3.3.2.1. Pseudowire Switching TLV

The format of the pseudowire switching TLV is as follows:

Pseudowire Switching Point Sub-TLVs

The following are details specific to pseudowire switching point sub-TLVs:

3.3.2.2. Static-to-Dynamic Pseudowire Switching

When one segment of the pseudowire cross-connect at the S-PE is static while the other is signaled using T-LDP, the S-PE operates much like a T-PE from a signaling perspective and as an S-PE from a data plane perspective.

The S-PE signals a label mapping message as soon as the local configuration is complete. The control word C-bit field in the pseudowire FEC is set to the value configured on the static spoke-SDP.

When the label mapping for the egress direction is also received from the T-LDP peer, and the information in the FEC matches that of the local configuration, the static-to-dynamic cross-connection is effected.

Note that it is possible that end nodes of a static pseudowire segment be misconfigured. In this case, an S-PE or T-PE node may be receiving packets with the wrong encapsulation. In this case, it is possible that an invalid payload will be forwarded over the pseudowire or the SAP respectively. Also, if the S-PE or T-PE node is expecting the control word in the packet encapsulation and the received packet comes with no control word but the first nibble below the label stack is 0x0001, the packet may be mistaken for a VCCV OAM packet and may be forwarded to the CPM. In that case, the CPM will perform a check of the IP header fields such as version, IP header length, and checksum. If any of this fails the VCCV packet will be discarded.

3.3.3.1. VLL Resilience with Two Destination PE Nodes

Figure 16 shows the application of pseudowire redundancy to provide Ethernet VLL service resilience for broadband service subscribers accessing the broadband service on the service provider BRAS.

Figure 16:  VLL Resilience 

If the Ethernet SAP on PE2 fails, PE2 notifies PE1 of the failure by either withdrawing the primary pseudowire label it advertised or by sending a pseudowire status notification with the code set to indicate a SAP defect. PE1 will receive it and will immediately switch its local SAP to forward over the secondary standby spoke-SDP. To avoid black holing of in-flight packets during the switching of the path, PE1 will accept packets received from PE2 on the primary pseudowire while transmitting over the backup pseudowire.

When the SAP at PE2 is restored, PE2 updates the new status of the SAP by sending a new label mapping message for the same pseudowire FEC or by sending pseudowire status notification message indicating that the SAP is back up. PE1 then starts a timer and reverts back to the primary at the expiry of the timer. By default, the timer is set to 0, which means PE1 reverts immediately. A special value of the timer (infinity) will mean that PE1 should never revert back to the primary pseudowire.

The behavior of the pseudowire redundancy feature is the same if PE1 detects or is notified of a network failure that brought the spoke-SDP operational status to DOWN. The following are the events which will cause PE1 to trigger a switchover to the secondary standby pseudowire:

The Nokia routers support the ability to configure multiple secondary standby pseudowire paths. For example, PE1 uses the value of the user configurable precedence parameter associated with each spoke-SDP to select the next available pseudowire path after the failure of the current active pseudowire (whether it is the primary or one of the secondary pseudowires). The revertive operation always switches the path of the VLL back to the primary pseudowire though. There is no revertive operation between secondary paths meaning that the path of the VLL will not be switched back to a secondary pseudowire of higher precedence when the latter comes back up again.

-The Nokia routers support the ability for a user-initiated manual switchover of the VLL path to the primary or any of the secondary be supported to divert user traffic in case of a planned outage such as in node upgrade procedures.

3.3.4.1. Overview

The following sections describe the end-to-end solution with BGP PW-routing, assuming appropriate platforms are used for various functions.

Dynamic Multi-Segment Pseudowire Routing (Dynamic MS-PWs) enable a complete multi-segment pseudowire to be established, while only requiring per-pseudowire configuration on the T-PEs. No per-pseudowire configuration is required on the S-PEs. End-to-end signaling of the MS-PW is achieved using T-LDP, while multi-protocol BGP is used to advertise the T-PEs, so allowing dynamic routing of the MS-PW through the intervening network of S-PEs. Dynamic multi-segment pseudowires are described in the IETF in draft-ietf-pwe3-dynamic-ms-pw-13.txt.

Figure 17 shows the operation of dynamic MS-PWs.

