stub region will use the Summary-LSA (Summary link status announcement) publish a default route to the region and only flood the traffic in the region. To further reduce the size of the router route table and the number of route information transmitted in the Stub region, you can configure this region as the Totally Stub (full-ended) region, the ABR of this region does not transmit route information and external route information between regions to this region.

1. Full Stub Area

The complete Stub region (Totally Stub, or Stub no-summary) mentioned here is based on the Stub region (that is, based on blocking the Type 5 LSA package) in addition, the network aggregation LSA (Type 3 LSA) advertised by other BRS is blocked and the inter-region routing announcement is not received. Its ABR only advertises a default route through the network summary LSA, and uses this default route to reach the destination address outside the OSPF autonomous system. In other words, Type 3, 4, or 5 LSA injection is not allowed in the full Stub region, except for the default summary route.

To define a region as the Stub region, use the "area xx stub" command in all vrouters in the region. To define a full Stub region, use the OSPF router configuration command "area xx stub no-summary.

2. NSSA Region

Although the Stub area depicts a bright future for a reasonable network planning, the utilization rate in the actual network is not high (Stub area generally only exists on the network edge. However, at this time, the OSPF protocol has been basically formed and cannot be modified. To compensate for the defects, the Protocol designer proposed a new concept NSSA (not-so-Stubby area, non-pure peripheral area ), the extended attributes of OSPF are described separately in RFC 1587. NSSA can be said to have relaxed requirements for the original Stub region so that it can be applied in more network environments.

The NSSA Region stipulates that ASE routes outside of AS cannot enter the NSSA region, but the ASE routes introduced by routers in the NSSA region (ASBR can be connected in The NSSA Region) it can be flood in NSSA and sent out of the region. In this way, the restrictions on the two-way transmission of ASE in the original Stub region are removed from the NSSA region (the areas cannot be inbound or outbound ), change to one-way restriction (out-of-area access is not allowed ). To define the NSSA region, use the OSPF router configuration command "area xx nssa.

To solve the problem of ASE one-way transmission, an LSA-Type 7 Type LSA (NSSA external LSA) is defined in NSSA and used as the router in the region to introduce external routes. Except for the Type ID and Type 5, the Type of LSA is basically the same. In this way, the router in the region can determine whether the route is from the region by using the LSA type. However, because Type 7 LSA is newly defined and cannot be identified for vrouters that do not support the NSSA attribute, the Protocol stipulates that: on The nssa api, convert the Type 7 LSA generated inside the NSSA to the Type 5 LSA and release it again. At the same time, change the LSA publisher to the API itself. In this way, vrouters outside The NSSA region do not need to support this attribute at all. All routers in the NSSA region (including NSSA's ABR) must support Type 7 LSA attributes, while other routers in the autonomous system do not.

In general, Type 5 LSA is not allowed in the NSSA region, but Type 7 LSA converted to Type 5 on The nssa abr still can pass.

The ID of the region corresponding to NSSA is 1, which is connected to the backbone region 0, and it is connected to the routing network of the other two Autonomous Systems (running the IGRP and RIP routing protocols. In this case, the routing information of the external region and external AS cannot be advertised to the NSSA region, but the routing information in The NSSA can be published externally.

Example of ospf nssa Region

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