Cisco ASR 9000 Series Configuration Manuallines page 449

Implementing Multiple Spanning Tree Protocol
(MSTP) root node. Thus, the access devices always use the gateway devices to reach the root, and the ports
on the gateway devices are always in the designated forwarding state.
In a mixed Layer 2-Layer 3 environment, the L2 access network is used to provide a Layer 2 service on certain
VLANs and a Layer 3 (L3) service on other VLANs. In the access network, a different MSTI is used for the
L2 service and the L3 service. For the L2 VLANs, the core or aggregation network provides L2 connectivity
between the gateway devices. However, for the L3 service, the gateway devices terminate the L2 network
and perform L3 routing. Typically, an L3 redundancy mechanism such as HSRP or VRRP is used to allow
the end hosts to route to the correct gateway.
In this scenario, the use of MSTAG alone does not achieve the desired behavior for the L3 MSTI. This is
because it results in one of the ports in the access network being blocked, even though there is actually no
loop. (This, in turn, is because there is no L2 connectivity between the gateway devices for the L3 VLANs.)
In fact, because the gateway devices terminate the L2 network for the L3 VLANs, the desirable behavior is
for the MSTP root to be located in the access network, and for the gateway devices to appear as leaf nodes
with a single connection. This can be achieved by reversing the MSTAG configuration; that is, setting the
gateway devices to advertise the worst possible path to the worst possible root. This forces the access devices
to elect one of the access devices as the root, and therefore, no ports are blocked. In this case, the ports on the
gateway devices are always in root forwarding state. The MSTAG Edge mode feature enables this scenario
by changing the role advertised by the gateway devices from designated to root. The following figure illustrates
this scenario.
Figure 60: MSTAG Edge Mode scenario
• D - Designated port (forwarding)
• R - Root port (forwarding)
• A - Alternate port (blocked)
For normal MSTAG, and for the L2 MSTIs, topology change notifications are propagated from one gateway
device to the other, and re-advertised into the access network. However, for the L3 MSTI, this is not desirable.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
MSTAG Edge Mode
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