Tag - Reliability

Routing Protocol Characteristics

Packets originating from a nonrouting device destined for another network are sent to their default gateway (Layer 3 device on segment). The router consults its routing table to dvcer-mine if the destination network can be reached. If not, the ICMP Destination Unreachable message is sent to the source. If so, packet is forwarded out interface associated with the des­tination network in routing table.

Routing Sources

Default Administrative Distances

Maximum Hop Counts
Routed Source
Default Distance
Static route
EIGRP (internal)
RIPv1 and v2
EIGRP (external)

Static and Default Routes

Static routes are useful in stub networks in which we want to control the routing behavior by manually configuring destination networks into the routing table:

Router(config)#ip route

A floating static route can be configured when redundant connections exist and you want to use the redundant link if the primary fails. This is configured by adding a higher administra­tive distance at the end of a static route:

Router(config)#ip route 2

A default route is a gateway of last resort for a router when there isn’t a specific match for an IP destination network in the routing table (such as packets destined for the Internet):

Router(config)#ip route 0/0

With routing protocols, you can specify a default network, which is a network in the routingtable that routing devices consider to be the gateway of last resort. Using their routing proto­cols, they dvcermine the best path to the default network:

Router(config)#ip default-network

Dynamic Routing Protocols

In complex networks with multiple pathways to destinations, dynamic routing protocols enable routers to advertise their networks to each other and dynamically react to topology changes.

Routing protocols dvcermine the best path based on the lowest metric.

 Routing Metrics

Because one of the core responsibilities of routing protocols is to build routing tables to dvcermineoptimal routing paths, we need to have some means of measuring which routes are preferred whenthere are multiple pathways to a destination. Routing protocols use some measure of metrics toidentify which routes are optimal to reach a destination network. The lowest cumulative metric toa destination is the preferred path and the one that ultimately enters the routing table. Different routing protocols use one or several of the following metrics to calculate the best path.

Routing Metrics

Hop count
The number of routing devices that the packet must travel to reach a destination network
The cumulative bandwidth of the links to the destination in kilobits per second
The length of time (measured in microseconds) a packet takes from source to destination
The consistency of the links and paths toward the destination based on error rates of the interfaces
The cumulative amount of congestion or saturation of the links toward the destination
The maximum frame size that is allowed to traverse the links to the destination
Cost   An arbitrary number typically based on the link’s bandwidth

Interior and Exterior Gateway Routing Protocols

  1. Interior gateway routing protocols: IG routing protocols advertise networks and metrics within an autonomous system.
  2. Exterior gateway routing protocols: EG routing protocols advertise networks in between autonomous systems.

Classful and Classless Routing Updates

  1. Classful routing: The routing updates only contain the classful networks without any subnet mask. Summarization is automatically done when a router advertises a network out an interface that is not within the same major subnet. Classful routing protocols must have a FLSM design and do not operate correctly with discontiguous networks.
  2. Classless routing: The routing updates can contain subnetted networks because the subnet mask is advertised in the updates. Route summarization can be manually config­ured at any bit boundary. Classless routing protocols support VLSM designs and dis­contiguous networks.

all routing protocol

Routing Protocol Classes

  1. Distance vector: The entire routing table is periodically sent to directly connected neighbors regardless of a topology change. These routing protocols manipulate the routing table updates before sending that information to their neighbors and are slow to converge when a topology change occurs.
  2. Link state: All possible link states are stored in an independent topology table in which the best routes are calculated and put into the routing table. The topology table is ini­tially synchronized with discovered neighbors followed by frequent hello messages. These routing protocols are faster to converge than distance vector routing protocols.
  3. Hybrid: By using the best characteristics from link-state and routing protocols, these advanced routing protocols efficiently and quickly build their routing information and converge when topology changes occur.


Redistribution is the method of configuring routing protocols to advertise networks from other routing protocols:

  1. One-way redistribution: Networks from an edge protocol are injected into a more robust core routing protocol, but not the other way around. This method is the safest way to perform redistribution.
  2. Two-way redistribution: Networks from each routing protocol are injected into the other. This is the least preferred method because it is possible that suboptimal routing or routing loops might occur because of the network design or the difference in con­vergence times when a topology change occurs.

Distance Vector Routing Loop Mitigation

Distance vector routing protocols contain several measures to prevent routing loops:

Maximum hop counts: To ensure that routing metrics do not increment until infinity in a routing loop, distance vector routing protocols have a maximum hop count.

Distance Vector/Link State/Hybrid
Maximum Hop Count
RI Pv1
Distance vector
RI Pv2
Distance vector
Link state

Split horizon:

Subnets learned from neighbor routers should not be sent back out the same interface from which the original update came.

