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Configuration Guide - IP Unicast Routing

S7700 and S9700 V200R011C10

This document describes IP Unicast Routing configurations supported by the switch, including the principle and configuration procedures of IP Routing Overview, Static Route, RIP, RIPng, OSPF, OSPFv3, IS-IS(IPv4), IS-IS(IPv6), BGP, Routing Policy ,and PBR, and provides configuration examples.

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OSPF Network Planning and Design

OSPF Network Planning and Design

OSPF Network Stability: Router ID Design

The first thing to consider in OSPF network design and implementation is router ID selection.

As a link state routing protocol, OSPF calculates routes based on LSAs. All OSPF routers on an OSPF network send and flood LSAs to the entire network. In this way, each OSPF router on the network collects LSAs received from other routers, saves the LSAs in its LSDB, and uses an SPF algorithm to calculate an SPT to other networks with itself as the root. Therefore, ensuring the stability of the LSDB on each OSPF router is a prerequisite for ensuring the stability of the OSPF network. In an LSDB, LSAs received from different OSPF routers are distinguished by the router ID. If the router ID of a router changes, the router floods its LSA again. As a result, all OSPF routers on the network update their LSDBs and perform SPF calculation again, leading to OSPF network flapping. Therefore, choosing stable router IDs is the first task in OSPF network design.

Router IDs can be configured manually. If no router ID is specified for a router, the system automatically selects an interface IP address as the router's router ID. The router ID selection rules are as follows:

  1. The system preferentially selects the largest IP address among loopback interface addresses as the router ID.

  2. If no loopback interface is configured, the system selects the largest IP address among interface addresses as the router ID.

  3. The system reselects the router ID only when the interface address used as the router ID is deleted or changed.

On actual networks, it is recommended that router IDs of OSPF routers be specified manually. First, plan a private network segment such as for router ID selection. Before starting the OSPF process, create a loopback interface on each OSPF router, and use a private IP address with a 32-bit mask as the IP address of the loopback interface. The private IP address is then used as the router's router ID. If there is no special requirement, this loopback interface address does not need to be advertised to the OSPF network.

Hierarchical Network Design: OSPF Area Design

OSPF is a network protocol that requires hierarchical design. The concept of area is used in an OSPF network. From a hierarchical perspective, areas on an OSPF network are classified into backbone and non-backbone areas. The number of the backbone area is 0 and that of a non-backbone area ranges from 1 to 4294967295. An OSPF router at the border between the backbone area and a non-backbone area is called an ABR.

Actually, OSPF area design is the process of classifying OSPF routers on the network. The first thing to consider in OSPF area design is the network scale. You can only plan the backbone area for small networks such as a network with several routers as the core and aggregation devices. However, hierarchical network design must be considered on large OSPF networks.

For large OSPF networks, the core, aggregation, and access layers must be considered in OSPF area design. In addition, OSPF backbone routers generally are egress routers and core switches. These devices are typically high-end routing devices such as Huawei NE series high-end routers and Huawei S series modular switches. Non-backbone area design depends on the geographical location and device performance. If many low-end Layer 3 switching devices are deployed in a non-backbone area, the number of routes should be minimized to reduce the area size or a special area can be used due to product positioning and performance limitations of the switching devices.

The numbers of non-backbone areas on an actual network should not be planned randomly. Consecutive numbers such as 1, 2, and 3 are not recommended. It is recommended that numbers in ascending order such as 10, 20, and 30 be used, facilitating area number addition in future network capacity expansion.

Routing Entry Optimization for Non-Backbone Areas: Special Areas

Using special areas can optimize routing entries on routers in non-backbone areas. Generally, the number of routing entries on routers in a non-backbone area needs to be reduced in the following scenarios:

  • The non-backbone area has only one ABR as the egress router and all traffic for accessing external areas passes through this egress router. In this case, non-ABR routers in this non-backbone area do not need to obtain detailed information about external areas, and only an egress is required to send traffic outside this area.

