Cisco ATM / DSL setup and troubleshooting

Layers in the stack:

• Layer 1 - DSL
• Layer 2.1 - ATM
• Layer 2.2 - RFC1483B
• Layer 3 - IP

Point-to-Point / Multipoint Interfaces

Like Frame Relay, ATM supports two types of interface: point-to-point and multipoint. The one you choose determines whether you need to use the configuration commands that ensure IP-to-ATM mappings. After configuring the PVC itself, you must tell the router which PVC to use in order to reach a specific destination. Consider these options:

1. Point-to-point subinterface - With point-to-point subinterfaces, each pair of routers has its own subnet. If you put the PVC on a point-to-point subinterface, the router assumes that there is only one point-to-point PVC configured on the subinterface. Therefore, any IP packets with a destination IP address in the same subnet are forwarded on this virtual circuit (VC). This is the simplest way to configure the mapping and is therefore the recommended method.
2. Multipoint networks - Multipoint networks have three or more routers in the same subnet. If you put the PVC in a point-to-multipoint subinterface or in the main interface (which is multipoint by default), you need to either configure a static mapping or enable inverse Address Resolution Protocol (ARP) for dynamic mapping.

Inverse ARP on ATM Connections

On Ethernet networks, IP-based network devices use ARP when they know the destination layer 3 address and need to discover the destination MAC address. Layer 2 network devices use inverse ARP (InARP) when they know the destination MAC address and need to discover the destination layer 3 address.
On ATM networks, RFC 1577 (Classical IP and ARP over ATM), specifies mechanisms for address resolution and defines the Inverse ATM Address Resolution Protocol (InATMARP).
With InATMARP, the ATM interface knows the layer 2 address. This is the PVC’s virtual path identifier (VPI) or virtual channel identifier (VCI). However, it still needs to discover which IP address is reachable at the remote end of a connection. To do this, the router sends an InATMARP request over a virtual connection for the address of the other end.
Note: InATMARP is the same protocol as Ethernet InARP. This is defined in RFC 1293, with additional extensions to support ARP in an ATM network.

Neither a static mapping nor InARP are required on a point-to-point subinterface since there is a single VC (sh atm vc) and a single path for the traffic. The router simply consults the routing table and makes a forwarding decision: 

sh atm map # with P2P ATM interface output of this command is empty

show run int a2/0.3 

interface ATM2/0.3 point-to-point

  ip address 192.168.3.1 255.255.255.252 

  no ip route-cache 

  no ip mroute-cache pvc 0/300 !

InARP is enabled on multipoint links by default.

show atm map 

 Map list ATM1/1/0.100_ATM_INARP : DYNAMIC 

  ip 1.1.1.2 maps to VC 19, VPI 2, VCI 100, ATM1/1/0.100 

 Map list ATM1/1/0.200_ATM_INARP : DYNAMIC 

  ip 2.2.2.2 maps to VC 20, VPI 2, VCI 200, ATM1/1/0.200

The show atm map command displays the dynamic mapping through InATMARP, while the show arp and show atm arp commands do not.

Static IP to ATM VC Mappings

Static map lists are a Cisco IOS Software feature that offers an alternative to using the ATMARP and InATMARP mechanisms. Using static maps, you can associate a protocol address with an ATM address on a switched virtual circuit (SVC), or with a VPI or VCI on a PVC.
Note: Static map lists do not relate to RFC 1483 or RFC 1577 .

interface ATM1/1/0.200 multipoint 

  ip address 2.2.2.1 255.255.255.0 

  no ip directed-broadcast 

  pvc 2/200 

    inarp 5 

    protocol ip 2.2.2.2 broadcast

You can check the mapping using the show atm map command. As you can see, the mapping of layer 3 to layer 2 addresses is permanent rather than dynamic, as it was when you used InARP.

show atm map 

 Map list ATM1/1/0.100_ATM_INARP : DYNAMIC
ip 1.1.1.2 maps to VC 19, VPI 2, VCI 100, ATM1/1/0.100
Map list ATM1/1/0.200pvc20 : PERMANENT
ip 2.2.2.2 maps to VC 20, VPI 2, VCI 200, ATM1/1/0.200, broadcast

Using Point-to-point subinterface with LLC/SNAP RFC1483

LLC/SNAP RFC1483 sometimes called MER (MAC Address Encapsulation) or Enet encapsulation in some modems. To setup Bridging in Cisco Router:

interface ATM0
 no ip address
 atm ilmi-keepalive
interface ATM0.1 point-to-point
 description ===WAN===
 ip address 172.16.16.16 255.255.255.0
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip verify unicast reverse-path
 atm route-bridged ip
 pvc 7/34
  encapsulation aal5snap
 !
end

To verify that router sees other end over ATM:
sh adjacency ATM0.1
Protocol Interface                 Address
IP       ATM0.1                    172.16.16.1(10)