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S9300, S9300E, and S9300X V200R012C00 Configuration Guide - Ethernet Switching

This document describes the configuration of Ethernet services, including configuring MAC address table, link aggregation, VLANs, VLAN aggregation, MUX VLAN, VLAN termination, Voice VLAN, VLAN mapping, QinQ, GVRP, STP/RSTP/MSTP, VBST, SEP, RRPP, ERPS, LBDT, HVRP, and Layer 2 protocol transparent transmission.
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Understanding VBST

Understanding VBST

VBST is equivalent to running STP or RSTP in each VLAN so that spanning trees in different VLANs are independent of each other. Though VBST does not provide multi-instance, VBST implements load balancing of traffic from different VLANs.

VBST inherits the following concepts of STP/RSTP:
Difference between VBST and STP/RSTP:
  • Bridge ID (BID)

    In VBST, the BID consists of the bridge priority, VLAN ID, and bridge MAC address. The sum of the bridge priority and VLAN ID occupies the leftmost 16 bits, and the bridge MAC address occupies the rightmost 48 bits.

    On a VBST network, the device with the smallest bridge ID will be selected as the root bridge.

  • VBST transmits VBST BPDUs in VLANs to determine the network topology. VBST BPDUs are based on STP/RSTP BPDUs and a 4-byte 802.1Q tag is added between the source MAC address and protocol length. Figure 15-1 shows the comparisons between the STP/RSTP BPDU and VBST BPDU.

    Figure 15-1  Comparisons between the formats of the STP/RSTP BPDU and VBST BPDU

    The DMAC identifies the destination MAC address of packets. The DMAC in a VBST BPDU is 0100-0CCC-CCCD; the Data field in a standard RSTP/STP BPDU is used as the Data field in a VBST BPDU. By default, the Data field in a standard RSTP BPDU is used as the Data field in a VBST BPDU.

    VBST implements VLAN-based spanning tree calculation, topology convergence, and interworking with spanning tree protocols of other vendors.

VBST Topology Calculation

VBST supports VLAN-based topology calculation. Tagged VBST BPDUs are sent in each VLAN except VLAN 1 and topology calculation is performed separately. The VBST topology calculation method is similar to the STP/RSTP calculation method. For details, see STP Topology Calculation. Different root bridges can be selected in VLANs. Figure 15-2 shows the topology calculation results of STP/RSTP and VBST.

Figure 15-2  Topology calculation results of STP/RSTP and VBST



In Figure 15-2:
  • Through topology calculation, STP/RSTP generates a spanning tree with the root bridge as S6. The links between S2 and S5 and between S1 and S4 are blocked. HostA and HostB belong to VLAN 2. The link between S2 and S5 does not permit packets of VLAN 2 to pass through because the link between S2 and S5 is blocked. Therefore, HostA fails to communicate with HostB.

  • Through topology calculation, VBST generates spanning trees VLAN 2 and VLAN 3 with root bridges as S4 and S6 respectively. Traffic in VLAN 2 and VLAN3 is forwarded through their respective spanning trees so that traffic is load balanced between paths S2-S5 and S3-S6.

Fast Convergence of VBST

VBST supports the Proposal/Agreement mechanism in common and enhanced modes:
  • Common mode

    The Proposal/Agreement mechanism in common mode supported by VBST is similar to that supported by RSTP. For details, see Technical Details of RSTP.

  • Enhanced mode

    The Proposal/Agreement mechanism in enhanced mode supported by VBST is similar to that supported by MSTP. For details, see MSTP Fast Convergence.

Protection Mechanisms of VBST

Similar to RSTP, VBST provides BPDU protection, TC protection, root protection, and loop protection. For details, see Protection functions.

Interworking Between VBST and Standard STP/RSTP

On a live network, VBST-enabled devices may connect to STP/RSTP-enabled devices. VBST and STP/RSTP use different BPDU formats, so there are interworking problems. To implement interworking between VBST and standard STP/RSTP, take the following measures:

  • On a trunk interface:

    • When a VBST-enabled device connects to an RSTP-enabled device, the VBST-enabled device uses standard RSTP BPDUs in VLAN 1 and VBST BPDUs with the Data field of RSTP BPDUs in other VLANs to exchange with the RSTP-enabled device.
    • When a VBST-enabled device connects to an STP-enabled device, the VBST-enabled device uses standard STP BPDUs in VLAN 1 and VBST BPDUs with the Data field of STP BPDUs in other VLANs to exchange with the STP-enabled device.

