From: Brian McGahan (brian@cyscoexpert.com)
Date: Sat Sep 07 2002 - 11:53:16 GMT-3
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
This archive was generated by hypermail 2.1.4 : Mon Oct 07 2002 - 07:43:46 GMT-3