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Configuration Guide - IP Unicast Routing

CloudEngine 8800, 7800, 6800, and 5800 V200R005C10

This document describes the configurations of IP Unicast Routing, including IP Routing, Static Route, RIP, RIPng, OSPF, OSPFv3, IPv4 IS-IS, IPv6 IS-IS, BGP, Routing Policy, and PBR.
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OSPF Mesh-Group

OSPF Mesh-Group

Definition

In scenarios with multiple concurrent links, you can deploy OSPF mesh-group to classify links into a mesh group. This allows OSPF to flood LSAs to only one link selected from the mesh group. OSPF mesh-group prevents repetitive flooding that burdens the system.

The mesh-group feature is disabled by default.

Purpose

After receiving or generating an LSA, an OSPF process floods the LSA. If there are multiple concurrent links, OSPF floods the LSA to each link and sends Update messages. Flooding more than one LSA is unnecessary as only one is valid.

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 this unnecessary burden on the system, 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 for flooding.

Implementation

As shown in Figure 5-23, Router A and Router B are connected through three links and establish an OSPF neighbor relationship. After receiving a new LSA from interface 4, Router A floods the LSA to Router B 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 5-23 LSA flooding with OSPF mesh-group disabled

If there are multiple concurrent links between a device with OSPF mesh-group enabled and its neighbor, the device floods received LSAs only to the primary link, as shown in Figure 5-24.

Figure 5-24 LSA flooding with OSPF mesh-group enabled

As defined in OSPF, LSAs can be flooded to a link only when the neighbor status reaches Exchange or a higher state. When the status of the interface on the primary link is lower than Exchange, OSPF reselects a primary link from concurrent links for flooding LSAs. After receiving LSAs flooded by Router A through link 1, Router B no longer floods the LSAs to Router A through link 2 and link 3.

The Router ID of a neighbor uniquely identifies a mesh group. Interfaces connected to the same neighbor with a state higher than Exchange belong to the same mesh group.

In Figure 5-25, a mesh group of Router A 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 added to the mesh group.

Figure 5-25 Scenario where an interface cannot be added to a mesh group

NOTE:

If a router with OSPF mesh-group enabled has the same router ID as its directly connected neighbor, LSDBs cannot be synchronized and routes cannot be calculated correctly. In such a scenario, you need to reconfigure the router ID of the neighbor. (The configuration with the same router ID for two different devices is a configuration error.)

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Updated: 2019-04-20

Document ID: EDOC1100074760

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