The following describes the events for switchover in Figure 1. This configuration uses operational groups. The nodes of interest are A, B, and C listed in Table 1.
A single G.8031 SAP that represents the control for a group of G.8031 SAPs is configured on the CE.
The Control SAP does not normally carry any data; however, it can if needed.
An Epipe service is provisioned on each PE node (B/C), only for control (no customer traffic flows over this service).
On CE A, there is an Epipe Ethernet tunnel (G.8031) control SAP.
The Ethernet tunnel has two paths:
one facing B
one facing C
PE B has an Epipe control SAP that is controlled by the BGP-MH site and PE C also has the corresponding SAP that is controlled by the same BGP-MH site.
At node A, there are MEPs configured under each path that check connectivity on the A-B and A-C links. At nodes B and C, there is a MEP configured under their respective SAPs with fault propagation enabled with the use of ifStatusTlv.
Initially, assume there is no link failure:
SAPs on node A have ifStatusTLV No Fault to B and C (no MEP fault detected at A); see Table 1 row 1 (Fault is signaled in the other direction PE to CE).
BGP-MH determines which is the master or Designated Forwarder (DF).
Assume SAP on node B is picked as the DF.
The MEP at Path A-B signals ifStatusTlv No Fault. Because of this signal, the MEP under the node A path facing node B detects the path to node B is usable by the path manager on A.
At the CE node A, Path A-C becomes standby and is brought down; see Table 1 row 2.
Because fault propagation is enabled under the SAP node C MEP, and ifStatusTLV is operationally Down, the Path remains in the present state.
Under these conditions, the MEP under the node A path facing node C detects the fault and informs Ethernet manager on node A.
Node A then considers bringing path A-C down.
ET port remains up ecause the path A-B is operationally up. This is a stable state.
On nodes B and C, each Epipe-controlled SAP is the sole (controlling) member of an operational group.
Other data SAPs may be configured for fate shared VLANs (Ethernet tunnels) and to monitor the control SAP.
The SAPs facing the CE node A share the fate of the control SAP and follow the operation.
If there is a break in path A-B connectivity (CCM timeout or LOS on the port for link A-B), then on node A the path MEP detects connectivity failure and informs Ethernet tunnel manager; see Table 1 row 4.
At this point, the Ethernet tunnel is down because both path A-B and path A-C are down.
The CE node A Ethernet tunnel goes down.
At node B on the PE, the SAP also detects the failure and the propagation of fault status goes to BGP-MH; see Table 1 row 4.
This in turn feeds into BGP-MH, which deems the site non-DF and makes the site standby.
Because the SAP at node B is standby, the service manager feeds this to CFM, which then propagates a Fault toward node A. This is a cyclic fault propagation. However, because path A-B is broken, the situation is stable; see Table 1 row 5.
There is traffic loss during the BGP-MH convergence.
Load sharing mode is recommended when using a 7450 as a CE node A device.
BGP-MH signals that node C is now the DF; see Table 1 row 3.
BGP-MH on node C elects a SAP and brings it up.
ET port transitions to port A-C, and is operationally up. This is a stable state. The A-C SAPs monitoring the operational group on C transitions to operationally up.
Unidirectional failures: at point 6 the failure was detected at both ends. In the case of a unidirectional failure, CCM times out on one side.
In the case where the PE detects the failure, it propagates the failure to BGP-MH and the BGP-MH takes the site down causing the SAPs on the PE to signal a Fault to the CE.
In the case where G.8031 on the CE detects the failure, it takes the tunnel down and signals a fault to the PE, and then the SAP propagates that to BGP-MH.