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In a Virtual Chassis, multiple switches—each with its own set of interfaces with unique MAC addresses—are connected together to form one chassis that can be managed as a single switch. The MAC address assigned to each network-facing interface on the switch changes when the switch joins a Virtual Chassis. Because all Layer 2 traffic decisions are based on an interface’s MAC address, understanding MAC address assignment is important to understanding how network traffic is forwarded and received by the Virtual Chassis. For additional information about how a network uses MAC addresses to forward and receive traffic, see Understanding Bridging and VLANs on EX Series Switches. http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/virtual-chassis-mac-address-assignment.html

The transit route POP's the outer label and swaps the inner label

The VLAN tag was invented to distinguish among different VLAN broadcast domains on a group of LAN switches. The VLAN tag is a two-byte field inserted between the source MAC address and the Ethertype (or length) field in an Ethernet frame. Another two-byte field, the Tag Protocol Identifier (TPI or TPID), precedes the VLAN tag field.  The Canonical Format Indicator (CFI) bit indicates whether the following 12 bits of VLAN identifier conform to Ethernet or not. For Ethernet frames, this bit is always set to 0. (The other possible value, CFI=1, is used for Token Ring LANs, and tagged frames should never be bridged between an Ethernet and Token Ring LAN regardless of the VLAN tag or MAC address.) http://www.juniper.net/documentation/en_US/junos15.1/topics/concept/layer-2-networking-ethernet-frame-forwarding-802-1q-vlan-tag-mx-solutions.html

Learning:A switch begins learning the local MAC addresses as soon as it is connected to other devices or to a network. This learning capability makes switches easy to use on a network. Flooding:As part of the learning process, a switch will flood the single frame out all of its other ports when it cannot find the destination MAC address in the switch’s lookup table. Aging:Whenever switch updates an entry in the MAC address table, the switch resets the timer for that entry. Timers are used in aging process of old entries. Aging helps to remove old entries and free memory of MAC address table to add new entries.

A trunk is a line or link designed to handle many VLANs simultaneously, and that connects major switching centers or nodes in a communications system.

VSTP on Juniper Networks routers has some interoperability with other proprietary per-VLAN vendor solutions; specifically, VSTP can interoperate with per-VLAN spanning tree (PVST) and Rapid-PVST on IEEE 802.1Q non-proprietary trunks. In PVST, the IEEE 802.1Q BPDUs are sent untagged on the common spanning-tree VLAN 1 for interoperability with other vendors. The CST BPDUs are sent to the IEEE standard bridge group: MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42.PVST BPDUs are tagged and sent to MAC address 01-00-0c-cc-cc-cd (SNAP HDLC protocol type 0x010b) for each VLAN on a trunk. PVST per-VLAN BPDUs are tunneled by pure IEEE 802.1Q bridges.

EX Series switches support spanning-tree protocols that prevent loops in a network by creating a tree topology (spanning-tree) of the entire bridged network. All spanning-tree protocols use a special type of frame called bridge protocol data units (BPDUs) to communicate with each other.  When BPDUs generated by other devices are transmitted to switches on which spanning-tree protocols are configured, a misconfiguration can occur in the spanning tree and a network outage can occur. Therefore, it is necessary to protect an interface in a spanning-tree topology from BPDUs generated from other devices. http://www.juniper.net/documentation/en_US/junos13.3/topics/task/configuration/spanning-trees-bpdu-block-cli.html

Multiple Spanning Tree Protocol (MSTP) was first specified in IEEE 802.1s and is standardized in IEEE 802.1Q. MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of spanning-tree instances needed to support a large number of VLANs. MSTP provides multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of the network because a failure in one instance, or forwarding path, does not affect other instances. MSTP has a backward compatibility mode in which it can fall back to STP or RSTP operation on links with bridges that support only STP or RSTP. The maximum number of MSTP instances supported in Juniper Networks MX Series 3D Universal Edge Routers is 64. https://www.juniper.net/documentation/en_US/junos12.3/topics/concept/mx-series-multiple-stp.html

Rapid Spanning Tree Protocol (RSTP–802.1w/802.1d 2004): Similar to legacy STP, RSTP implements a single instance of Spanning Tree but dramatically improves the convergence time  For example, in legacy STP, a host port coming online can take up to 30 seconds (2 x FWD_Delay) before it begins forwarding . Direct or indirect node/link failure convergence can even take up to 50 seconds (MAX_Age + 2 x FWD_Delay) with legacy STP. Such convergence times are not acceptable in today’s high-performance enterprise networks RSTP overcomes slow convergence times by introducing new port roles (edge port, alternate port and backup port) and new convergence behavior under specific connectivity conditions (for example, point-to-point or shared medium) . These enhancements enable RSTP to achieve sub-second forwarding and network convergence RSTP is backward-compatible with legacy STP; if legacy STP BPDU is detected on a link, then an RSTP-capable bridge will revert to legacy STP on that given port . Understanding RSTP for EX Series Switches http://www.juniper.net/documentation/en_US/junos13.2/topics/concept/spanning-trees-ex-series-rstp-understanding.html

Bridge Protocol Data Units (BPDUs) are frames that contain information about the Spanning tree protocol (STP). Switches send BPDUs using a unique MAC address from its origin port and a multicast address as destination MAC (01:80:C2:00:00:00, or 01:00:0C:CC:CC:CD for Per VLAN Spanning Tree). For STP algorithms to function, the switches need to share information about themselves and their connections. What they share are bridge protocol data units (BPDUs). BPDUs are sent out as multicast frames to which only other layer 2 switches or bridges are listening.There are three kinds of BPDUs:    Configuration BPDU, used by Spanning tree protocol to provide information to all switches.     TCN (Topology Change Notification), tell about changes in the topology.     TCA (Topology Change Acknowledgment), confirm the reception of the TCN. https://en.wikipedia.org/wiki/Bridge_Protocol_Data_Unit