The IGP and Label Distribution Protocol on the Asynchronous Transfer Mode Label Switch Routers cannot run directly over the Asynchronous Transfer Mode interface and establish a neighborship. A control VC is needed for the IGP and Label Distribution Protocol to run on between two adjacent Asynchronous Transfer Mode Label Switch Routers. When the IGP adjacency is built, the IGP can exchange IP prefixes which are put in the routing table. After Label Distribution Protocol forms a session across the control VC, it can exchange label bindings. This in turn enables the Asynchronous Transfer Mode Label Switch Routers to populate the LIB with bindings. As you recall, a binding is a prefix and an associated label. Each IGP prefix in the routing table must be assigned a label. Each label value is mapped to a VPI/VCI value, and a virtual circuit is built for each label. Such a virtual circuit is called a label switched controlled virtual circuit (LVC) or tag switching controlled virtual circuit (TVC). To create these LVCs, you must configure the Asynchronous Transfer Mode interfaces on the Asynchronous Transfer Mode switches and routers to be Label Switching Controlled-Asynchronous Transfer Mode (LC-Asynchronous Transfer Mode) interfaces. Each such LC-Asynchronous Transfer Mode interface must have the control virtual circuit. On routers and Asynchronous Transfer Mode switches that are running Cisco IOS, this is by default the virtual circuit 0/32. The encapsulation for it must be LLC/SNAP. Figure 5-5 shows a typical Multiprotocol Label Switching network with Asynchronous Transfer Mode Label Switch Routers in the core. Remember that each prefix that is present in the routing table creates a virtual circuit through the network. Therefore, in the interest of scalability, it is better to limit the number of prefixes in the routing table. One way of doing this, which is highly advisable, is to have the Asynchronous Transfer Mode interfaces as IP unnumbered interfaces. You need a loopback interface anyway as Label Distribution Protocol router ID and IGP router ID, and the IP unnumbered interfaces can point to the loopback interface. When you do not use IP unnumbered interfaces, you allocate a label and a virtual circuit to each IP prefix that is configured on a link. These unimportant prefixes do not forward traffic through the Asynchronous Transfer Mode network, so unused LVCs are set up.

Downstream-on-Demand Label Advertisement

To avoid unnecessary label advertisement for prefixes in the routing table, the Asynchronous Transfer Mode Label Switch Router does not operate in Unsolicited Downstream (UD) label advertisement mode. Rather, it operates in Downstream-on-Demand (DoD) label advertisement mode. This means that an Asynchronous Transfer Mode Label Switch Router only advertises a label (binding) when it is requested to. The upstream Label Switch Router requests the downstream Label Switch Router for a label for a particular prefix.

The upstream Asynchronous Transfer Mode Label Switch Router knows who the downstream Label Switch Router is by looking up the next hop for the prefix in the routing table. Asynchronous Transfer Mode Label Switch Routers (and routers with LCAsynchronous Transfer Mode interfaces) use the Ordered Label Switch Routers Control mode  by default, whereas routers (non-LC-Asynchronous Transfer Mode interfaces) use the Independent Label Switch Routers Control mode. With Ordered Label Switch Routers Control mode, the downstream Asynchronous Transfer Mode Label Switch Router only replies with a label if it has received a label for the prefix from its downstream Label Switch Router. The egress Label Switch Router at the end of the Label Switch Routers is When a new prefix is learned throughout the network via the router brussels, the routers denver and washington send an Label Distribution Protocol Label Request message to their downstream Asynchronous Transfer Mode Label Switch Router neighbor, requesting a label for the prefix. They in turn send an Label Distribution Protocol Label Request message to the downstream Label Switch Routers. This continues until the Label Request reaches the edge Asynchronous Transfer Mode Label Switch Router brussels.

This router returns an Label Distribution Protocol Label Mapping message to its upstream neighbor. The upstream neighbor sends an Label Distribution Protocol Label Mapping message upstream and so on, until the edge Asynchronous Transfer Mode Label Switch Router is reached. At that point, every Asynchronous Transfer Mode Label Switch Router has the label binding for the destination. Because the Asynchronous Transfer Mode Label Switch Routers run in DoD mode, they only request a label from the next-hop Label Switch Router as indicated by the routing table. Thus, the label retention mode is conservative for Label Switch Routers that are running DoD label advertisement mode. When the routing adjacencies are up, the IP prefixes are propagated, the routing table is built on the Label Switch Routers, and Label Distribution Protocol forms the Label Distribution Protocol neighborships and advertises the label bindings for the prefixes, the Asynchronous Transfer Mode Label Switch Routers can build the LVCs between them.