  • Some low-end Layer 3 switches are deployed in the non-backbone area and their routing tables cannot contain too many routing entries because of performance limitations. Special areas can be configured to reduce the number of routing entries on the devices.

Switches support four types of special areas defined in OSPF: stub area, totally stub area, NSSA, and totally NSSA.

In most cases, routers in non-backbone areas on a typical OSPF network only need to know the outbound interface of the default route. Therefore, it is recommended that non-backbone areas be planned as totally NSSAs. This configuration significantly reduces the number of routing entries on internal routers of the non-backbone areas and the number of OSPF packets exchanged among these routers. For some networks with special requirements, the four types of special areas can be used flexibly based on actual requirements.

Routing Entry Optimization for the Backbone Area: IP Subnetting and Route Summarization in Non-Backbone Areas

Because routers in the backbone area are responsible for route exchange among areas, these routers have large routing tables. You can properly plan IP subnets used by non-backbone areas and summarize routes on routers at area borders to optimize routing entries on routers in the backbone area.

It is recommended that an IP network facilitating route summarization be used as a new OSPF network, and IP addresses be planned again for expanded OSPF networks. Route summarization reduces the number of routing entries in routing tables of routers in the backbone area and the number of OSPF packets exchanged among these routers. After routes are summarized, a single-point link failure or network flapping does not affect route update on the entire network. Therefore, route summarization improves network stability.

Uplink Traffic Diversion: OSPF Default Route Import and Route Selection Optimization

Most service traffic on a large OSPF network is not transmitted inside the network, but is forwarded to the Internet egress. Therefore, default route design is a key design point for OSPF networks.

An actual OSPF network may have more than one egress. Configuring multiple links to load share egress traffic is a difficult point in OSPF network design. Although there are many load sharing methods, the most simple and secure method is to use the internal route selection mechanism of OSPF. An OSPF router determines whether a route is optimal by calculating its cost, and preferentially adds a route with a smaller cost to its routing table. Therefore, you can adjust the costs of OSPF interfaces to make a router select different outbound interfaces for load sharing.

OSPFv2 was developed earlier and rapid bandwidth development was not considered at that time. By default, the reference bandwidth for OSPF cost calculation is 100 Mbit/s. By default, OSPF considers that the cost of an interface with bandwidth larger than 100 Mbit/s is 1. The reference bandwidth is outdated because the network backbone bandwidth develops towards 10 Tbit/s. The switch provides the function of changing the reference bandwidth. In OSPF network construction, you can run the bandwidth-reference command on the switch to configure a proper reference bandwidth. For route selection optimization on an OSPF network, it is recommended that you select a proper reference bandwidth and then change costs of OSPF interfaces.

Loop Prevention in Route Summarization Scenarios: Blackhole Route

Route summarization reduces the number of routes and improves network stability in many scenarios. However, routing loops may occur in route summarization. Blackhole routes can solve this problem. A route discards packets that match a blackhole route, and does not send any error information to the packet sender. Therefore, route summarization and blackhole routes are often used together in OSPF network design.

OSPF Network Security: Silent Interface

Security must be considered for large OSPF networks. In OSPF network design, OSPF packets are often prohibited from being sent to users, preventing end users from obtaining OSPF packet information. If a user can intercept OSPF packets, the user knows how to connect to the OSPF network, making the OSPF network prone to attacks or damages. For example, if a router is connected to an OSPF network and the router's OSPF process is unstable, the OSPF network flaps or even breaks down.

To ensure security and stability of an actual OSPF network, it is recommended that silent interfaces be configured on edge devices of the OSPF network to prevent OSPF packets from being sent to users. These silent interfaces are prohibited from receiving and sending OSPF packets. These interfaces can still advertise direct routes, but cannot establish neighbor relationships because Hello packets are blocked.

Updated: 2020-02-04

Document ID: EDOC1000178324

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