ATM and ARP resolution

[andysaykao]

Scenario:

End user's gateway on ISP's core-router: 203.x.x.1

End user's IP: 203.x.x.2

Core-router -> ATM cloud -> end user cpe

Can anyone explain how the core-router determines the Layer 2 address to use of the end user's cpe through a ATM network? The core-router knows the end user ip is 203.x.x.2 but it also needs the MAC address to forward to. How does this process work?

At the moment the ARP table of the core-router shows no entry for 203.x.x.2:

core-router#sh arp | inc 203.x.x.2

Internet 203.x.x.2 0 Incomplete ARPA

I have an idea how ARP works on a LAN segment, but not quite sure how ARP resolution is achieved through the ATM network from a Service Provider's point of view.

Any help or reference material would be much appreciated.

Thanks.

[Richard Burts]

Andy

What you are asking about is essentially how the router maps the layer 3 address to the layer 2 address. Depending on how the ATM is set up the mechanism might be ARP or it might be something else. If the ATM is configured for LAN emulation then there is a function within LAN emulation that registers IP addresses and how to get to them through the ATM cloud. If the ATM is not set up for LAN emulation then there really is not a MAC and there is no ARP but there are other mechanism for mapping the IP address to the virtual circuit which get to that destination.

If you can provide some details about how the ATM is set up then maybe we can describe the mechanism for mapping layer 3 to layer 2 addresses.

HTH

Rick

[andysaykao]

Rick,

Here's a sample config of how we set up the ATM side on our core router.

interface ATM1/0.xxxxxx point-to-point

description SHDSL connection to end user

ip address 203.x.x.1 255.255.255.252

ip mtu 1500

atm route-bridged ip

pvc xxx/yyy

oam-pvc manage

encapsulation aal5snap

When I do a sh arp, I get the MAC address of the end user's cisco 828 which has it's ATM interface set to ip address 203.x.x.2.

core-router#sh arp | inc 203.x.x.2

Internet 203.x.x.2 68 0000.0cd5.f078 ARPA ATM1/0.xxxxxx

So my question is, how is the core router able to determine the MAC address of the Cisco 828? What mechanism does it use to get the MAC address?

Hope I make sense....

Thanks.

Andy

[Richard Burts]

Andy

The key part of the configuration for this is the line:

atm route-bridged ip

This allows the routers to transport frames with an Ethernet header over the ATM connection.

I found this on CCO and thought it might be helpful:

routed bridge encaps---The process by which a stub-bridged segment is terminated on a point-to-point routed interface. Specifically, the router is routing on an IEEE 802.3 or Ethernet header carried over a point-to-point protocol such as PPP, RFC 1483 ATM, or RFC 1490 Frame Relay.

here is the URL is you want to go further with this:

http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps5014/products_feature_guide09186a0080087970.html

Since it supports routing logic on a frame that is bridged with an Ethernet header, I assume that the functions like ARP are supported in this configuration.

HTH

Rick

[andysaykao]

Thank you Rick.

You've given me a start and I shall research further :)