12.7 LDP-Based Dynamic-LSP Configuration

This section shows how to configure LDP LSPs on a Juniper Networks M-Series router. LDP created LSPs are not used for traffic engineering in the general sense of the term . They are sometimes used in VPN scenarios, therefore LDP configuration does merit mention.

This example also uses the topology shown in Figure 12-10. In this example, each router will be configured to run LDP. To configure LDP, issue the set ldp interface all and set ldp interface fxp0 disable commands under the [edit protocols] level of hierarchy. This will activate LDP on the router. Once again, the signaling protocol is disabled across fxp0 , as the management LAN will not need LSPs.

 [edit protocols]  lab@Chicago#  set ldp interface all  [edit protocols] lab@Chicago#  set ldp interface fxp0 disable  [edit protocols] lab@Chicago# show ldp {  interface all;   interface fxp0.0 {   disable;  } } 

The example below shows the output from a show route command issued on Chicago. To save space, the output has been truncated, and only the output for prefixes 192.168.24/24 and 192.168.8/24 is shown.

Some interesting information is learned from this output. There is no label associated with the 192.168.8 . x / n prefixes owing to their being only one hop away. There is a label associated with the 192.168.24 . x / n prefix. In this case BGP has preferred routes from inet.3 as was discussed earlier.

 lab@Chicago> show route  inet.0: 26 destinations, 26 routes (25 active, 0 holddown, 1 hidden) + = Active Route, - = Last Active, * = Both  192.168.8.0/24  *[BGP/170] 5d 23:32:52, MED 0, localpref 100, from 192.168.8.1                       AS path: I                     > to 10.0.15.1 via ge-3/0/0.0 192.168.8.1/32     *[IS-IS/18] 01:24:42, metric 10, tag 2                     > to 10.0.15.1 via ge-3/0/0.0 192.168.24.0/24    *[BGP/170] 5d 23:42:42, MED 0, localpref 100, from 192.168.24.1                       AS path: I                     > to 10.0.15.1 via ge-3/0/0.0,  Push 251047  192.168.24.1/32    *[IS-IS/18] 01:24:42, metric 30, tag 2                     > to 10.0.15.1 via ge-3/0/0.0 inet.3: 6 destinations, 6 routes (6 active, 0 holddown, 0 hidden) + = Active Route, - = Last Active, * = Both  192.168.8.1/32  *[  LDP/9  ] 00:14:18, metric 1                     > to 10.0.15.1 via ge-3/0/0.0 192.168.24.1/32    *[LDP/9] 00:13:55, metric 1                     > to 10.0.15.1 via ge-3/0/0.0,  Push 251047  

LDP is considered an LSPs-gone-wild situation in that they are created based upon the IGP routes installed already. When LDP is configured, LSPs are created for each of the destination prefixes listed in the routing table.

This section showed the steps necessary to build a minimal configuration for static, RSVP-based, and LDP- based LSPs. This section also showed that static LSPs are configured at every hop along the LSP, whereas signaled LSPs are configured only at the point of ingress. The case studies at the end of this chapter offer more details on configuring LSPs including the use of constraints.