LDP Principle Configuration

DC011001 LDPAll rights reserved
In MPLS network, we often adopt LDP as the label distribution protocol in an MPLS domain to set up LSPs, this slide introduces the configuration of how to establish LDP sessions and how to perform label mapping, release, and so on.
HUAWEI TECHNOLOGIES CO., LTD.
Upon completion of this course, you will be able to:
Describe the LDP configuration commands
Know typical applications of LDP
Describe how to monitor the information of LDP
Optional Configuration
Preconfigured Tasks
LDP configuration
LDP’s Configuration include mandatory parts and optional parts. The mandatory parts have several steps as follows, the first two steps are preconfigured tasks:
1.Configure MPLS LSR-ID
Configure MPLS LSR ID should be configured before configuring any other MPLS commands.
The LSR ID is usually the IP address of a loopback interface and which should be unique in the MPLS domain.
By default, LSR ID is not specified. You must specify the LSR ID, and the LSR ID need to be reachable as an IP address by other peer routers.
2. Enable MPLS in system view
All the routers participating in MPLS forwarding need to be configured with MPLS basic capability.
3.Enable MPLS LDP in system view
Adopt LDP as the label distribution protocol in a MPLS domain to setup LSP, LDP sessions should be established between LSRs along the LSP to perform label mapping, release, and so on
4.Enable MPLS&LDP in interface view
Enable interface’s MPLS and LDP function, to receive and send MPLS and LDP’s PDUs.
The optional parts are:
Configuring Label Distribution and Retention Mode (optional)
Configuring Loop Detection (optional)
Configuring LDP MD5 Authentication (optional)
Configuring LDP Support MTU Signaling
Proper functioning of path MTU detection requires that IP routers have knowledge of the MTU for each link to which they are connected
By default, MTU signaling is enabled in the system.
Note:
Enabling or disabling the MTU signaling feature might cause the original session to be recreated. The LSP(s) created on this session will also be deleted and hence will be recreated.
Next slides will introduce some of the optional configurations.
Enable mpls ldp in system view
mpls ldp enable LDP for a public instance or for a VPN instance.
Use this command after the MPLS LSR ID has been configured and MPLS is globally enabled.
[Quidway]
Enable mpls ldp in interface view
Disabling LDP on the interface can result in interruption of all LDP sessions on the interface. All the LSPs based on these sessions will be deleted accordingly.
Configure Remote LDP
Configure remote peer IP address
There is only one LDP session between two LSRs, and the priority of local LDP is higher than remote LDP
Up to now, remote LDP is only used to transmit message for L2VPN
Remote peer is used to set up a remote session with remote router and exchange some information which shouldn’t be known by middle routes. Up to now, remote LDP is only used to transmit message for L2VPN, when configure L2VPN, we need to configure remote peer.
When a local adjacency already exists with the remote peer, the remote adjacency will not be created. On the other hand, when a remote adjacency exists and a local adjacency is created for the remote peer, the remote-peer will be deleted. Hence, between two routers, either a remote session or a local session can exist with the higher preference for the local session.
[Quidway-Serial1]
[Quidway-Serial1]
mpls ldp timer keepalive-hold seconds
Configure the session hold timer.
By default, the keepalive hold value is 45 seconds for both local and remote sessions.
Configure Parameters for LDP Session
Using the mpls ldp advertisement command, you can configure label advertisement mode to distribution either on demand or distribution unsolicited.
By default, the label advertisement mode is configured as Downstream Unsolicited.
It should be noted that the modification of advertisement mode might cause the original session(s) to be recreated. The LSP(s) created on the basis of this session will also be deleted and hence will be recreated.
Parameters:
dod: configures label advertisement mode to be in downstream on demand. This mode allows the LSR to distribute a FEC label binding or mapping in response to an explicit request from another LSR.
du: configures label advertisement mode to be in downstream unsolicited. This mode allows the LSR to distribute label binding or mapping to LSRs that have not explicitly requested them.
Using the mpls ldp timer keepalive-hold command, you can configure keepalive hold timer value
This command in interface view is for local keepalive-hold timer, and this command in remote-peer view is for remote keepalive-hold timer.
By default, the keepalive hold value is 45 seconds.
In case of multi-link, where there is more than one link between two LSRs with LDP enabled on all, it is important that the keepalive timer value on all the links is kept the same. If not, it is possible that the session may be unstable
Configure Label Distribution and Retention Mode
[Quidway]
Configure label retention mode.
By default, label distribution control mode is ordered; label retention mode is liberal; re-advertising labels in DU mode is enabled.
Parameter:
independent: configures label distribution control mode to be independent. The LSR advertises label mappings to its neighbors at any desired time.
ordered: configures label distribution control mode to be ordered. An LSR initiates the transmission of a label mapping only for a FEC, for which it has received a label mapping from the downstream LSR or for which it is the egress LSR.
liberal: configures label retention mode to be liberal. In Liberal mode every label mapping received from a peer LSR is retained regardless of whether the peer LSR is the next hop for the advertised mapping.
conservative: configures label retention mode to be conservative. In conservative mode the advertised label mappings are retained only if they will be used to forward packets (that is, if they are received from a valid next hop according to routing information).
Note:
When the label retention mode is changed from conservative to liberal, it will have an effect on the entries due to du-readvertisement.
When the mode is changed from liberal to conservative, the liberal entries will not be deleted. If the lsp's are configured or changed later, the lsp content is unpredictable.
By default, loop detection is disabled.
To perform loop detection for the MPLS domain, you must configure loop detection on all LSRs.
