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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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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).
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. The function is that a router that supports only IPv4 or IPv6 can 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 should 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. Some routers and links in a mixed topology do not support IPv6. However, routers that support the IPv4/IPv6 dual stack in the mixed topology cannot sense the routers or links, and still 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 cannot be forwarded.

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.

Principles

IS-IS MT refers to multiple separate IP topologies that are run in an IS-IS AS, such as IPv4 topology and IPv6 topology. The separate IP topologies are not considered as an integrated and single 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, only the single topology is considered during SPF calculation. The shortest path from RouterA to RouterC is 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

IS-IS MT is implemented as follows:

  1. Setting up topologies: Neighbors are set up by exchanging various packets for setting up MTs.

  2. Performing the SPF calculation: The SPF calculation is performed for different MTs.

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Updated: 2019-05-17

Document ID: EDOC1000174069

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