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NE40E V800R010C10SPC500 Feature Description - Network Reliability 01

This is NE40E V800R010C10SPC500 Feature Description - Network Reliability
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BFD for Perlink

BFD for Perlink

Background

When two devices are directly connected over an Eth-Trunk, BFD sessions can be bound to VLANIF interfaces to monitor the connectivity of the Eth-Trunk between the two devices. If BFD for IP single-hop sessions are used, BFD will select one of the member links of the Eth-Trunk to monitor the status of the Eth-Trunk. Once BFD detects that the selected member link is Down, it considers the Eth-Trunk to be Down even if other member links of the Eth-Trunk are Up.

To prevent BFD from incorrectly detecting the Eth-Trunk state, you can deploy BFD for Perlink sessions instead of BFD for IP single-hop sessions. After a BFD for Perlink session is bound to a VLANIF interface, a unicast dynamic BFD sub-session is established for each Eth-Trunk member interface. Each unicast dynamic BFD sub-session independently negotiates and monitors the link status and reports the monitored link status to a MPU. BFD also establishes a management session, which processes the link status reported by the sub-sessions but does not negotiate or monitor the link status. The management session maintains its status based on the link status reported by the sub-sessions and then reports its status to the APP. With BFD for Perlink sessions, BFD considers the management session to be Down only when all the BFD sub-sessions go Down.

Implementation

The NE40E can use BFD sub-sessions to detect link faults in one-arm BFD echo mode. Figure 2-9 shows BFD for Perlink networking.

Figure 2-9 BFD for Perlink networking

Router1 is directly connected to Switch1 over an Eth-Trunk, and Router2 is directly connected to Switch2 over another Eth-Trunk. The two Eth-Trunks join the same VLAN respectively on Router1 and Router2. BFD is used to detect link faults between Router1 and Router2. After a BFD for Perlink session is established on Router1:
  1. Router1 establishes a BFD sub-session for each Eth-Trunk member link, constructs a BFD echo packet for each BFD sub-session, and sends the BFD echo packet to Switch1 through a specified member interface. In the BFD echo packet, the destination IP address is the IP address of Router1's VLANIF interface, and the destination MAC address is Router2's MAC address. MDs and YDs are dynamically established for BFD sub-sessions.
  2. Switch1 receives the BFD echo packet and forwards it to Switch2.
  3. After receiving the BFD echo packet, Switch2 uses a hash algorithm to select a link from the Eth-Trunk member links and then forwards the BFD echo packet to Router2 over the link.
  4. After receiving the BFD echo packet and finding that the packet's destination IP address is the IP address of Router1's VLANIF interface, Router2 immediately loops back the BFD echo packet to Switch2 at the forwarding layer.
  5. Switch2 receives the BFD echo packet and forwards it to Switch1.
  6. After receiving the BFD echo packet, Switch1 uses a hash algorithm to select a link from the Eth-Trunk member links and then forwards the BFD echo packet to Router1 over the link.

If Eth-Trunk member link 1 between Router1 and Switch1 fails, BFD sub-session 1 of the BFD for Perlink session goes Down. However, BFD sub-session 2 remains Up. Therefore, the status of the management session is Up, which prevents BFD from incorrectly reporting link faults.

In Step 6, Switch1 may select another Eth-Trunk member link that is not bound to the BFD sub-session to forward the BFD echo packet. In this case, because the preceding process takes less time to complete than a fiber fault-triggered traffic switchover, if Eth-Trunk member link 2 between Router1 and Switch1 fails, BFD sub-session 1's BFD echo packet from Router2 may be forwarded over Eth-Trunk member link 2. As BFD sub-sessions 1 and 2 are both Down, BFD will incorrectly consider the Eth-Trunk between Router1 and Router2 to be Down. However, there is only a low probability that this situation occurs.

When using BFD for Perlink, disable URPF on the device that sends BFD echo packets. Otherwise, BFD echo does not take effect.

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Updated: 2019-01-03

Document ID: EDOC1100055045

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