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| Question ID 18489 | Which two mechanisms can be used to eliminate Cisco Express Forwarding polarization?
(Choose two.)
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| Option A | alternating cost links
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| Option B | the unique-ID/universal-ID algorithm
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| Option C | Cisco Express Forwarding antipolarization
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| Option D | different hashing inputs at each layer of the network
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Explanation Explanation: This document describes how Cisco Express Forwarding (CEF) polarization can cause suboptimal use of redundant paths to a destination network. CEF polarization is the effect when a hash algorithm chooses a particular path and the redundant paths remain completely unused. How to Avoid CEF Polarization ✑ Alternate between default (SIP and DIP) and full (SIP + DIP + Layer4 ports) hashing inputs configuration at each layer of the network. ✑ Alternate between an even and odd number of ECMP links at each layer of the network.The CEF load-balancing does not depend on how the protocol routes are inserted in the routing table. Therefore, the OSPF routes exhibit the same behavior as EIGRP. In a hierarchical network where there are several routers that perform load-sharing in a row, they all use same algorithm to load-share. The hash algorithm load-balances this way by default: 1: 1 2: 7-8 3: 1-1-1 4: 1-1-1-2 5: 1-1-1-1-1 6: 1-2-2-2-2-2 7: 1-1-1-1-1-1-1 8: 1-1-1-2-2-2-2-2 The number before the colon represents the number of equal-cost paths. The number after the colon represents the proportion of traffic which is forwarded per path. This means that: ✑ For two equal cost paths, load-sharing is 46.666%-53.333%, not 50%-50%. ✑ For three equal cost paths, load-sharing is 33.33%-33.33%-33.33% (as expected). ✑ For four equal cost paths, load-sharing is 20%-20%-20%-40% and not 25%-25%- 25%-25%. This illustrates that, when there is even number of ECMP links, the traffic is not load- balanced. ✑ Cisco IOS introduced a concept called unique-ID/universal-ID which helps avoid CEF polarization. This algorithm, called the universal algorithm (the default in current Cisco IOS versions), adds a 32-bit router-specific value to the hash function (called the universal ID - this is a randomly generated value at the time of the switch boot up that can can be manually controlled). This seeds the hash function on each router with a unique ID.
| Question ID 18490 | What is a cause for unicast flooding?
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| Option A | Unicast flooding occurs when multicast traffic arrives on a Layer 2 switch that has directly connected multicast receivers.
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| Option B | When PIM snooping is not enabled, unicast flooding occurs on the switch that interconnects the PIM-enabled routers.
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| Option C | A man-in-the-middle attack can cause the ARP cache of an end host to have the wrong MAC address. Instead of having the MAC address of the default gateway, it has a MAC address of the man-in-the-middle. This causes all traffic to be unicast flooded through the man-in-the-middle, which can then sniff all packets.
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| Option D | Forwarding table overflow prevents new MAC addresses from being learned, and packets destined to those MAC addresses are flooded until space becomes available in the forwarding table.
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| Correct Answer | D |
Explanation Explanation: Causes of Flooding The very cause of flooding is that destination MAC address of the packet is not in the L2 forwarding table of 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). Below case studies display most common reasons for destination MAC address not being known to the switch. Cause 1: Asymmetric Routing 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 Cause 2: Spanning-Tree Protocol Topology Changes Another common issue caused by flooding is Spanning-Tree Protocol (STP) Topology Change Notification (TCN). TCN is designed to correct forwarding tables after the forwarding topology has changed. This is necessary to avoid a connectivity outage, as after a topology change some destinations previously accessible via particular ports might become accessible via different ports. TCN operates by shortening the forwarding table aging time, such that if the address is not relearned, it will age out and flooding will occur 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, since most modern switches have large enough forwarding tables to accommodate MAC addresses for most designs. Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23563- 143.html