    The following describes spanning tree implementation, as shown in Figure 15-3.

    As shown in Figure 15-3, STP/RSTP is deployed on S1 and S2, and VBST is deployed on S3 and S4. Devices are connected through trunk interfaces, and interfaces on S1 through S4 allow packets from VLAN 1 and VLAN 10 to pass through.

    Figure 15-3  Interworking between VBST and STP/RSTP on a trunk interface

    An STP/RSTP-enabled device can only send and receive STP/RSTP BPDUs, and transparently transmit VBST BPDUs, so a spanning tree is formed in VLAN 1 as defined by STP/RSTP.

    Assume that the congestion point of the spanning tree in VLAN 1 is on S4. Because VBST runs on S4, so the congestion point exists in VLAN 1. S4 can still receive and forward VBST BPDUs in VLAN 10. Loops occur in VLAN 10, so spanning tree calculation in VLAN 10 is triggered. S1 and S2 transparently transmit VBST BPDUs in VLAN 10, so only four interfaces on S3 and S4 participate in spanning tree calculation in VLAN 10. Then the spanning trees in VLAN 1 and VLAN 10 are formed, as shown in Figure 15-3.

    Assume that the blocking point of the spanning tree in VLAN 1 is on S2. STP/RSTP runs on S2, so the blocking port exists on S2. S2 cannot forward VBST BPDUs from VLAN 10 and no loop occurs in VLAN 10, so spanning tree calculation in VLAN 10 is not triggered. VBST BPDUs from VLAN 10 can be forwarded along the spanning tree in VLAN 1, that is, VLAN 10 and VLAN 1 share the spanning tree. as shown in Figure 15-3.

  • On an access interface, a VBST-enabled device uses standard STP or RSTP BPDUs to exchange with the remote end according to the VLAN that the access interface belongs to. Topology calculation is performed as defined by STP/RSTP. Because STP/RSTP does not differentiate VLANs, a spanning tree shared by VLANs is formed.

When a VBST-enabled device connects to an STP/RSTP-enabled device, the trunk interface must be used to connect the two devices and the blocking point must be located on the VBST-enabled device to implement load balancing.

Interworking Between VBST and PVST/PVST+/Rapid PVST+

On a live network, a VBST-enabled device may connect to a device enabled with PVST/PVST+/Rapid PVST+.

  • Trunk interface

    • When a VBST-enabled device connects to a device enabled with Rapid PVST+, the VBST-enabled device sends standard RSTP BPDUs (or VBST BPDUs with the Data field of RSTP BPDUs) and VBST BPDUs with the Data field of RSTP BPDUs in other VLANs to exchange with the device enabled with Rapid PVST+.

    • When a VBST-enabled device connects to a device enabled with PVST+, the VBST-enabled device sends standard STP BPDUs (or VBST BPDUs with the Data field of STP BPDUs) and VBST BPDUs with the Data field of STP BPDUs in other VLANs to exchange with the device enabled with PVST+.

    • When a VBST-enabled device connects to a PVST-enabled device, packet exchange is similar to that in the scenario where a VBST-enabled device connects to a device enabled with PVST+. The difference is that the VBST-enabled device and PVST-enabled device send only VBST BPDUs with the Data field of STP BPDUs in VLAN 1.

    The two devices can identify the BPDUs carrying VLAN information, so a VLAN-based spanning tree is formed. The connection between a VBST-enabled device and a device enabled with PVST/PVST+/Rapid PVST+ through a trunk interface is similar to the connection between two VBST-enabled devices.

  • Access interface

    A VBST-enabled device uses standard STP BPDUs to exchange with the device enabled with PVST/PVST+ or RSTP BPDUs to exchange with the device enabled with Rapid PVST+ according to the VLAN that the access interface belongs to. Topology calculation is performed as defined by STP/RSTP. Because STP/RSTP does not differentiate VLANs, a spanning tree shared by VLANs is formed.

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Updated: 2019-01-18

Document ID: EDOC1100038290

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