[nsp] does it switch or route

Tue May 20 17:31:04 EDT 2003

On Tue, 20 May 2003, gab.seun jones.ewulomi wrote:

> Hi Steve,
> 
> you are acyually speaking my language thanks.
> 
> when the router first hears a conversation it hasnt heard before between a 
> 10 host and 69 host at this poit it will be routed and has you have 
> explained the router will then like any end host will store the arp(mac 
> address) of the destination host.

correct!

> Now again when if the same 10 host and 69 host then send a packet to each 
> other(assuming the aging timer for the arp entry hasnt expired). It will be 
> a straight layer 2 conversation.

no it will follow exactly the same procedure as before, the only thing missing 
is that the devices already have the arp cached and dont need to rerequest it.

> So yes it does both. Routes it when it hasnt heard of the converstaion 
> between two end point before and after that it swithes or the converstaion 
> is transmitted on the data link

what you may be thinking of here is the "Layer3 switching" that the routers can 
do, also known as multilayer switching, mls and closely related to cef and fast 
switching?

you've not said what your router is but i assume its relatively low end in which 
case its a clear case of simple routing each time

on the more complicated high end routers they are able to cache the path that 
the packets take through the router so that subsequent packets can be forwarded 
on by the switching ASICs without going up to the router CPU.. do you think this 
is your confusion?

even on the high end stuff you may as well still think of the packets as routed 
every time, the clever stuff which people call "layer 3 switching" is a neat 
trick that gives great performance but from the users point of view its still 
normal routing

Steve

 > 
> regards,
> gab
> 
> 
> >From: "Stephen J. Wilcox" <steve at telecomplete.co.uk>
> >To: "gab.seun jones.ewulomi" <seun_ewulomi at hotmail.com>
> >CC: cisco-nsp at puck.nether.net
> >Subject: Re: [nsp] does it switch or route
> >Date: Tue, 20 May 2003 16:05:36 +0100 (BST)
> >
> >
> >On Tue, 20 May 2003, gab.seun jones.ewulomi wrote:
> >
> > > The 69 adresses and the 10 addresses are all on the same Lan(all in the 
> >same
> > > physical building)
> >
> >Yes but they are different IP Networks that you happen to have on the same
> >physical systems, to the software they are separate.
> >
> > > When I do a sh arp I see arp entries for 69 and 10 addresses.
> >
> >Yes but your router has an IP on the interface configured in both networks,
> >again think as tho they are two different networks
> >
> > > An as far as I know when the router as learnt the ip to mac address(as 
> >long
> > > as the aging time hasn't expired) and if
> > > it then hears a packet going to a 10 network in which it has already 
> >heard a
> > >
> > > conversation(arp entry in router) the router will switch it or it will 
> >be a
> > > layer 2 conversation
> >
> >ok its both! your looking too deep..
> >
> >think of your network like this
> >
> ><--L3-->Rtr<--L3-->
> ><--L2-->   <--L2-->
> ><-Host->   <-Host->
> >69/8       10/8
> >
> >right the data starts at the host on the left src 69/8 destination 10/8. 
> >the
> >host has no idea where the 10/8 net is so it sends it to the router default
> >gateway on 69/8 address
> >
> >to send to the router 69/8 the host and router have to have arp to 
> >determine how
> >the L3 Ip address maps to the L2 network card address (MAC)
> >
> >so the packet (L3) is put into an ethernet frame (L2), sent over the 
> >ethernet
> >(L2) to the router, the router takes the packet (L3) out of the frame (L2)
> >decides it is going to a host on the 10/8 net which it has arp for already 
> >so it
> >puts the packet (L3) into a frame (L2) and transmits onto the ethernet
> >
> >so the receiving host gets the frame (L2) extracts the packet (L3) and 
> >theres
> >your data.. so you see it has been routed over the router and the L2 arp 
> >and mac
> >info is needed by the hardware to do the actual transmitting on the 
> >ethernet
> >
> >.. any help or have i just really muddied the water for you!
> >
> >Steve
> >
> > >
> > > please advice/correct if im wrong
> > >
> > > regards,
> > > gab
> > >
> > >
> > >
> > > >From: "Stephen J. Wilcox" <steve at telecomplete.co.uk>
> > > >To: "gab.seun jones.ewulomi" <seun_ewulomi at hotmail.com>
> > > >CC: cisco-nsp at puck.nether.net
> > > >Subject: Re: [nsp] does it switch or route
> > > >Date: Tue, 20 May 2003 15:38:07 +0100 (BST)
> > > >
> > > >
> > > >On Tue, 20 May 2003, gab.seun jones.ewulomi wrote:
> > > >
> > > > > H people,
> > > > >
> > > > > my apologies if this is somewhat a simple and trivial question.
> > > > >
> > > > > On my Lan we have end stations configured with a 10 and 69 network 
> >/8.
> > > > > On the default gateway(router) we have the 69.x.x.x ip adress on the
> > > > > interface and a 10.x.x.x secondary ip address on the same interface.
> > > > >
> > > > > My question is if the end stations on the the same lan e.g if a 69
> > > >address
> > > > > pc wants to talk to a 10 adress pc is this switched or routed by the
> > > >router
> > > >
> > > >I'm guessing your confused by the two terms..
> > > >
> > > >Ok to get from 10/8 to 69/8 the packets need to go via a router which 
> >is a
> > > >layer
> > > >3 device capable of forwarding in the IP layer (this IP 
> >forwarding/routing
> > > >is
> > > >required as the devices are in different networks)
> > > >
> > > >The end stations or even switches in the network are not capable of 
> >routing
> > > >packets between networks so the router is needed, all the end stations 
> >can
> > > >do is
> > > >talk to other devices in the same network or push the packets at their
> > > >default
> > > >gateway....
> > > >
> > > >hth
> > > >
> > > >  >
> > > > > your opinions and answers will be most appreciated
> > > > >
> > > > > regards,
> > > > > gab
> > > > >
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