Targeted LDP Session
Normally, LDP sessions are set up between directly connected LSRs. In a network in which the IGP routes need to be labeled, this is sufficient, because the label switching of packets is hop per hop. Therefore, if the label bindings are advertised hop per hop for the IGP routes, the LSPs are set up. However, in some cases, a remote or targeted LDP session is needed. This is an LDP session between LSRs that are not directly connected. Examples in which the targeted LDP session is needed are AToM networks and TE tunnels in an MPLS VPN network. In the case of AToM, an LDP session must exist between each pair of PE routers. The remote LDP session is set up when configuring the xconnect command on the PE routers of the AToM network. In the case of TE tunnels in an MPLS VPN network, with the TE tunnels ending on a P router, the head-end and the tail-end LSR of the TE tunnel need a targeted LDP session between them to get the MPLS VPN traffic correctly label-switched through the MPLS VPN network. Refer to Chapter 8, "MPLS Traffic Engineering," for a detailed explanation on this. For directly connected neighbors, you only need to enable mpls ip on the interface; the LDP peers discover each other and create the LDP TCP session between them. For LDP neighbors that are not directly connected, the LDP neighborship needs to be configured manually on both the routers with the mpls ldp neighbor targeted command.
The syntax of the command is as follows:
mpls ldp neighbor [vrf vpn-name] ip-addr targeted [ldp | tdp]
The vrf refers to the Carrier's Carrier (CsC) scenarios in which the LDP sessions are established across VRF interfaces.
A targeted LDP neighbor can improve the label convergence time compared to the convergence time with directly connected LDP peers when there are flapping links. That is because when the link between two LSRs goes down, the LDP session is lost. With a targeted LDP session and an alternative path to get the LDP TCP packets from one LSR to the other, the LDP session stays up when the link between the two LSRs goes down. If the LDP session stays up, the labels are retained, improving the installment of the labels from the LIB into the LFIB when the link comes back up. Refer to the section "MPLS LDP Session Protection" later in this chapter to configure targeted LDP sessions with one command to protect the LDP sessions.
To change the LDP Hello interval and the Hold time for targeted LDP sessions, you can use the following command:
mpls ldp discovery {hello {holdtime | interval} seconds | targeted-hello {holdtime | interval} seconds | accept [from acl]}
Look at Figure 4-6. The routers new-york and sydney are not directly connected; however, you want them to have an LDP session between them. You can configure on both routers the LDP neighbor as targeted. Another way of achieving the same result is to configure the targeted LDP neighbor on one router only and to configure the other router to accept targeted LDP sessions from specific LDP routers. You do this by configuring the mpls ldp discovery targeted-hello accept [from acl] command. To prevent just any router from setting up an LDP session with this router, you can use the command with an access list so that you can specify which routers are allowed to set up a targeted LDP session.
Figure 4-6 Targeted Hello Accept Example Network
MPLS LDP Discovery Targeted-hello Accept From Accept-LDP !
IP Access-List Standard Accept-LDP Permit 10.200.254.4
MPLS LDP Neighbor 10.200.254.1 Targeted LDP
Targeted LDP Session
Loopback 0 10.200.254.1/32
Loopback 0 10.200.254.2/32
Loopback 0 10.200.254.3/32
"A
Loopback 0 10.200.254.4/32
new-york
10.200.210.1 Eth 1/2
10.200.218.1 10.200.218.2
10.200.211.1 10.200.211.2
sydney
POS 0/1
rome
Examples 4-14 and 4-15 show the configuration needed on the new-york and sydney routers to set up a targeted LDP session between them.
Example 4-14 Sydney Configuration for Targeted LDP
hostname sydney
mpls label protocol ldp mpls ldp neighbor 10.200.254.1 targeted ldp mpls ldp router-id Loopback0 force
Example 4-15 New-York Configuration for Targeted LDP new-york#conf t new-york(config)#mpls ldp discovery targeted-hello accept from accept-ldp new-york(config)#ip access-list standard accept-ldp new-york(config-std-nacl)#permit host 10.200.254.4
new-york(config-std-nacl)#~Z
new-york#
mpls ldp discovery targeted-hello accept from accept-ldp mpls ldp router-id Loopback0 force mpls label protocol ldp ip access-list standard accept-ldp permit 10.200.254.4
Example 4-16 shows the output of the show mpls ldp neighbor command for the targeted LDP session.
Example 4-16 Targeted LDP Session on Router New-York
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new-york#show mpls ldp neighbor 10.200.254. |
4 detail |
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Peer LDP Ident: 10.200.254.4:0; Local LDP Ident 10.200.254.1:0 |
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TCP connection: 10.200.254.4.22262 |
- 10.200.254.1.646 |
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State: Oper; Msgs sent/rcvd: 20/20; |
Downstream; Last TIB rev |
sent 120 |
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Up time: 00:03:10; UID: 5; Peer Id |
1; |
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LDP discovery sources: |
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Targeted Hello 10.200.254.1 -> H |
.200.254.4, passive; |
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holdtime: 90000 ms, hello interval: 10000 ms |
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Addresses bound to peer LDP Ident: |
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10.200.254.4 10.200.214.2 1 |
0.200.217.1 10.200.216 |
2 |
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Peer holdtime: 180000 ms; KA interval: 60000 ms; Peer state: |
estab |
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