Loop Detection by Label Distribution Protocol

Loop detection in Label Distribution Protocol is optional. It consists of the usage of a Hop Count TLV and a Path Vector TLV to find out if an Label Switch Routers is looping or if Label Request messages are looping. Routing loops can be permanent, but these are rather rare or are the result of a configuration error. Transient loops do occur more often and can be short in nature. They are often the result of the routing protocol converging and one Label Switch Router converging faster than the other. If labeled packets are looping, the label TTL eventually reaches 0, and the packet is dropped.

However, Asynchronous Transfer Mode Label Switch Routers forward cells instead of frames. The Asynchronous Transfer Mode cells do not have a TTL value, so Asynchronous Transfer Mode Label Switch Routers cannot use this mechanism. Because an Label Switch Routers is a VC on Asynchronous Transfer Mode Label Switch Routers, a mechanism is needed to make sure that the VCs do not loop. Cisco Asynchronous Transfer Mode Label Switch Routers use both the Hop Count TLV and Path Vector TLV to prevent a looped Label Switch Routers from being signaled in the first place. When a loop is detected in Cisco IOS, the Label Switch Router periodically resends Label Request messages to try to set up the Label Switch Routers.

Loop Detection by Hop Count TLV

A Hop Count TLV holds the number of Label Switch Routers that the Label Distribution Protocol message has traversed. Every Label Switch Router that sees this TLV must increment the hop count by 1. A loop is detected when a configured maximum hop count value is reached. Following is the command to enable loop detection by means of the Hop Count TLV in Cisco IOS:

Multiprotocol Label Switching ldp maxhops number

The default value for the maximum hop count argument (number) is 254. You can configure the maximum hop count to be n. The ingress Label Switch Router of a Forwarding Equivalence Class sends an Label Distribution Protocol Label Request message with a hop count of 1. The next Label Switch Router that receives this request must increase the hop count by 1 in its request, and so on. The same is true for Label Mapping messages. There, the egress Label Switch Router must send the first Label Distribution Protocol Label Mapping message with a hop count of 1.

Subsequent Label Switch Routers increase the hop count value by 1. If the Multiprotocol Label Switching network consists of a part with Asynchronous Transfer Mode Label Switch Routers and a part with router Label Switch Routers, the Label Switch Routers at the edge of the Asynchronous Transfer Mode domain reset the hop count value to 1, because the Asynchronous Transfer Mode Label Switch Routers are not “hop count-capable.” When an Asynchronous Transfer Mode Label Switch Router detects that the hop count has reached the maximum configured value n, it returns a Loop Detected Notification message to the source of the Label Request or Label Mapping message. The Label Request or Label Mapping message is not answered with a Label Mapping message. It also is not propagated or used.

TTL Manipulation

Asynchronous Transfer Mode Label Switch Routers cannot decrement the TTL value at each hop. You can use the mechanism described in the previous section by means of the hop count propagated by Label Distribution Protocol to set the TTL value of a labeled packet before it enters the Asynchronous Transfer Mode domain. As you can see in Example 5-14, the hop count is present for each binding that is received. You can determine the incoming TTL of a packet on the ingress Asynchronous Transfer Mode Label Switch Router either from the IP TLL if the packet is received as an IP packet or from the TTL in the top label if the packet is received as a labeled packet. This incoming TLL is decremented by 1 to arrive at the new TTL.

Then two things are possible: You can use this TTL on the ingress Asynchronous Transfer Mode Label Switch Router to send the packet, or you can use this TTL minus the reported hop count in the label binding for the prefix to send the packet. The result of the latter is that the TTL set on the packet when it leaves the ingress Asynchronous Transfer Mode Label Switch Router already has the number of hops through the Asynchronous Transfer Mode domain calculated in. If, however, the result of that subtraction is 0 or less, the packet is discarded on the ingress Asynchronous Transfer Mode Label Switch Router. If a packet arrives with a TTL of 3, the ingress Asynchronous Transfer Mode Label Switch Router does not forward it, because the hop count through the Asynchronous Transfer Mode domain is deemed too large. If, however, a packet that has a TTL of 200 arrives on the ingress Asynchronous Transfer Mode Label Switch Router, it is forwarded with a TTL of 197. The egress Asynchronous Transfer Mode Label Switch Router then sees an incoming packet with a TTL of 197.