JEX - Chapter 2: Layer 2 Switching Flashcards
Ethernet Frame Format
Preamble: 7 bytes Start Frame Deliminator: 1 byte Destination Address: 6 bytes Source Address: 6 bytes Type: 2 bytes Data: 64 - 1500 bytes Frame Check Sequence: 4 bytes
Bridging Mechanisms
Learning Forwarding Flooding Filtering Aging
Bridge Mechanism: Learning
The process by which a switch learns a MAC address and puts it in the bridge table.
- The switch examines the Ethernet frame header of all received frames looking for a Source MAC of sending nodes.
- The switch adds MAC Address, Time the MAC was learned to the Bridging Table.
Bridge Mechanism: Forwarding
The process used by the switch to deliver traffic, passing it from an incoming interface to an outgoing interface that leads to (or toward) the destination.
- The switch checks the bridge table for a corresponding MAC and forwards the traffic out the interface associated with that MAC.
- If a MAC doesn’t exist in the bridging table, the switch floods the frame out all interfaces in the same broadcast domain as the ingress interface.
Note: The Frame is not sent back out the ingress interface.
Bridge Mechanism: Flooding
A transparent mechanism used to deliver packets to unknown MAC addresses.
- If the bridge table has no MAC entry for a destination or if the packet received is a broadcast or multicast packet, the switch floods the traffic out all interfaces except the ingress interface.
- If traffic originates on the switch, the switch floods traffic out all its interfaces.
- When an unknown destination responds to traffic that has been flooded, the switch learns the MAC of that node and updates the bridge table.
Bridge Mechanism: Filtering
The process of limiting traffic to its associated segment or switch port; this is how broadcast traffic is limited to the local VLAN whenever possible.
- As the number of entries in the Ethernet switching table increases, the switch gets an increasingly complete picture of the VLAN and the larger LAN.
- The switch uses this information to filter traffic.
Bridge Mechanism: Aging
The process of ensuring that only active MAC address entries are in the bridge table.
- The switch records a timestamp in the bridge table of the time the MAC was learned.
- Each time the switch detects traffic from a MAC address, it updates the timestamp.
- If the timestamp is older than a user-configured value, the switch removes the MAC; The default aging timer is 300 seconds (5 minutes) and can be configured for all VLANs.
Core Layer Switches
Relay packets between aggregation switches and function as the gateway to the WAN edge device.
Aggregation (or Distribution) Layer Switches
Connect access switches and often provide inter-VLAN routing and policy-based connectivity.
Access Layer Switches
Facilitate end-user access, device access and enforce access policy.
Enhanced Layer 2 Software (ELS)
A standard Layer 2 configuration syntax that ensures a common Layer 2 configuration for Junos OS.
This combines the best of the EX Series with the best of the MX Series platforms.
Control Plane: Routing Engine (RE)
The brains of the platform. It is connected to the PFE through an internal link.
It performs:
- protocol updates
- system management
The RE maintains:
- Routing Table
- Bridging Table,
- Primary Forwarding Table
Forwarding Plane:
Packet Forwarding Engine (PFE)
Responsible for forwarding the following through the switch:
- Transit frames
- Packets
- Both
The PFE is implemented using ASICs on the EX Series platforms.
Receives Layer 2 and Layer 3 forwarding tables from the RE via an internal link.
Frame Processing: Unknown Source
- Frame enters ingress port and attached ingress PFE
- Ingress PFE performs MAC address lookup and determines source MAC is unknown
- Ingress PFE sends header information to RE, where MAC is added or discarded (MAC limiting).
- If RE adds new Source MAC address to bridge table, newly added MAC entry is sent to and programmed into all PFEs.
Frame Processing: Known Destination
- Frame enters ingress port and attached ingress PFE.
- Ingress PFE performs a MAC address lookup and determines the egress PFE and port.
- Ingress PFE forwards frame to egress PFE
- Egress PFE forwards frame out egress port towards its destination. No additional lookup is needed.
Frame Processing: Unknown Destination
- Frame enters ingress port and attached ingress PFE.
- Ingress PFE performs MAC address lookup, determines no entry exists then replicates the frame out to other PFEs and all other local ports in the same broadcast domain (VLAN).
- All other PFEs replicate frame and forward those frames out all egress ports in the same broadcast domain. No additional lookup is needed.
- Once the switch sees return traffic from this MAC address, it adds the address to the bridge table.
Frame Processing: Routed Packet
- Frame enters ingress port and attached ingress PFE
- Ingress PFE performs MAC address lookup; Because the destination MAC address belongs to the switch, PFE performs a Layer 3 lookup.
a. If the destination IP belongs to the switch, the decapsulated packet is sent to RE for processing.
b. If the destination IP does not belong to the switch, the packet is forwarded to the egress PFE. - Egress PFE forwards packet out egress port towards destination. No additional lookup needed.
What is the EX Series switch’s management interface?
me0
How do you run EZsetup?
From UNIX shell prompt:
% ezsetup
To run the ezsetup the switch must have the default-configuration as the active configuration.
What must be specified in order to enable Layer 2 operation on an interface?
family ethernet-switching
What command shows the contents of the bridging table?
> show ethernet-switching table
What command shows the Layer 2 forwarding table?
> show route forwarding-table family ethernet-switching
Which command clears bridge table entries?
> clear ethernet-switching table
