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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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IS-IS MT

IS-IS MT

During the transition from IPv4 networks to IPv6 networks, IPv4 topologies and IPv6 topologies must coexist for a long time. The IPv4/IPv6 dual stack is a widely used technology that is applicable to IPv4 networks and IPv6 networks. This technology allows a router that supports only IPv4 or IPv6 to communicate with a router that supports both IPv4 and IPv6.

Background

IS-IS implements IPv6 by extending TLV and complies with the rules for establishing and maintaining neighbor databases and topology databases as defined in ISO 10589 and RFC 1195. As a result, IPv4 networks and IPv6 networks have the same topology. The mixed topology of IPv4 and IPv6 is considered as an integrated topology, which utilizes the SPT to perform the SPF calculation. This requires that IPv6 and IPv4 topology information be consistent.

In actual applications, the deployment of IPv4 and IPv6 may be different on the network; therefore, information about IPv4 topologies may be different from information about IPv6 topologies. In a mixed topology, routers that support the IPv4/IPv6 dual stack cannot sense the routers or links that do not support IPv6 and may forward IPv6 packets to them. As a result, the IPv6 packets are discarded. Similarly, when routers and links that do not support IPv4 exist in the topology, IPv4 packets discarded.

IS-IS multi-topology (MT) can be used to solve the preceding problems. IS-IS MT is an extension of IS-IS to support multiple topologies, complying with draft-ietf-IS-IS-wg-multi-topology. IS-IS MT defines new TLVs in IS-IS packets, transmits MT information, and performs separate SPF calculation in different topologies.

Implementation

IS-IS MT refers to multiple separate IP topologies (for example, IPv4 and IPv6 topologies) that are run in an IS-IS AS. These separate IP topologies are not considered as an integrated topology. This is helpful for calculating IS-IS routes of separate IPv4 networks and IPv6 networks. Based on the IP protocols supported by links, separate SPF calculation is performed in different topologies to shield networks from each other.

Figure 8-1 shows the IS-IS MT. Values in Figure 8-1 indicate link costs. RouterA, RouterC, and RouterD support the IPv4/IPv6 dual stack. RouterB supports only IPv4 and cannot forward IPv6 packets.

If RouterA does not support IS-IS MT, it considers the network as a single topology during SPF calculation. The calculated shortest path from RouterA to RouterC would be RouterA->RouterB->RouterC. However, RouterB does not support IPv6. IPv6 packets sent from RouterA cannot be forwarded by RouterB to RouterC.

If IS-IS MT is enabled on RouterA, RouterA performs SPF calculation in different topologies. When RouterA needs to send IPv6 packets to RouterC, RouterA chooses only IPv6 links to forward IPv6 packets. The shortest path from RouterA to RouterC changes to RouterA->RouterD->RouterC. IPv6 packets are then forwarded.

Figure 8-1  IS-IS MT networking

Switches implement IS-IS MT as follows:

  1. Set up topologies by exchanging packets to establish neighbor relationships.

  2. Perform the SPF calculation for different MTs.

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Updated: 2019-04-01

Document ID: EDOC1000178324

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