Huawei Quidway NetEngine80E Configuration Manual page 524

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
A physical network can be divided into different logical topologies as needed. Each logical
topology maintains its own routing table and uses the Shortest Path First (SPF) algorithm to
calculate routes. Traffic of different services (including traffic of different IP topologies) can be
forwarded along different paths. With logical topologies, IS-IS MT can help customers better
utilize network resources and reduce network construction costs.
IS-IS MT has the following functions:
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IS-IS GR
GR is a function used to restart a router gracefully. It ensures uninterrupted traffic forwarding
during the restart of a router in a short time.
If IS-IS restarts in a non-graceful mode, IS-IS sessions are reset and Link State Protocol Data
Units (LSPs) are regenerated and flooded. This triggers the SPF calculation in the entire area,
which causes route flapping and forwarding interruption in the area. The IETF defines IS-IS GR
in RFC 3847, in which the specifications of protocol restart with FIB tables reserved and
unreserved are stated.
Issue 02 (2014-09-30)
Separation between IPv4 and IPv6 topologies
IS-IS implements IPv6 using extended TLVs. IS-IS establishes and maintains neighbor
databases and topology databases as defined in ISO 10589 and RFC 1195. IPv4 and IPv6
networks have the same topology. The IPv4 and IPv6 hybrid topology can be regarded as
an integrated topology, which uses the SPF algorithm to calculate routes. It is required that
all IPv4 and IPv6 topology information be the same.
In applications, however, not all devices on the same network support both IPv4 and IPv6.
This may result in inconsistency between IPv4 and IPv6 topology information. In an IPv4
and IPv6 hybrid topology, some routers and links do not support IPv6. Routers supporting
IPv4/IPv6 dual-stack still forward IPv6 packets to them. As a result, these routers and links
discard the IPv6 packets. IPv4 packets also fail to be forwarded if some routers and links
do not support IPv4.
IS-IS MT uses the SPF algorithm on an IPv6 topology, and sets up a routing table on each
device in the IPv6 topology. This solves the problem of inconsistency between IPv4 and
IPv6 topology information.
Separation between unicast and multicast topologies
If an MPLS TE tunnel is deployed in a unicast topology, the outbound interface of the route
calculated by IS-IS may not be a physical interface but a TE tunnel interface. No multicast
forwarding entry is created on the router that the TE tunnel traverses. The router will discard
packets sent from a multicast source, causing multicast service interruption.
With IS-IS MT, a separate multicast topology can be set up for multicast services, and the
multicast topology is separated from the unicast topology. TE tunnels are excluded from a
multicast topology, ensuring continuity of multicast services.
NOTE
For details about IS-IS MT, see the IS-IS MT of "IS-IS" chapter in the HUAWEI NetEngine80E/
40E Router Feature Description-IP Routing.
NOTE
For details about IS-IS GR, see the "IS-IS" chapter in the HUAWEI NetEngine80E/40E Router Feature
Description-IP Routing.
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