Discussion

The first line of the output shows that the LDP session is up and running—this is
essentially the same information as the basic show ldp session command. The second
line reports the LDP session ID, which is a concatenation of the LDP IDs for the local
router and its LDP neighbor. Each router creates a 6-byte LDP ID. The first four
bytes are the router ID or IP address of the router itself. The next two bytes define
the type of labels that LDP is allocating. The value 0 is the default and means that
LDP assigns labels on a per-router basis rather than on a per-interface basis.
The Next keepalive field shows how long before the LDP sends a keepalive message
to its neighbors. A couple of lines down, you see that keepalive messages are sent
every 10 seconds, which is the LDP default. The fourth line indicates that the session
is active and can carry packets up to 4,096 bytes long. The last two fields show the
default hold time and how many neighbors are participating in this LDP session.
The next several lines provide information about the session to the LDP peer, including
the IP addresses and how long the session has been up. The Local, Remote, and
Local maximum recovery time lines report provide information about graceful restart
(see Recipe 8.12). The last section lists the next-hop addresses that the router has
learned from the LDP session. You see that the router has learned the address to
interface t1-4/0/0, the address of the subnet to the neighbor (10.0.0.2), and the
address of the subnet between the neighbor and the egress router (10.0.8.1).
A reliable way to check that the LSP is up is to look for a route for the FEC:
aviva@RouterG> show route protocol ldp table inet.3
inet.3: 2 destinations, 2 routes (2 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both
192.168.16.1/32 *[LDP/9] 1d 21:53:52, metric 1
> via t1-4/0/0.0, Push 100000
192.168.17.1/32 *[LDP/9] 1d 21:53:52, metric 1
> via t1-4/0/0.0
These two routes are the LDP FECs, and there is one for each LDP neighbor. By
default, the JUNOS LDP software advertises an FEC for its loopback address. The
first FEC is to the LSP’s egress point, 192.168.16.1/32, through the t1-4/0/0/0 interface.
The second line also shows the label value and operation associated with this
FEC. The label value is 100000, and LDP pushes this label onto the label stack of all
packets destined for 192.168.16.1/32. Recipe 14.2 describes the contents of the
inet.3 and mpls.0 routing tables.
LDP also keeps track of its FECs in a database. Here are the entries on the egress
router:
aviva@RouterF> show ldp database
Input label database, 192.168.16.1:0--192.168.17.1:0
Label Prefix
100000 192.168.16.1/32
3 192.168.17.1/32
100032 192.168.19.1/32
Output label database, 192.168.16.1:0--192.168.17.1:0
Label Prefix
Configuring LSPs Using LDP as the Signaling Protocol | 491
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