OSPF inter-region Routing Technology

OSPF inter-region Routing Technology

This section describes the OSPF inter-region routing technology, including the principle of inter-region routing. Network-Summary-LSA is used to describe inter-region routing information, virtual connection technology, and inter-region Routing aggregation.

Objective:

1. Understand the routing principles between regions

2. Understand Network-Summary-LSA

3. Understand Virtual Connection

4. Understand route aggregation between regions

Multiple LSDB instances exist on the Regional Border Router (ABR), and each region maintains an LSDB. ABR abstracts the link status information in the connected non-backbone areas into routing information and publishes it to the backbone areas. The backbone areas are further released to other backbone areas.

The ABR also abstracts the link status information of the backbone area into the routing information and publishes it to the connected non-backbone area.

Medium: RTA generates information about the N1 link status and floods it to RTB. RTB generates abstract routing information about N1 and flood the traffic in the backbone area. RTC flood the received abstract routing information to RTD.

Medium: If routing information published between Area2 and Area3 is allowed, a loop between regions will be formed.

To avoid inter-region loops, OSPF does not allow route information to be published directly between two non-backbone regions, but only within one region or between backbone and non-backbone regions. Therefore, each region's border router (ABR) must be connected to the backbone region.

 

Network-Summary-LSA mainly includes the following content:

The Link State ID is set to the IP address of the destination CIDR block.

. Net mask is set as the network mask of the destination CIDR block.

Metric is set to the overhead value from this ABR to the destination CIDR block.

Take the CIDR Block 10.1.1.0/24 as an example. The route publishing process between regions is as follows:

First, RTB (ABR in Area 1) publishes the route information of this network segment to the backbone Area.

Then, RTC learns the route information about the network segment 10.1.1.0/24 published by RTB through the backbone area.

Finally, RTC generates a new Network-Summary-LSA based on the learning from the backbone Area to Network-Summary-LSA, and releases it to Area 2. In this new LSA: the Advertising Router is changed to the RTC's own Router ID. the overhead of the destination CIDR Block needs to be recalculated and changed to the overhead from RTC to the destination CIDR block.

The backbone area must be continuous, but it does not require physical continuity. You can use virtual connections to make the backbone area logically continuous.

Virtual connection can be established on vbrs in any two regions, but both regions require that the vbrs have ports connected to a common non-backbone region.

Virtual Connection is a virtual link in the backbone Area (Area 0.

In this example, a virtual connection is established between RTA and RTB to connect RTB to the backbone area.

Problem:

1. Is the link status information transmitted between regions?

2. How to avoid the generation of inter-regional loops?

3. How to Determine the peer IP address of the virtual connection?

4. On which vrouters does the Routing aggregation function between regions be configured?

A:

1. No. routes are transmitted between regions, not detailed link status information.

2. Route information can only be published between the backbone and non-backbone regions. Route information cannot be directly published between non-backbone regions.

3. Find the IP address of the Peer router on the virtual connection by calculating the Shortest Path Tree of the Peer router.

4. Configure on the Regional Border Router (ABR)