Figure 17:  Dynamic MS-PW Overview 

The FEC 129 AII Type 2 structure shown in Figure 18 is used to identify each individual pseudowire endpoint:

Figure 18:  MS-PW Addressing using FEC129 AII Type 2 

A 4-byte global ID followed by a 4 byte prefix and a 4 byte attachment circuit ID are used to provide for hierarchical, independent allocation of addresses on a per service provider network basis. The first 8 bytes (Global ID + Prefix) may be used to identify each individual T-PE or S-PE as a loopback Layer 2 Address.

This new AII type is mapped into the MS-PW BGP NLRI (a new BGP AFI of L2VPN, and SAFI for network layer reachability information for dynamic MS-PWs. As soon as a new T- PE is configured with a local prefix address of global id:prefix, pseudowire routing will proceed to advertise this new address to all the other T- PEs and S-PEs in the network, as shown in Figure 19.

Figure 19:  Advertisement of PE Addresses by PW Routing 

In step 1 a new T-PE (T-PE2) is configured with a local prefix.

Next, in steps 2-5, MP-BGP will use the NLRI for the MS-PW routing SAFI to advertise the location of the new T-PE to all the other PEs in the network. Alternatively, static routes may be configured on a per T-PE/S-PE basis to accommodate non-BGP PEs in the solution.

As a result, pseudowire routing tables for all the S-PEs and remote T-PEs are populated with the next hop to be used to reach T-PE2.

VLL services can then be established, as shown in Figure 20.

Figure 20:  Signaling of Dynamic MS-PWs Using T-LDP 

In step 1 and 1' the T-PEs are configured with the local and remote endpoint information, Source AII (SAII), Target AII (TAII). On the 7210, the AIIs are locally configured for each spoke-SDP, according to the model shown in Figure 21. The 7210 therefore provides for a flexible mapping of AII to SAP. That is, the values used for the AII are through local configuration, and it is the context of the spoke-SDP that binds it to a specific SAP.

Figure 21:  Mapping of AII to SAP 

Before T-LDP signaling starts, the two T-PEs decide on an active and passive relationship using the highest AII (comparing the configured SAII and TAII) or the configured precedence. Next, the active T-PE (in the IETF draft this is referred to as the source T-PE or ST-PE) checks the PW Routing Table to determine the next signaling hop for the configured TAII using the longest match between the TAII and the entries in the PW routing table

This signaling hop is then used to choose the T-LDP session to the chosen next-hop S-PE. Signaling proceeds through each subsequent S-PE using similar matching procedures to determine the next signaling hop. Otherwise, if a subsequent S-PE does not support dynamic MS-PW routing and therefore uses a statically configured PW segment, the signaling of individual segments follows the procedures already implemented in the PW Switching feature. Note that BGP can install a PW AII route in the PW routing table with ECMP next-hops. However when LDP needs to signal a PW with matching TAII, it will choose only one next-hop from the available ECMP next-hops. PW routing supports up to 4 ECMP paths for each destination.

The signaling of the forward path ends when the PE matches the TAII in the label mapping message with the SAII of a spoke-SDP bound to a local SAP. The signaling in the reverse direction can now be initiated, which follows the entries installed in the forward path. The PW Routing tables are not consulted for the reverse path. This ensures that the reverse direction of the PW follows exactly the same set of S-PEs as the forward direction.

This solution can be used in either a MAN-WAN environment or in an Inter-AS/Inter-Provider environment as shown in Figure 22.

Figure 22:  VLL Using Dynamic MS-PWs, Inter-AS Scenario 

Note that data plane forwarding at the S-PEs uses pseudowire service label switching, as per the pseudowire switching feature.

3.3.4.2. Pseudowire Routing

Note: The platforms as described in this document can be configured as S-PE nodes.

The following sections describe the end-to-end solution with BGP PW-routing, assuming appropriate platforms are used for various functions.

Each S-PE and T-PE has a pseudowire routing table that contains a reference to the T-LDP session to use to signal to a set of next hop S-PEs to reach a specific T-PE (or the T-PE if that is the next hop). For VLLs, this table contains aggregated AII Type 2 FECs and may be populated with routes that are learned through MP-BGP or that are statically configured.

