Preempt Delay; Supported Access Gateway Protocols; Mstag Edge Mode - Cisco ASR 9000 Series Configuration Manuallines

Preempt Delay

Preempt Delay
One of the assumptions underpinning access gateway is that the gateway devices are always available to
provide connectivity to the core or aggregation network. However, there is one situation where this assumption
may not hold, which is at bringup time. At bringup, it may be the case that the access facing interface is
available before all of the necessary signaling and convergence has completed that means traffic can successfully
be forwarded into the core or aggregation network. Since access gateway starts sending BPDUs as soon as
the interface comes up, this could result in the access devices sending traffic to the gateway device before it
is ready to receive it. To avoid this problem, the preempt delay feature is used.
The preempt delay feature causes access gateway to send out inferior BPDUs for some period of time after
the interface comes up, before reverting to the normal values. These inferior BPDUs can be configured such
that the access network directs all traffic to the other gateway device, unless the other gateway device is also
down. If the other gateway device is unavailable, it is desirable for the traffic to be sent to this device, even
if it is only partially available, rather than being dropped completely. For this reason, inferior BPDUs are sent
during the preempt delay time, rather than sending no BPDUs at all.

Supported Access Gateway Protocols

Access Gateway is supported on Cisco ASR 9000 Series Routers when the following protocols are used in
the access network
Table 4: Protocols
Access Network Protocol
MSTP
REP
PVST+
PVRST
1. REP Access Gateway is supported when the access device interfaces that connect to the gateway devices
are configured with REP MSTP Compatibility mode.
2. Topology Change Propagation is not supported for PVSTAG.
3. Topology Change Propagation is not supported for PVRSTAG.

MSTAG Edge Mode

An access gateway is used in a Layer 2 (L2) environment to ensure that for each Multiple Spanning Tree
Instance (MSTI), each access device has one path to the core or aggregation network. The core or aggregation
network provides L2 (Ethernet) connectivity between two gateway devices. Therefore, when there are no
failures, there must be at least one blocked port in the access network for each MSTI. In the case of an access
ring, there should be one blocked port in the access ring. For each MSTI – this is typically one of the uplink
ports that connects to one of the gateway devices. This is achieved by configuring MSTAG in such a way
that the gateway devices appear to have the best path to the best possible Multiple Spanning Tree Protocol
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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be sending traffic in the right direction, or will learn about the topology change from the access device
that originates it.
Implementing Multiple Spanning Tree Protocol
Access Gateway Variant
MST Access Gateway (MSTAG)
REP Access gateway (REPAG)
PVST+ Access Gateway (PVSTAG)
PVRST Access Gateway (PVRSTAG)
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