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[Transport Bit] Talking about Broadcast Storm (1) - Scenarios

l00302393 Senior Huawei Staff

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E-LAN services are convenient for native Ethernet transmission. But an E-LAN loop may cause MAC address flapping and even a broadcast storm. If a broadcast storm occurs, a large amount of port bandwidth is occupied, interrupting services and resulting in protocol exceptions. Therefore, services must be carefully planned and deployed to avoid E-LAN loops.

Introduction

E-LAN service

E-LAN services are Layer 2 services switched based on MAC addresses. Equipment implements E-LAN services through virtual bridges (VBs) or VSIs.
Broadcast packet

MAC-layer broadcast packets refer to Ethernet packets whose destination MAC address is all 1s (that is, FF-FF-FF-FF-FF-FF).
Broadcast storm

Within a VB (or router) for E-LAN services, broadcast packets are copied and forwarded to all ports except the source port. Layer 2 packets do not carry any fields similar to the TTL field in IP packets to limit the number of forwarding times. Therefore, if an E-LAN loop exists, broadcast packets will be copied and forwarded repeatedly and a broadcast storm occurs.

E-LAN Loop Scenario 1: Port Self-Loop

In this scenario, a port configured for E-LAN services is looped through an optical fiber, a self-loop connector, or software. As a result, the port receives all packets that are transmitted by itself.

When a single port is self-looped, the number of broadcast packets multiples, but generally no broadcast storm will occur. However, because broadcast packets are looped, the source MAC address flaps between the source port and self-looped port in the forwarding table. As a result, packets cannot be properly forwarded and packet loss occurs.

Figure 1 A single port being self-looped
When multiple ports are self-looped, a loop is formed, causing a broadcast storm.

Figure 2 Multiple ports being self-looped

E-LAN Loop Scenario 2: Incorrect Port Interconnection

If two ports on an NE configured for an E-LAN service are connected by a network cable, a loop is formed and a broadcast storm occurs.

Figure 3 A loop caused by incorrect port interconnection

E-LAN Loop Scenario 3: Redundant Links

If two E-LANs are connected to each other through multiple links and no LAG/PLA, RSP, or split horizon group is configured, redundant links for broadcast packets are formed. E-LAN services in this scenario include IEEE 802.1d-based E-LAN services, IEEE 802.1q- or IEEE 802.1ad-based E-LAN services with the same VLAN ID, and VPLS services configured with multiple PWs.

NOTE:

Redundant links between NEs may be microwave or Ethernet links.

Figure 4 A loop caused by a redundant link

E-LAN Loop Scenario 4: Abnormal ERPS Protocol

On an ERPS-protected ring network, all ports are in the unblocked state due to an incorrect protocol configuration or abnormal protocol running state, forming a loop.

Figure 5 A loop caused by the abnormal ERPS protocol

E-LAN Loop Scenario 5: Incorrect Configuration on an External Network

Incorrect configurations or connections on an external network may cause an E-LAN loop. For example, E-LAN+E-Line services are looped at the remote end, or a third-party redundant link exists between two E-LANs.

Figure 6 A loop caused by incorrect configurations on an external network