Refer to these documents for more information:Īsymmetric Routing with Bridge Groups on Catalyst 2948G-LG-元 SwitchesĪsymmetric Routing and HSRP (Excessive Flooding of Unicast Traffic in Network with Routers Running HSRP) There are different approaches to limit the flooding caused by asymmetric routing. Since the switch is not receiving traffic constantly, this forwarding entry will eventually age out and flooding will resume. Switch A will flood this packet to VLAN 1 and switch B will receive and learn the MAC address of S1. This flooding is due to asymmetric routing, and may stop when server S1 sends a broadcast packet (for example Address Resolution Protocol (ARP)). This will cause a complete connectivity outage to the PCs or slow them down considerably. This amount of traffic will saturate 10 Mbps links. Suppose the server backup takes 50 Mbps of bandwidth. This means every connected port (workstation W in this example) in VLAN 1 on switch B will receive all packets of conversation between S1 and S2. Returning to the above example, the result is that packets of the data transfer between S1 and S2 will mostly be flooded to VLAN 2 on switch A and to VLAN 1 on switch B. Asymmetric routing is one of the two most common causes of flooding. Packets follow different paths depending on the direction.
This behavior is called asymmetric routing. The same situation will occur with the S1 MAC address on switch B. This means that every time switch A needs to send the packet to the S2 MAC address, the packet will be flooded to VLAN 2. Note that with such an arrangement, switch A will not "see" traffic from the S2 MAC address in VLAN 2 (since the source MAC address will be rewritten by router B and the packet will only arrive in VLAN 1). S2-VLAN 2-switch B-router B-VLAN 1-switch A-flooded to VLAN 1-S1 (red line) Packets from S2 to S1 go along the following path: S1-VLAN 1-switch A-router A-VLAN 2-switch B-VLAN 2-S2 (blue line) Packets from S1 to S2 will follow this path: Server S2 has its default gateway pointing to router B's VLAN 2 interface. Server S1 has its default gateway pointing to router A's VLAN 1 interface. In the diagram above, server S1 in VLAN 1 is running backup (bulk data transfer) to server S2 in VLAN 2. Large amounts of flooded traffic might saturate low-bandwidth links causing network performance issues or complete connectivity outage to devices connected across such low-bandwidth links. Below case studies display most common reasons for destination MAC address not being known to the switch. In this case the packet will be flooded out of all forwarding ports in its VLAN (except the port it was received on).
The very cause of flooding is that destination MAC address of the packet is not in the L2 forwarding table of the switch. This document explains what issues can arise due to flooding, and the most common reasons why certain traffic might constantly be flooded. There are situations, however, when continuous flooding can cause adverse performance effects on the network. Limited flooding is part of the normal switching process. When there is no entry corresponding to the frame's destination MAC address in the incoming VLAN, the (unicast) frame will be sent to all forwarding ports within the respective VLAN, which causes flooding. LAN switches use forwarding tables (Layer 2 (L2) tables, Content Addressable Memory (CAM) tables) to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. Conventionsįor more information on document conventions, refer to Cisco Technical Tips Conventions.
WHAT IS MAC ADDRESS FLOODING SOFTWARE
This document is not restricted to specific software and hardware versions. There are no specific requirements for this document. This document discusses possible causes and implications of unicast packet flooding in switched networks.
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