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S2700, S3700, S5700, S6700, S7700, and S9700 Series Switches Interoperation and Replacement Guide

This document provides typical configuration examples for interoperation between Huawei switches and mainstream IP phones, firewalls, routers, Microsoft NLB servers, multi-NIC servers, Cisco switches, and SolarWinds.
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Overview of OSPF and EIGRP

Overview of OSPF and EIGRP

OSPF

The Open Shortest Path First (OSPF) protocol is a link-state Interior Gateway Protocol (IGP) developed by the Internet Engineering Task Force (IETF). It is also a dynamic routing protocol.

  • Mechanism
    1. After OSPF is run on switches, these switches send Hello packets on all OSPF-enabled interfaces. If two switches share a data link and can successfully negotiate certain parameters specified in their Hello packets, they can establish an OSPF neighbor relationship.
    2. Switches that have established an OSPF adjacency can exchange link-state advertisements (LSAs). LSAs describe information about a switch, including all links, interfaces, neighbors, and link state of the switch. Switches exchange the link information to learn about the whole network topology.
    3. A switch floods LSAs and records received LSAs in its link state database (LSDB). Subsequently, all switches have the same LSDB. An LSA describes the surrounding network topology of a switch, whereas an LSDB describes the network topology of the entire autonomous system (AS) and is the summary of LSAs.
    4. After LSDB synchronization is complete, each switch uses a shortest path first (SPF) algorithm to calculate a loop-free topology with itself as the root to describe the shortest path (with the minimum path cost) to each destination. The topology is the shortest path tree (SPT), which shows the optimal paths to nodes in an AS.
    5. After each switch uses an SPF algorithm to calculate the SPT, it installs the shortest paths in its routing table as routing entries to guide data forwarding and updates the routing table in real time. Meanwhile, neighbors exchange Hello packets to maintain their neighbor relationships or adjacencies and periodically retransmit LSAs.
  • DR/BDR and Area
    • Designated Router (DR)/Backup Designated Router (BDR): On broadcast and non-broadcast multiple access (NBMA) networks, any two switches need to establish an OSPF adjacency and exchange routing information, wasting bandwidth resources. To solve this problem, a DR and a BDR are elected and establish an OSPF adjacency with other switches (DR others) on the same network segment. DR others do not establish an OSPF adjacency or exchange any routing information with each other. This reduces the number of OSPF adjacencies established between switches on broadcast and NBMA networks, saving bandwidth resources.
    • Area: When a large number of switches run OSPF, their LSDBs become large, complicating SPF calculations. Each route change causes route recalculations on all switches. OSPF resolves this problem by partitioning an AS into different areas. An area is regarded as a logical group of switches and is identified by an area ID. A network segment belongs to only one area. Area partitioning reduces the LSDB size, simplifies SPF calculations, and increases network efficiency.

For more details about OSPF of switches, see "OSPF Configuration" in Configuration Guide - IP Unicast Routing of the required product version.

EIGRP

The Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary dynamic routing protocol that uses a distance-vector algorithm.

  • Mechanism
    1. An EIGRP-enabled switch periodically sends Hello packets on each interface, establishes a neighbor relationship with the switch that receives the packet, and adds the neighbor relationship to its neighbor table.
    2. The two switches that establish a neighbor relationship can exchange routing information only after a three-way handshake. They exchange all routing information when exchanging routing information for the first time and update the changed routing information only when the network structure or route changes. EIGRP routing information is exchanged using Update, Query, Reply, and Ack packets.
    3. EIGRP stores all received routing information in the topology table, including the destination IP address, mask, next hop, and metric of each route. An EIGRP switch selects the optimal loop-free routes from the topology table using the Diffusing Update Algorithm (DUAL) and adds the routes to its routing table.
    4. The EIGRP switch maintains neighbor relationships through Hello packets. If the switch does not receive any Hello packet from a peer switch within the specified period, it considers the peer switch to be unreachable, deletes the peer switch from the neighbor table, and notifies other neighbors that the route to the peer switch is unreachable.
  • DUAL
    • DUAL is used by EIGRP to calculate loop-free routes.
    • The algorithm is a distance-vector algorithm. Using this algorithm, a switch calculates the shortest distance to a destination by calculating its distance to each neighbor plus the distance from this neighbor to the destination and obtaining the minimum distance to the destination from the distance calculation result. The distance, known as a metric, is calculated based on parameters such as the transmission delay, link bandwidth, and effective bandwidth to reflect the advantages and disadvantages of each route.
    • DUAL calculates loop-free routes based on feasible successors. A successor is a neighboring switch used for packet forwarding that has a least cost path to a destination that is guaranteed not to be part of a routing loop. When a network change occurs, DUAL first detects whether a feasible successor for the destination network exists. If a feasible successor exists, DUAL selects it as the next hop. If no feasible successor exists and there is no routing entry to the destination address in the topology table, the destination network is unreachable. If the topology table contains such a routing entry, the neighboring switch is not eligible to become the feasible successor. In this case, DUAL needs to recalculate the feasible successor.
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Updated: 2019-05-15

Document ID: EDOC1000114005

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