Many router vendors are now adding in-service software upgrade support
to address this problem. Nonstop routing or forwarding techniques are used to enable
upgrade of the active control card; however, the most complex problem is upgrading the
line cards, especially where these have programmable forwarding ASICs.
MPLS Scaling
Scaling a connectionless network such as IP is largely an issue of increasing the forwarding
performance of the nodes as link speeds increase, and of scaling the routing
tables (either by increasing the memory used to store the routing tables or by aggregating
routes to reduce the memory required). Scaling a connection-orientated network
adds issues of scaling forwarding state. Since MPLS has both connectionless and
connection-orientated characteristics both sets of issues must be addressed, as must
some specific issues relating to MPLS itself.
Routing State In order to establish an RSVP-TE signalled LSP, each router along the
LSP must know how to reach the egress LSR. Thus, routes may be summarized without
impacting LSP signalling. However, the traffic engineering extensions to OSPF and
IS-IS operate only within a single IGP area (to restrict the scope over which their
messages are flooded), and thus when CSPF is used, RSVP-TE LSPs may only be
432 Chapter 14
signaled across a single IGP area.
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