The inherent design limitations of conventional, stand-alone routers
are starting to show in the Internet and large IP networks.
While there are numerous variations in bus-based router designs, they
all share a common weakness: slow next-hop route lookup. One solution is to
use hardware, rather than software, to eliminate this bottleneck.
Ascend Communications, Inc. invented such a hardware-based offering,
called Quick Branch Routing Technology (QBRT), for its family of
multigigabit routers. QBRT, when combined with other high-performance
design elements, allows the multigigabit router to forward packets at wire
speed on a fully configured chassis under heavy traffic.
The fundamental design for the QBRT-based multigigabit router is a
distributed architecture with parallel processing of a router's two main
functions: packet forwarding and route table management.
Packet forwarding must occur in real time and requires a forwarding
decision and the actual routing or forwarding of the packet. With QBRT, the
forwarding decision is based on a full route table lookup to quickly and
accurately determine each packet's next-hop route. Routing tables keep
track of network destinations. Route table management is a process that
helps users get a handle on the thousands of network routes that are
available.
Individual media cards in the multigigabit router perform both packet
forwarding operations. To attain the full potential of parallel processing,
there is a separate data path for each card. This is achieved with a
crosspoint switch that has a nonblocking path for each card in either a
4-by-4 or 16-by-16 matrix for a four or 16-slot chassis, respectively.
All four- or 16-slot bidirectional paths operate independently at 1G
bit/sec, which means each path through the switch is able to accommodate
multiple ports and today's fastest LAN and WAN media. The use of a
crosspoint switching engine makes the design a true multigigabit router.
Each of the hot-swappable forwarding media cards in the QBRT-based
multigigabit router is effectively a separate router. All cards contain one
or more LAN or WAN ports; a forwarding table, which is a simplified version
of the full route table; the QBRT hardware; packet-forwarding hardware;
buffering capacity; and separate inter-faces to the route manager and the
switch engine on the control board.
Unlike QBRT-based routers, conventional routers employ a shared bus
and central processing unit that handle all functions serially, rather than
in parallel. The usual bottleneck in a conventional router is the
relentless next-hop route table lookup for the constant stream of incoming
packets. To keep pace, most
conventional routers employ a cache of recently used addresses. If a
packet's next hop is in the cache, the lookup is reasonably fast. But when
cache hits drop to 50%, which is fairly common, the router's overall
throughput can drop by as much as 90%.
The throughput limitation of conventional routers is causing a rather
interesting self-destruct sequence in the Internet. With limited
performance, the only way to increase the capacity of the Internet is to
segment the network and add more routers. But the proliferation of routers
creates scaling problems for routing protocols as the route tables become
increasingly complex and unwieldy.
Exchanging large table up-dates consumes precious Internet capacity;
processing them regularly knocks out some routers. When a router goes down,
alternate routers come to the rescue, but at the expense of another route
table update. This 'route flapping' can bring any conventional router -
and even entire segments of the Internet - to a grinding halt.
The QBRT hardware on each media card is fast enough to look up every
packet's next-hop address on its own replicated version of the full route
table, so there is no need for an address cache and the traditional routing
bottleneck is eliminated.
Even with minimum-size packets, each with a different destination
address, arriving at wire speed on a 622M bit/sec WAN interface, QBRT is
able to keep up with the traffic. Specifically, the patent-pending QBRT
hardware can determine the next hop in about 3 microsec in a table as large
as 250,000 routes, or five times the size of those in today's Internet.
Because the route manager is able to quickly and concurrently update
the replicated route tables, the packet forwarding process continues
undisturbed. In this way, switch performance neither contributes to nor is
hindered by route flapping.
Elimination of the next-hop route lookup bottleneck - even under
worst-case traffic conditions - combined with the nonblocking 1G bit/sec
forwarding paths allows the multigigabit router to achieve wire-speed
performance in a fully loaded chassis on OC-12 lines operating at 622M
bit/sec.
The main benefits of QBRT-based routers include a major improvement in
price/performance over conventional routers, linear scalability as parallel
media cards are added and greater port density in a more compact chassis.
Kachelmeyer is director of product marketing with GRF Ascend
Communications Core Systems Division. He can be reached at (510) 769-6001. |