MP-BGP is used to automatically distribute T-PE prefixes using the new MS-PW NLRI, or static routes can be used. The MS-PW NLRI is composed of a Length, an 8-byte RD, a 4-byte Global-ID, a 4-byte local prefix, and (optionally) a 4-byte AC-ID. Support for the MS-PW address family is configured in CLI under config>router>bgp>family ms-pw.

MS-PW routing parameters are configured in the config>service>pw-routing context.

To enable support for dynamic MS-PWs on a 7210 node to be used as a T-PE or S-PE, a single, globally unique, S-PE ID, known as the S-PE Address, is first configured under config>service>pw-routing on each 7210 to be used as a T-PE or S-PE. The S-PE Address has the format global-id:prefix. It is not possible to configure any local prefixes used for pseudowire routing or to configure spoke SPDs using dynamic MS-PWs at a T-PE unless an S-PE address has already been configured. The S-PE address is used as the address of a node used to populate the switching point TLV in the LDP label mapping message and the pseudowire status notification sent for faults at an S-PE.

Each T-PE is also be configured with the following parameters:

For each local prefix, BGP then advertises each global ID/prefix tuple and unique RD and community pseudowire using the MS-PW NLRI, based on the aggregated FEC129 AII Type 2 and the Layer 2 VPN/PW routing AFI/SAFI 25/6, to each T-PE/S-PE that is a T-LDP neighbor, subject to local BGP policies.

The dynamic advertisement of each of these pseudowire routes is enabled for each prefix and RD using the advertise-bgp command.

An export policy is also required to export MS-PW routes in MP-BGP. This can be done using a default policy, such as the following.

However, this would export all routes. A recommended choice is to enable filtering per-family, as follows.

The following command is then added in the config>router>bgp context.

Local preference for iBGP and BGP communities can be configured under such a policy.

3.3.4.3. Configuring VLLs using Dynamic MS-PWs

One or more spoke SDPs may be configured for distributed Epipe VLL services. Dynamic MS-PWs use FEC129 (also known as the Generalized ID FEC) with Attachment Individual Identifier (AII) Type 2 to identify the pseudowire, as opposed to FEC128 (also known as the PW ID FEC) used for traditional single segment pseudowires and for pseudowire switching. FEC129 spoke SDPs are configured under the spoke-sdp-fec command in the CLI.

FEC129 AII Type 2 uses a Source Attachment Individual Identifier (SAII) and a Target Attachment Individual Identifier (TAII) to identify the end of a pseudowire at the T-PE. The SAII identifies the local end, while the TAII identifies the remote end. The SAII and TAII are each structured as follows:

3.3.4.4. Pseudowire Redundancy

Pseudowire redundancy is supported on dynamic MS-PWs used for VLLs. It is configured in a similar manner to pseudowire redundancy on VLLs using FEC128, whereby each spoke-sdp-fec within an endpoint is configured with a unique SAII/TAII.

Figure 23 shows the use of pseudowire redundancy.

Figure 23:  Pseudowire Redundancy 

The following is a summary of the key points to consider in using pseudowire redundancy with dynamic MS-PWs:

If the primary MS-PW fails, fail-over to a standby MS-PW, as per the normal pseudowire redundancy procedures. A configurable retry timer for the failed primary MS-PW is then started. When the timer expires, attempt to reestablish the primary MS-PW using its original path, up to a maximum number of attempts as per the retry count parameter. The T-PE may then optionally revert back to the primary MS-PW on successful reestablishment.

Note that because the SDP ID is determined dynamically at signaling time, it cannot be used as a tie breaker to choose the primary MS-PW between multiple MS-PWs of the same precedence. The user should therefore explicitly configure the precedence values to determine which MS-PW is active in the final selection.

3.3.4.5. VCCV OAM for Dynamic MS-PWs

The primary difference between dynamic MS-PWs and those using FEC128 is support for FEC129 AII type 2. As in PW Switching, VCCV on dynamic MS-PWs requires the use of the VCCV control word on the pseudowire. Both the vccv-ping and vccv-trace commands support dynamic MS-PWs.

3.3.4.6. VCCV-Ping on Dynamic MS-PWs

VCCV-ping supports the use of FEC129 AII type 2 in the target FEC stack of the ping echo request message. The FEC to use in the echo request message is derived in one of two ways: Either the user can specify only the spoke-sdp-fec-id of the MS-PW in the vccv-ping command, or the user can explicitly specify the SAII and TAII to use.

