Bpdu Guard; Flush Containment; Bringup Delay - Cisco ASR 9000 Series Configuration Manuallines
L2vpn and ethernet services configuration guide
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Also See for ASR 9000 Series:
- Command reference manual (1138 pages) ,
- Routing configuration manual (702 pages) ,
- Reference manual (696 pages)
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Implementing Multiple Spanning Tree Protocol
BPDU Guard
BPDU Guard is a Cisco feature that protects against misconfiguration of edge ports. It is an enhancement to
the MSTP port fast feature. When port fast is configured on an interface, MSTP considers that interface to be
an edge port and removes it from consideration when calculating the spanning tree. When BPDU Guard is
configured, MSTP additionally shuts down the interface using error-disable if an MSTP BPDU is received.
Flush Containment
Flush containment is a Cisco feature that helps prevent unnecessary MAC flushes due to unrelated topology
changes in other areas of a network. This is best illustrated by example. The following figure shows a network
containing four devices. Two VLANs are in use: VLAN 1 is only used on device D, while VLAN 2 spans
devices A, B and C. The two VLANs are in the same spanning tree instance, but do not share any links.
Figure 56: Flush Containment
If the link AB goes down, then in normal operation, as C brings up its blocked port, it sends out a topology
change notification on all other interfaces, including towards D. This causes a MAC flush to occur for VLAN
1, even though the topology change which has taken place only affects VLAN 2.
Flush containment helps deal with this problem by preventing topology change notifications from being sent
on interfaces on which no VLANs are configured for the MSTI in question. In the example network this would
mean no topology change notifications would be sent from C to D, and the MAC flushes which take place
would be confined to the right hand side of the network.
Note
Flush containment is enabled by default, but can be disabled by configuration, thus restoring the behavior
described in the IEEE 802.1Q standard.
Bringup Delay
Bringup delay is a Cisco feature that stops MSTP from considering an interface when calculating the spanning
tree, if the interface is not yet ready to forward traffic. This is useful when a line card first boots up, as the
system may declare that the interfaces on that card are Up before the dataplane is fully ready to forward traffic.
According to the standard, MSTP considers the interfaces as soon as they are declared Up, and this may cause
it to move other interfaces into the blocking state if the new interfaces are selected instead.
Bringup delay solves this problem by adding a configurable delay period which occurs as interfaces that are
configured with MSTP first come into existence. Until this delay period ends, the interfaces remain in blocking
state, and are not considered when calculating the spanning tree.
Bringup delay only takes place when interfaces which are already configured with MSTP are created, for
example, on a card reload. No delay takes place if an interface which already exists is later configured with
MSTP.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
BPDU Guard
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