From: Brian McGahan (brian@cyscoexpert.com)
Date: Sat Sep 07 2002 - 15:15:47 GMT-3
Paul,
The host routes are installed dynamically, then redistributed
into IGP to maintain connectivity. The reason the usage is limited is
because of the host routes. If you have lots of mobile hosts, your
routing tables will be populated with tons of host routes.
> Also I have always wondered if you just moved a host to another
subnet
> with a static address and did absolutly nothing what exacly
happens...??
Nothing happens. The host loses connectivity.
> Or could you force the router to route it by chaging the static
default
> gateway to the gateway of the new subnet, and having proxy arp
enabled..
I'm not sure what you mean by this, but proxy ARP is enabled by
default. However, a host can only request an ARP resolution for a host
it believes to be on its own subnet; otherwise it must send traffic to
the default gateway. You can fool the host into thinking this is true
by giving it a very small subnet mask, such as 128.0.0.0, then proxy ARP
would do all the work. This would create excessive ARP traffic however.
Try these:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
fipr_c/ipcprt1/1cfipadr.htm#xtocid11
ftp://ftp.isi.edu/in-notes/rfc1027.txt
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
http://www.cyscoexpert.com
Voice: 847.674.3392
Fax: 847.674.2625
> -----Original Message-----
> From: Casey, Paul (6822) [mailto:Paul.Casey@o2.com]
> Sent: Saturday, September 07, 2002 12:55 PM
> To: 'brian@cyscoexpert.com'
> Subject: Re: Need help/ideas?
>
> Seems a good idea,
> Why is it the the usage is limited??
> Seems a clever solution to the problem..
>
> Is it that the host routes have to installed manually.
> And wouldn't this work without doing anything by just putting a host
route
> on router 2 for this address.. And enabling proxy arp..??
>
>
> Also I have always wondered if you just moved a host to another
subnet
> with
> a static address and did absolutly nothing what exacly happens...?? Or
> could
> you force the router to route it by chaging the static default gateway
to
> the gateway of the new subnet, and having proxy arp enabled..
>
> Could the packets possibly be routed.
>
> Any help appreciated.
> Kind regards
> Paul.
>
>
>
>
> --------------------------
> Sent from my BlackBerry Wireless Handheld
>
>
> -----Original Message-----
> From: Brian McGahan <brian@cyscoexpert.com>
> To: 'Phil Virnoche' <p.virnoche@verizon.net>; ccielab@groupstudy.com
> <ccielab@groupstudy.com>
> Sent: Sat Sep 07 15:53:16 2002
> Subject: RE: Need help/ideas?
>
> Phil,
>
> This is called "Local Area Mobility", which is basically an
> optimization on ARP. Let's suppose you have the following situation:
>
> | |
> |---R1---R2---|
> | |
>
> We have three networks 10.0.0.0/8, 12.0.0.0/8, and 20.0.0.0/8.
> Host 10.0.0.1 exists on R1's Ethernet segment. For some reason or
> other, host 10.0.0.1 (which is statically addressed), leaves R1's
> Ethernet segment and is relocated on R2's Ethernet segment. Since
this
> host has a static IP address, and is now located in a different subnet
> than it has configured, connectivity to this host is lost. Local Area
> Mobility (LAM) fixes this problem through the usage of optimized ARP
> timers and host routes (/32).
>
> When host 10.0.0.1 (now located on the 20.0.0.0/8 segment)
> wishes to send traffic, it must resolve layer3 to layer2 addresses
> through ARP. When R2 implements LAM, it starts listening on its
> Ethernet segment for traffic sourced from hosts whose addresses are
not
> part of the Ethernet segment's subnet. R2 hears host 10.0.0.1's ARP
> requests, and realizes that it is a mobile host. A host route is
> installed in R2's routing table for this host, and a mobile ARP entry
is
> created.
>
> ARP timeout on the router defaults to 4 hours, which is quite a
> bit of time. Since with LAM we are assuming that are hosts are
roaming
> around the network, we do not want to have to wait 4 hours before we
> realize that a host has left our local segment. Therefore, when a
> mobile host is discovered, the router running LAM begins to unicast
ARP
> requests to this mobile host every few minutes. The default keepalive
> time is 5 minutes, and the default hold time is 15 minutes. This
means
> that the router must only wait 15 minutes, not 4 hours, to realize
that
> the mobile host has left the segment.
>
> Reachability is maintained to this host through the usage of
> host routes. Assuming you're running an IGP that supports host
routes,
> we can redistribute mobile host routes into IGP. This allows us to
> maintain connectivity due to the longest match lookup that the router
> performs. Since R1 and R2 now have a host route 10.0.0.1/32, they can
> both do an exact lookup that is longer than 10.0.0.0/8, which R1 has
> directly connected. At the same time, hosts on R1's Ethernet segment
of
> 10.0.0.0/8 can still transparently communicate with this mobile host.
>
> When hosts on 10.0.0.0/8 send out an ARP for 10.0.0.1, R1 can
> respond with its own MAC address using proxy ARP. This means that the
> hosts are now actually sending their traffic to R1, and R1 is routing
> the traffic to host 10.0.0.1 via its host route.
>
> As you can imagine, this feature has limited usage, as each
> mobile host requires a host route in your IGP table. Syntax is very
> basic to implement, which is the following:
>
> Interface Ethernet 0
> Ip mobile arp [timers | access-group]
>
> Router OSPF 1
> Redistribute mobile subnets
>
>
> See this white paper for more details:
>
> http://www.cisco.com/warp/public/cc/pd/iosw/ioft/lam/tech/lamso_wp.htm
>
>
> HTH
>
> Brian McGahan, CCIE #8593
> Director of Design and Implementation
> brian@cyscoexpert.com
>
> CyscoExpert Corporation
> Internetwork Consulting & Training
> http://www.cyscoexpert.com
> Voice: 847.674.3392
> Fax: 847.674.2625
>
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Phil Virnoche
> > Sent: Saturday, September 07, 2002 8:41 AM
> > To: ccielab@groupstudy.com
> > Subject: OT: Need help/ideas?
> >
> > Good morning all-
> >
> > Looking to find a Cisco solution that aids in controlling a hosts
> > IPCONFIG parameters. For instance, when you are in a hotel room, you
> > plug your computer in that is configured using a static IP. Chances
> are
> > very good that you don't have an IP that belongs to the local
segment,
> > but some magic transition takes place and you still are able to
attach
> > and operate.
> >
> > Is there anything S/W or H/W from Cisco that would do this? My
thought
> > is that it is essentially doing NAT at the port level..... Ideas?
> >
> > Any info is appreciated....
> >
> > Phil
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