If the SAII:TAII is entered by the user in the vccv-ping command, then those values are be used for the VCCV ping echo request, but their order is be reversed before being sent so that they match the order for the downstream FEC element for an S-PE, or the locally configured SAII:TAII for a remote T-PE of that MS-PW. Note that is SAII:TAII is entered in addition to the spoke-sdp-fec-id, then the system will verify the entered values against the values stored in the context for that spoke-sdp-fec-id.

Otherwise, if the SAII:TAII to use in the target FEC stack of the VCCV ping message is not entered by the user, and if a switching point TLV was previously received in the initial label mapping message for the reverse direction of the MS-PW (with respect to the sending PE), then the SAII:TAII to use in the target FEC stack of the VCCV ping echo request message is derived by parsing that switching point TLV based on the user-specified TTL (or a TTL of 255 if none is specified). In this case, the order of the SAII:TAII in the switching point TLV is maintained for the VCCV ping echo request message.

If no pseudowire switching point TLV was received, then the SAII:TAII values to use for the VCCV ping echo request are derived from the MS-PW context, but their order is reversed before being sent so that they match the order for the downstream FEC element for an S-PE, or the locally configured SAII:TAII for a remote T-PE of that MS-PW. Note that the use of spoke-sdp-fec-id in vccv-ping is only applicable at T-PE nodes, because it is not configured for a specific MS-PW at S-PE nodes.

3.3.4.7. VCCV-Trace on Dynamic MS-PWs

The 7210 supports the MS-PW path trace mode of operation for VCCV trace, as per pseudowire switching, but using FEC129 AII type 2. As in the case of VCCV ping, the SAII:TAII used in the VCCV echo request message sent from the T-PE or S-PE from which the VCCV trace command is executed is specified by the user or derived from the context of the MS-PW. Note that the use of spoke-sdp-fec-id in vccv-trace is only applicable at T-PE nodes, since it is not configured for a specific MS-PW at S-PE nodes.

3.4.1.1. VLL Resilience

Figure 26 shows a VLL resilience path example. A sample configuration follows.

Figure 26:  VLL Resilience 

Note that a revert-time value of zero (default) means that the VLL path will be switched back to the primary immediately after it comes back up

PE3

3.4.1.2. VLL Resilience for a Switched PW Path

Note: The 7210 SAS platforms as described in this document can be configured as S-PE nodes.

Figure 27 shows a VLL resilience for a switched pseudowire path example. A sample configuration follows.

Figure 27:  VLL Resilience with Pseudowire Switching 

Configuration

S-PE1

VC switching indicates a VC cross-connect so that the service manager does not signal the VC label mapping immediately but will put this into passive mode.

3.5.2.1. Processing Endpoint SAP Active/Standby Status Bits

The advertised admin forwarding status of active/standby reflects the status of the local LAG SAP in MC-LAG application. If the SAP is not part of a MC-LAG instance, the forwarding status of active is always advertised.

When the SAP in endpoint “X” is part of a MC-LAG instance, a node must send T-LDP forwarding status bit of “SAP active/standby” over all “Y” endpoint spoke SDPs, except the ICB spoke-SDP, whenever this status changes. The status bit sent over the ICB is always zero (active by default).

When the SAP in endpoint “X” is not part of a MC-LAG instance, then the forwarding status sent over all “Y” endpoint spoke-SDPs should always be set to zero (active by default).

3.5.2.2. Processing and Merging

Endpoint “X” is operationally up if at least one of its objects is operationally up. It is down if all its objects are operationally down.

If the SAP in endpoint “X” transitions locally to the down state, or received a SAP down notification by SAP-specific OAM signal, the node must send T-LDP SAP down status bits on the “Y” endpoint ICB spoke-SDP only. Note that Ethernet SAP does not support SAP OAM protocol. All other SAP types cannot exist on the same endpoint as an ICB spoke-SDP since non Ethernet SAP cannot be part of a MC-LAG instance.

If the ICB spoke-SDP in endpoint “X” transitions locally to down state, the node must send T-LDP SDP-binding down status bits on this spoke-SDP.

If the ICB spoke-SDP in endpoint “X” received T-LDP SDP-binding down status bits or pseudowire not forwarding status bits, the node saves this status and takes no further action. The saved status is used for selecting the active transmit endpoint object.

