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BGP Interview Questions and Answers [CCNP]



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General Network Challenges (Unicast flooding ) [ CCNP]

                                               Unicast flooding

It refers to the unintentional behavior of a switch treating a unicast packet as a broadcast packet; The cause of flooding is that the destination MAC address of the packet is not in the L2 forwarding table of the switch. Normally occurs when the router needs to deliver a packet; it has an ARP entry for a destination host, but the switch has no CAM entry.   The result is a packet that needs to be flooded to all of the ports in the VLAN In order to locate that MAC address port/VLAN.

Common reasons for destination MAC address not being known to the switch:

Cause 1: Asymmetric Routing.

With asymmetric routing, transmit and receive packets follow different paths between a host and the peer with which it communicates, at some point in the packet delivery path a Switch may not have that MAC address destination on its CAM table and would need to flood the frame in order to discover which port/MAC address is.

Cause 2: Spanning-Tree Protocol Topology Changes.

Since TCNs are triggered by a port that is transitioning to or from the forwarding state we may remember that what TCN does is to age out the CAM table in order to relearn the Active MAC address,
A final cause of unicast floods are topology changes. When a link state changes on a network port which participates in rapid spanning tree, the address cache on that switch will be flushed causing all subsequent packets to be flooded out of all ports until the addresses are learned by the switch.

Cause 3: Forwarding Table Overflow.

Another possible cause of flooding can be overflow of the switch forwarding table. In this case, new addresses cannot be learned and packets destined to such addresses are flooded until some space becomes available in the forwarding table. New addresses will then be learned. This is possible but rare,
A switch that has no room left in its address cache will flood the packet out to all ports. This is a common problem on networks with many hosts. Less common is the artificial flooding of address tables—this is termed MAC flooding.

Another common cause are hosts with ARP timers longer than the address cache timeout on switches—the switch forgets which port connects to the host.

Remedies -

There are several remedies discussed in the links above.  But for many situations, a low end switch needs to be replaced with a higher end switch—one that has a larger address table and one that can block unicast floods. Blocking unicast floods on a Cisco switch is easy to do, but it is not enabled by default. After ensuring that timeouts and/or security features have been configured to maintain table entries on client access ports longer than typical host ARP cache timeouts, this command is used to quiet down the unicast floods on those ports

By default unknown unicast traffic is flooded to all Layer 2 ports in a Vlan. We can use UUFB and UMFB, features to prevent or limit this traffic.

The UUFB [unknown unicast flooding blocking] and UMFB [unknown multicast flooding blocking ] features block unknown unicast and multicast traffic flooding at a specific port, only permitting egress traffic with MAC addresses that are known to exist on the port. The UUFB and UMFB features are supported on all ports that are configured with the switchport command, including private VLAN (PVLAN) ports.

Router(config-if)# switchport    --> Configures the port for Layer 2 switching.
Router(config-if)# switchport block {unicast | multicast} -->Enables unknown unicast or multicast flood blocking on the port.

Note: Enter the switchport block multicast command only on ports where all unknown multicast flooded traffic needs to be completely blocked. UMFB disrupts protocols that make use of local subnetwork multicast control groups in the range, for example:

•IPv6 neighbor discovery (IPv6 ND)
•Network Time Protocol (NTP)

Do not enter this command on nonreceiver (router) ports or host ports that rely on dynamic ARP. Use IGMP snooping or other rate-limiting options to restrict, rather than completely block, unknown multicast flooded traffic.

Other techniques involve isolating hosts at Layer 2, which blocks intra-LAN communication not destined to the router. A handy tool (available in lower end switches ) is the

Switch(config-if)# switchport protected

or a more robust, cross-switch solution than 'switchport protected' is the use of Private VLANs.

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