Huawei S6700 Series Configuration Manual page 257

S6700 Series Ethernet Switches
Configuration Guide - IP Routing
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:
l
l
IS-IS TE
IS-IS TE supports MPLS to set up and maintain the label switched paths (LSPs).
When establishing constraint-based routed (CR) LSPs, MPLS needs to learn the traffic attributes
of all the links in the local area. CR-LSPs can acquire the TE information of the links using IS-
IS.
Administrative Tag
The use of administrative tags simplifies management. Administrative tags can advertise IP
address prefixes in the IS-IS area to control routes. The administrative tag carries the
administrative information about an IP address prefix. It is used to control the routes of different
levels and routes imported from different areas, various routing protocols, multiple IS-IS
instances running on a switch, and carrying of tags.
Each administrative tag is associated with certain attributes. If the prefix of the reachable IP
address to be advertised by IS-IS has this attribute, IS-IS adds the administrative tag to the
reachability TLV in the prefix. In this manner, the tag is advertised throughout the entire IS-IS
area.
Issue 01 (2012-03-15)
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. If a client
sends a Join packet with the specified outbound interface as a TE tunnel interface, a router
that the TE tunnel traverses does not parse the Join packet, but adds an MPLS label to the
packet before forwarding it. No multicast forwarding entry is created on the router. 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.
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