3Com Switch 8800 Advanced Software V5 Configuration Guide

320 CHAPTER 30: OSPF CONFIGURATION
When a router shuts down, its neighbors will delete it from their neighbor tables
and inform other routers, resulting in SPF recalculation. If the router restarts in
several seconds, it is unnecessary to perform SPF recalculation, and reestablish
adjacencies.
To avoid unnecessary SPF calculation, when a router restarts, it will inform
neighboring routers the shutdown is temporary. Then these routers will not delete
the router from their neighbor tables, and other routers have no idea about this
restart.
After recovering to normal, the router obtains the Link State Database from
neighboring routers via the GR related synchronization mechanism.
n
For OSPF GR configuration, refer to “Configuring OSPF-based Graceful Restart” on
page 797.
TE and DS-TETE
OSPF Traffic Engineering (TE) provides for the establishment and maintenance of
Label Switch Paths (LSPs) of TE.
When establishing Constraint-based Routed LSPs (CR LSPs), MPLS obtains the TE
information of links in the area via OSPF.
OSPF has a new LSA, Opaque LSA, which can be used for carrying TE information.
DiffServ Aware TE (DS-TE) provides for network resource optimization and
allocation, flow classification, and indication of network bandwidth consumption
of each flow in a link. TE is implemented on the classified type (thin granularity
summarization type) rather than the summarized type (thick granularity
summarization type) to improve performance and bandwidth utilization.
To support DS-TE application in MPLS, OSPF supports Local Overbooking Multiplier
TLV and Bandwidth Constraint (BC) TLV.
IGP Shortcut and Forwarding Adjacency
IGP Shortcut and Forwarding Adjacency enable OSPF to use a LSP as the outbound
interface for a destination. Without them, OSPF cannot use the LSP as the
outbound interface.
Differences between IGP Shortcut and Forwarding Adjacency:
■ If Forwarding Adjacency is enabled only, OSPF can also use an LSP as the
outbound interface for a destination
■ If LGP Shortcut is enabled only, only the router enabled with it can use LSPs for
routing.
VPN
OSPF supports multi-instance, which can run on PEs in VPN networks.
In BGP MPLS VPN networks, multiple sites in the same VPN can use OSPF as the
internal routing protocol, but they are treated as different ASs. An OSPF route
learned by a site will be forwarded to another site as an external route, which
leads to heavy OSPF routing traffic and management issues.