--- yuxiao jia <yxjia@site.uottawa.ca> wrote: >
Hi,>
> I try run script to create a IP routing table (with
> entry 5000) on my core
> LSR by useing "IP routre add IPaddress/mask via
> gateawy"
>
> I expected to see the differnecte of latercy with or
> without large routing
> table for IP forwarding
>
> But result show no difference. why?
AFAIK, There shouldn''t be for small networks. Remember
5000 is still very small compared to the reason MPLS
came into picture.
Forwarding is a process of moving packets from input
to output based on:
- Forwarding Table
- Information in the Packet.
Routing: The Process by which the Forwarding table is
built and maintianed.
- One or more routing protocols
- Procedures to converting routing information to
forwarding table.
Forwarding is handled by Forwarding Information Bases
(FIB) or Forwarding Tables, which runs quite
independent of the rouitng protocol''s Tables. The FIBs
are derived from the routing tables ( on lesser
systems, the routing table can be directly used for
forwarding)
On Large (edge/backbone) routers forwarding takes
place literally at wire speed. A routing Table Manager
usually handles the synchronization /convertions
between the Routing table of particular protocol and
Forwarding base. There are more complex and advanced
techniques like route processor architecture etc. Look
into White papers at Cisco, Juniper, Proctel, Caspian
Netwroks, Vicace Networks for further details on them.>
> I want ask you the algorithm complexity with IP
> lookup ?
It depends of the data structure implementing the
routing table. On average for a hashed based IP
routing table lookup takes O(k) to O(logn) for Routing
table maintained using balanced trees.
> any differnece if I use static routing and OSPF when
> IP look up? I believe
> no difference...not sure?
>
No...Not as far as forwarding is concerned. But the
latency could come in building this forwading tables.
Static routing can be used in small networks. Infact
it''s the static route in dynamic protocols that is in
use most of the time. Real hard core dynamic routing
happens when one moves near the edge/ boundary of
Autonomous Systems.
When one is on pubic network like internet and the
cost of a route going down is high, we go in for more
advanced routing protocols like ospf etc, Which help
in resolving such issues more reliably, quickly and
efficently. How ? To put it simple... It is dynamic
and these routing procols like OSPF propogate such
information many times faster and routers are able to
update their routing tables more quickly. (For e.g
While using OSPF as IGP on networks, routers learn
about a route going down in 1/6th of time taken by the
same router for same information to propogate when RIP
is used. This makes huge difference in large
Autonomous Systems)
The advanced techniques of forwarding and combined
with QoS have seperated into a seperate wonderful
domain called "Traffic Engineering" where is one
comes across MPLS, Traffic control, the capabilities
discussed on LARTC etc etc and many other technologies
some of them are open and other propritary.
True MPLS features can be seen on core routing. If you
really want to check out the latency advantages of
MPLS forwarding ... you need to pump in 100 thousand
routes and probably push everything to their limits,
Like measure forwarding rate of one million packets
per second and then compare the two. you''ll definitely
get clear latency numbers.
Just like in very small networks OSPF and RIP become
equal, may be even OSPF having disadvantages like
memory requirement etc, MPLS forwarding also equals
the usual IP forwarding.
Any pointers to erros in my reply is greatly
appreciated.
Hope I have answered to your satisfaction.
-Deepak
>
> thanks
>
> yuxiao
>
>
> ----- Original Message -----
> From: "deepak patwardhan" <p99deepu@yahoo.co.uk>
> To: "yuxiao jia" <yxjia@site.uottawa.ca>
> Sent: Tuesday, June 17, 2003 10:25 AM
> Subject: Re: [LARTC] how to emulate large IP routing
> table
>
>
> > I forgot to CC the mail to LARTC, I don''t have the
> > copy of my reply, could you forward it there for
> me ?
> > Thanks,
> > Deepak
> >
> >
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