Loop detection includes two modes, the maximum hop count mode and the path vector mode.
Configure Loop Detection
ranges from 1 to 32
When the maximum hop count is adopted for loop detection, if the maximum value is exceeded, a loop happens and the LSP establishment fails. By default, the maximum hop count is 32.
When the path vector mode is adopted for loop detection, it is also necessary to specify the maximum hops of the LSP. By default, the maximum hop count of path vector is 32. When one of the following conditions is met, it is considered that a loop happens and the LSP setup fails. The record of the local LSR already exists in the path vector record table; the path hop count exceeds the maximum value set here.
Note:
Loop detection configuration takes effect only on the LSP established after modification. That is, the modification of loop detection does not take effect on the LSP that is already set up. You must restart LDP to make new loop detection configuration take effect on all the LDPs.
By default, the MD5 authentication is disabled.
To improve the security of LDP sessions, MD5 authentication can be used for the setup of TCP connections used by LDP.
[Quidway-mpls-ldp]
Configure LDP MD5 Authentication
By default, the md5 password feature is disabled in the system for an instance, and when configured, the password must be the same as in with the peer’s configuration.
Parameters:
plain: displays the current configuration information of the password directly as plain text.
cipher: displays the current configuration information of the password in an encrypted format.
peer-lsr-id: specifies the LSR ID of the peer with which the LSR shares the password.
password: specifies the password string. The length of the string ranges from 8 to 16. If the password type is cipher, a 24 character encrypted cipher password can be input.
Network Requirements
RTA, RTB and RC support MPLS, and run routing protocol between them.
Establish local LDP sessions between RTA and RTB, also between RTB and RTC. Establish a remote LDP session between RTA and RTC.
RTA
RTB
RTC
L0:1.1.1.9
L0:2.2.2.9
L0:3.3.3.9
POS:10.1.1.0/24
POS:10.2.2.0/24
Local LDP
Local LDP
Remote LDP
Look about the network, the requirement that RTA, RTB and RTC should support MPLS, first configure IP address to connect with each other, and then configure IGP to make sure that all the Loopback interface is reachable.
Configure two local LDP and a remote LDP between RTA and RTC, the configuration steps please refer to next slide.
[RouterA] mpls lsr-id 1.1.1.9
“mpls ldp enable”
Basic Configuration
lsp-trigger { all | host | ip-prefix prefix-name | none }
Using the lsp-trigger command, you can set to establish LSP according to the 32-bit IP route or IP prefix list.
Parameters:
all: specifies all static routes and IGP routing entries triggering to establish an LSP . .
host: specifies the 32-bit host IP routes triggering to establish an LSP.
prefix-name: specifies the name of the IP address prefix list.
none: specifies no needs to establish LSP.
By default, all IGP route entry cannot trigger LDP to setup LSP.
The 32-bit IP routes and IP prefix list are only valid for IGP and static route, not for BGP route. If the parameters of host and ip-prefix prefix-name are not configured, all static routes and IGP routes can trigger LDP to setup LSP.
[RouterA-mpls-ldp-remote-peerc] remote-ip 3.3.3.9
Configuration Procedure
Basic Configuration
remote-ip ip-address
This command configures the IP address of a remote peer. The address should be the LSR ID of the remote LSR. For remote peers, as they adopt LSR ID as their transport address, two remote peers take their LSR ID as their transport addresses for creating TCP connection.
Label Advertisement Mode
the router’s role in the session, the larger LSR ID router will be Activethe less one will be Passive
[RouterA] display mpls ldp session
LDP Session(s) in Public Network
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Reference:
LDP Session(s) in Public Network
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TCP Connection : 1.1.1.9 <- 2.2.2.9
Session FT Flag : On MD5 Flag : Off
Reconnect Timer : 100 Sec Recovery Timer : 300 Sec
Negotiated Keepalive Timer : 45 Sec
Keepalive Message Sent/Rcvd : 24/24 (Message Count)
Label Advertisement Mode : Downstream Unsolicited
Label Resource Status(Peer/Local) : Available/Available
Session Age : 002:20:39 (DDD:HH:MM)
10.1.1.2 2.2.2.9
Session FT Flag: The GR FT flag
Addresses received from peer: The address list, which is received from the peer on this session
LDP Peer Information in Public network
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display mpls ldp peer verbose can display more detailed information
Common Commands
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Recovery Timer : 300 Sec Reconnect Timer : 300 Sec
Peer Label Advertisement Mode : Downstream Unsolicited
Peer Discovery Source : Pos6/0/0
Peer FT Flag : Peer’s GR FT flag.
Reconnect Timer : Peers reconnect timer value. (This is applicable only GR FT flag is set)
Recovery Timer : Peer’s recovery timer value. . (This is applicable only GR FT flag is set)
Peer Discovery Source : All sources through which this peer has been discovered
LDP LSP Information
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A '*' before an LSP means the LSP is not established
A '*' before a Label means the USCB or DSCB is stale
Common Commands
Some Item Description:
SN : The serial number of this LSPA * character preceding the SN indicated a non-established LSP
DestAddress/Mask : The destination address and mask of the FEC of this LSP
In/OutLabel : The in and out label value. A * value before the IN-label or out-label indicates that the LSP is in GR process
Next-Hop : The next hop address
In/Out-Interface : The in interface and out interface names respectively separated by a ‘/ ’
USCB/DSCB : UpStream Control Block / Down Stream Control Block
All rights reserved
This slide includes LDP basic configuration, local and remote LDP configuration and one case, at last introduces some common commands for information monitor.
Summary
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LDP Principle Configuration