Signaling the Pseudowire
A targeted LDP session between the PE routers signals the pseudowires. In essence, the signaling protocol LDP sets up and maintains the pseudowires between the PE routers, as shown in Figure 10-4. LDP has been extended with new Type Length Value fields (TLVs) to perform this job. The main purpose of this LDP session between the PE routers is to advertise the VC label that is associated with the pseudowire. This label is advertised in a Label Mapping message using the downstream unsolicited label advertisement mode.
Figure 10-4 Signaling by Targeted LDP Session
Targeted LDP Session
P P PE
LSR LSR LSR Attachment
Circuit (AC)
Attachment Circuit (AC)
You can see in Figure 10-5 how the VC and tunnel labels are advertised. The VC label is advertised by the egress PE to the ingress PE for the AC (VC ID 100) over the targeted LDP session. The tunnel label is the label advertised for the egress PE router to the ingress PE by LDP. Notice that the egress PE advertises label 3, which indicates that PHP is used.
Figure 10-5 Advertised Labels for AToM
Label 33 (Targeted LDP)
VC ID 100 PE P P PE VC ID 100
_ ^ r < LDP < LDP ^ r < LDP ^ _
* wC * Label 121 * * Label 88 * * Label 3 * ^ ^
Attachment LSR (LDP) LSR (LDP) LSR (LDP) LSR Attachment Circuit (AC) Circuit (AC)
The Label Mapping message that is advertised on the targeted LDP session contains some TLVs:
■ Pseudowire identifier (PW ID) FEC TLV
The PW ID FEC TLV identifies the pseudowire that the label is bound to. The Label TLV is the TLV that LDP uses to advertise the MPLS label.
The PW ID FEC TLV contains the following elements:
■ Interface Parameters
Figure 10-6 shows the format of the PW ID FEC TLV. Figure 10-6 The PW ID FEC TLV
0 12 3
01234567890123456789012345678901
PW tlv
PW Type
PW Info Length
Group ID
PW ID
Interface Parameters
The PW TLV value is set to 128 to indicate that this is the PW ID FEC TLV.
The C-bit—if set to 1—indicates that a control word is present. Refer to the section "The Control Word" for an explanation on the control word.
PW Type
The PW Type is a 15-bit field that represents the type of the pseudowire. Table 10-1 lists the assigned values for the PW Type field.
|
PW Type |
Description |
|
0x0001 |
|
|
0x0002 |
ATM AAL5 SDU2 VCC3 transport |
|
0x0003 |
ATM transparent cell transport |
|
0x0004 |
Ethernet Tagged Mode |
|
0x0005 |
Ethernet |
|
0x0006 |
HDLC |
|
0x0007 |
PPP |
|
0x0008 |
SONET/SDH CEM4 |
|
0x0009 |
ATM n-to-one VCC cell transport |
|
0x000A |
ATM n-to-one VPC cell transport |
|
PW Type |
Description |
|
0x000B |
IP Layer 2 Transport |
|
0x000C |
ATM one-to-one VCC cell mode |
|
0x000D |
ATM one-to-one VPC cell mode |
|
0x000E |
ATM AAL5 PDU5 VCC transport |
|
0x000F |
Frame-Relay Port mode |
|
0x0010 |
SONET/SDH CEP6 |
|
0x0011 |
Structure-Agnostic E1 over Packet (SAToP) |
|
0x0012 |
Structure-Agnostic T1 (DS1) over Packet (SAToP) |
|
0x0013 |
Structure-Agnostic E3 over Packet (SAToP) |
|
0x0014 |
Structure-Agnostic T3 (DS3) over Packet (SAToP) |
|
0x0015 |
CESoPSN7 basic mode |
|
0x0016 |
TDMoIP8 basic mode |
|
0x0017 |
CESoPSN TDM9 with CAS10 |
|
0x0018 |
TDMoIP TDM with CAS |
|
0x0019 |
Frame Relay DLCI |
1 DLCI = data-link connection identifier
2 SDU = service data unit
3 VCC = virtual channel connection
4 CEM = Circuit Emulation Service over MPLS
5 PDU = protocol data unit
6 CEP = Circuit Emulation over Packet
7 CESoPSN = Circuit Emulation Service over Packet Switched Network
8 TDMoIP = time-division multiplexing over IP
9 TDM = time-division multiplexing
10 CAS = channel associated signaling
Many pseudowire types can be configured. You can recognize the widely used protocols HDLC, PPP, Ethernet, ATM, and Frame Relay. Others refer to the possibility to carry time-division multiplexing (TDM) and circuit emulation service (CES) over MPLS. This book does not cover these last two services.
Group ID
The Group ID identifies a group of pseudowires. Cisco IOS assigns the same Group ID to all ACs on the same interface. The PE can use the Group ID to withdraw all of the VC labels that are associated with that Group ID in one LDP Label Withdraw message. This is referred to as a wild card label withdrawal.
392 Chapter 10: Any Transport over MPLS PW ID
The PW ID is a 32-bit connection identifier that together with the PW Type completely identifies the pseudowire. You can specify the PW ID in Cisco IOS on both PE routers with the xconnect peer-router-id vcid interface command. The PW ID is the VC ID as seen in the output of Cisco IOS commands.
Interface Parameters
The Interface Parameters describe some interface-specific parameters such as maximum transmission unit (MTU) of the interface toward the CE router, an optional interface description string, requested VLAN ID, and so on. If the MTU parameter does not match on both sides, the pseudowire is not signaled.
Because an LSP is unidirectional, a pseudowire can be formed only if another LSP exists in the opposite direction between the same pair of PE routers. The PW ID FEC TLV is used to identify and match the two opposite LSPs between a pair of PE routers.
Continue reading here: Signaling the Status of the Pseudowire
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