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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010

This document describes the concepts and configuration procedures of IP Service features on the device, and provides the configuration examples.
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OSPF Mesh-Group

OSPF Mesh-Group

Definition

In the scenario where there are multiple concurrent links, you can deploy OSPF mesh-group to classify links into a mesh group. Then, OSPF floods LSAs to only a link selected from the mesh group. Using OSPF mesh-group prevents unnecessary burden on the system caused by repetitive flooding.

The mesh-group feature is disabled by default.

Purpose

After receiving or generating an LSA, an OSPF process floods the LSA. When there are multiple concurrent links, OSPF floods the LSA to each link and sends Update messages.

In this scenario, if there are 2000 concurrent links, OSPF floods each LSA 2000 times. Only one flooding, however, is valid. The other 1999 times are useless repetition.

To prevent burden on the system caused by repetitive flooding, you can enable mesh-group to classify multiple concurrent links between a router and its neighbor into a group and then select a primary link to use for flooding.

Principles

As shown in Figure 5-21, RouterA and RouterB, which are connected through three links, establish an OSPF neighbor relationship. After receiving a new LSA from interface 4, RouterA floods the LSA to RouterB through interfaces 1, 2, and 3.

This flooding causes a heavy load on the concurrent links. For the neighbor with concurrent links, only a primary link is selected to flood the LSA.

Figure 5-21 LSA flooding with OSPF mesh-group disabled

When multiple concurrent links exist between a device enabled with OSPF mesh-group and its neighbor, the device selects to flood the received LSAs, as shown in Figure 5-22.

Figure 5-22 LSA flooding with OSPF mesh-group enabled

As defined in OSPF, LSAs can be flooded to a link only when the neighbor status is not lower than Exchange. In this case, when the status of the interface on the primary link is lower than Exchange, OSPF reselects a primary link from the concurrent links and then floods the LSA. After receiving the LSA flooded by RouterA from link 1, RouterB no longer floods the LSA to RouterA through interfaces 2 and 3.

As defined by the mesh-group feature, the Router ID of a neighbor uniquely identifies the mesh group. Interfaces connected to the same neighbor that have a status greater than Exchange belong to the same mesh group.

In Figure 5-23, a mesh group of RouterA resides in Area 0, which contains the links of interface 1 and interface 2. More than one neighbor of interface 3 resides on the broadcast link. Therefore, interface 3 cannot be defined as part of the mesh group.

Figure 5-23 Interface not added to mesh group

NOTE:

After a router is enabled with mesh-group, if the Router IDs of the router and its directly connected neighbor are the same, LSDBs cannot be synchronized and routes cannot be calculated correctly. In this case, you need to reconfigure the Router ID of the neighbor.

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Updated: 2019-05-20

Document ID: EDOC1100034072

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