If all objects in endpoint “X” transition locally to down state, and/or received a SAP down notification by remote T-LDP status bits or by SAP specific OAM signal, and/or received status bits of SDP-binding down, and/or received status bits of pseudowire not forwarding, the node must send status bits of SAP down over all “Y” endpoint spoke SDPs, including the ICB.

Endpoint “Y” is operationally up if at least one of its objects is operationally up. It is down if all its objects are operationally down.

If a spoke-SDP in endpoint “Y”, including the ICB spoke-SDP, transitions locally to down state, the node must send T-LDP SDP-binding down status bits on this spoke-SDP.

If a spoke-SDP in endpoint “Y”, including the ICB spoke-SDP, received T-LDP SAP down status bits, and/or received T-LDP SDP-binding down status bits, and/or received status bits of pseudowire not forwarding, the node saves this status and takes no further action. The saved status is used for selecting the active transmit endpoint object.

If all objects in endpoint “Y”, except the ICB spoke-SDP, transition locally to down state, and/or received T-LDP SAP down status bits, and/or received T-LDP SDP-binding down status bits, and/or received status bits of pseudowire not forwarding, the node must send status bits of SDP-binding down over the “X” endpoint ICB spoke-SDP only.

If all objects in endpoint “Y” transition locally to down state, and/or received T-LDP SAP down status bits, and/or received T-LDP SDP-binding down status bits, and/or received status bits of pseudowire not forwarding, the node must send status bits of SDP-binding down over the “X” endpoint ICB spoke-SDP, and must send a SAP down notification on the “X” endpoint SAP by the SAP specific OAM signal if applicable. An Ethernet SAP does not support signaling status notifications.

3.8.1.1. SAP Encapsulations

The Epipe service is designed to carry Ethernet frame payloads, so it can provide connectivity between any two SAPs that pass Ethernet frames. The following SAP encapsulations are supported on the Epipe service:

Note that while different encapsulation types can be used, encapsulation mismatch can occur if the encapsulation behavior is not understood by connecting devices and are unable to send and receive the expected traffic. For example if the encapsulation type on one side of the Epipe is dot1q and the other is null, tagged traffic received on the null SAP will potentially be double tagged when it is transmitted out of the dot1q SAP.

3.8.1.2. QoS Policies

Traffic Management - Traffic management of Ethernet VLLs is achieved through the application of ingress QoS policies to SAPs and access egress QoS policies applied to the port. All traffic management is forwarding-class aware and the SAP ingress QoS policy identifies the forwarding class based on the rules configured to isolate and match the traffic ingressing on the SAP. Forwarding classes are determined based on the Layer 2 (Dot1p, MAC) or Layer 3 (IP, DSCP) fields of contained packets and this association of forwarding class at the ingress will determine both the queuing and the Dot1P bit setting of packets on the Ethernet VLL on the egress.

SAP ingress classification and Policing - The traffic at the SAP ingress is classified and metered according to the SLA parameters. All the traffic ingressing on the SAP is classified to a particular forwarding class. All the forwarding class is metered through and marked in-profile or put-profile based on the Meter parameters.

When applied to Epipe services, service ingress QoS policies only create the unicast defined in the policy. The multipoint are not created on the service. Note that both Layer 2 or Layer 3 criteria can be used in the QoS policies for traffic classification in a service.

Egress Network DOT1P Marking - Marking of IEEE DOT1P bits in VLAN tag is as per the FC-to-Dot1p map. For details see the default network QoS policy in the 7210 SAS-R6, R12 Quality of Service Guide. This marking is applied at the port level on access ports and access uplink ports.

Ingress Network Classification - Ingress network classification is based on the Dot1p bits in the outer VLAN tag received on the access uplink port. Dot1p-to-FC mapping is based on the network ingress QoS policy.

3.8.1.3. Filter Policies

7210 SAS Epipe services can have a single filter policy associated on both ingress and egress. Both MAC and IP filter policies can be used on Epipe services.

3.8.1.4. MAC Resources

Epipe services are point-to-point Layer 2 VPNs capable of carrying any Ethernet payloads. Although an Epipe is a Layer 2 service, the 7210 SAS-R6 and 7210 SAS-R12 Epipe implementation does not perform MAC learning on the service, so Epipe services do not consume any MAC hardware resources.