Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
8
Ingress and Egress Rules
The BPDU transmission on the 802.1Q port of a PVST+ router will be implemented in compliance with
the following rules:
• The CST BPDU (of VLAN 1, by default) is sent to the IEEE address.
• All the other BPDUs are sent to Shared Spanning Tree Protocol (SSTP)-Address and encapsulated
with Logical Link Control-Subnetwork Access Protocol (LLC-SNAP) header.
• The BPDU of the CST and BPDU of the VLAN equal to the PVID of the 802.1Q trunk are sent
untagged.
• All other BPDUs are sent tagged with the VLAN ID.
• The CST BPDU is also sent to the SSTP address.
• Each SSTP-addressed BPDU is also tailed by a Tag-Length-Value for the PVID checking.
The BPDU reception on the 802.1Q port of a PVST+ router will follow these rules:
• All untagged IEEE addressed BPDUs must be received on the PVID of the 802.1Q port.
• The IEEE addressed BPDUs whose VLAN ID matches the Native VLAN are processed by CST.
• All the other IEEE addressed BPDUs whose VLAN ID does not match the Native VLAN and whose
port type is not of 802.1Q are processed by the spanning tree of that particular VLAN ID.
• The SSTP addressed BPDU whose VLAN ID is not equal to the TLV are dropped and the ports are
blocked for inconsistency.
• All the other SSTP addressed BPDUs whose VLAN ID is not equal to the Native VLAN are
processed by the spanning tree of that particular VLAN ID.
• The SSTP addressed BPDUs whose VLAN ID is equal to the Native VLAN are dropped. It is used
for consistency checking.
Integrated Routing and Bridging
IRB enables a user to route a given protocol between routed interfaces and bridge groups or route a given
protocol between the bridge groups. Integrated routing and bridging is supported on the following
protocols:
• IP
• IPX
• AppleTalk
VLAN Colors
VLAN switching is accomplished through frame tagging where traffic originating and contained within
a particular virtual topology carries a unique VLAN ID as it traverses a common backbone or trunk link.
The VLAN ID enables VLAN switching devices to make intelligent forwarding decisions based on the
embedded VLAN ID. Each VLAN is differentiated by a color, or VLAN identifier. The unique VLAN
ID determines the frame coloring for the VLAN. Packets originating and contained within a particular
VLAN carry the identifier that uniquely defines that VLAN (by the VLAN ID).
The VLAN ID allows VLAN switches and routers to selectively forward packets to ports with the same
VLAN ID. The switch that receives the frame from the source station inserts the VLAN ID and the
packet is switched onto the shared backbone network. When the frame exits the switched LAN, a switch