Route poisoning with poison reverse:

When a route to a subnet fails, the subnet is advertised with an infinite metric. Routers receiving the poisoned route override the split horizon rule and send a poison reverse back to the source.

Hold-down timers:

The amount of time a router ignores any information about an alternative route with a higher metric to a poisoned subnet.

Flash updates/triggered updates:

When a route fails, the router immediately shoots out an update as opposed to waiting for a normal update interval.

CCNA Switching Dumps

Question 1

Refer to the exhibit. The following commands are executed on interface fa0/1 of 2950Switch.

2950Switch(config-if)#switchport port-security
2950Switch(config-if)#switchport port-security mac-address sticky
2950Switch(config-if)#switchport port-security maximum 1

The Ethernet frame that is shown arrives on interface fa0/1. What two functions will occur when this frame is received by 2950Switch? (Choose two)












A. The MAC address table will now have an additional entry of fa0/1 FFFF.FFFF.FFFF.
B. Only host A will be allowed to transmit frames on fa0/1.
C. This frame will be discarded when it is received by 2950Switch.
D. All frames arriving on 2950Switch with a destination of 0000.00aa.aaaa will be forwarded out fa0/1.
E. Hosts B and C may forward frames out fa0/1 but frames arriving from other switches will not be forwarded out fa0/1.
F. Only frames from source 0000.00bb.bbbb, the first learned MAC address of 2950Switch, will be forwarded out fa0/1.

Answer: B D


Please read the explanation at http://www.9tut.net/icnd2/icnd2-operations

Question 2

Which Cisco Catalyst feature automatically disables the port in an operational PortFast upon receipt of a BPDU?

A. BackboneFast
B. UplinkFast
C. Root Guard
D. BPDU Guard
E. BPDU Filter


Answer: D


We only enable PortFast feature on access ports (ports connected to end stations). But if someone does not know he can accidentally plug that port to another switch and a loop may occur when BPDUs are being transmitted and received on these ports.

With BPDU Guard, when a PortFast receives a BPDU, it will be shut down to prevent a loop -> D is correct.

Question 3

Why will a switch never learn a broadcast address?

A. Broadcast frames are never sent to switches.
B. Broadcast addresses use an incorrect format for the switching table.
C. A broadcast address will never be the source address of a frame.
D. Broadcasts only use network layer addressing.
E. A broadcast frame is never forwarded by a switch.


Answer: C

Question 4

Which three statements accurately describe layer 2 Ethernet switches? (choose three)

A. Microsegmentation decreases the number of collisions on the network.
B. If a switch receives a frame for an unknown destination.it uses ARP to resolve the address.
C. Spanning Tree Protocol allows switches to automatically share vlan information.
D. In a property functioning network with redundant switched paths, each switched segment will contain one root bridge with all its ports in the forwarding state. All other switches in that broadcast domain will have only one root port.
E. Establishing vlans increases the number of broadcast domains.
F. Switches that are configured with vlans make forwarding decisions based on both layer 2 and layer 3 address information.


Answer: A D E

Question 5

Switch ports operating in which two roles will forward traffic according to the IEEE 802.1w standard? (Choose two)

A. alternate
B. backup
C. designated
D. disabled
E. root


Answer: C E


IEEE 802.1w is the standard of Rapid Spanning Tree Protocol (RSTP). There are 5 port roles in this standard: Root port, Designated port, Alternative port, Backup port and Disabled port. In these 5 port roles, only Root port and Designated port can forward traffic.

Question 6

Select the action that results from executing these commands:

Switch(config-if)# switchport port-security
Switch(config-if)# switchport port-security mac-address sticky

A. A dynamically learned MAC address is saved in the startup-configuration file.
B. A dynamically learned MAC address is saved in the running-configuration file.
C. A dynamically learned MAC address is saved in the VLAN database.
D. Statically configured MAC addresses are saved in the startup-configuration file if frames from that address are received.
E. Statically configured MAC addresses are saved in the running-configuration file if frames from that address are received.


Answer: B


The full syntax of the second command is:

switchport port-security mac-address sticky [MAC]

If we don’t specify the MAC address (like in this question) then the switch will dynamically learn the attached MAC Address and place it into your running-configuration -> B is correct.

Question 7

What is valid reason for a switch to deny port access to new devices when port security is enabled?

A. The denied MAC addresses have already been learned or configured on another secure interface in the same VLAN.
B. The denied MAC address are statically configured on the port.
C. The minimum MAC threshold has been reached.
D. The absolute aging times for the denied MAC addresses have expired.


Answer: A


A security violation occurs in either of these situations:

* When the maximum number of secure MAC addresses is reached on a secure port and the source MAC address of the ingress traffic is different from any of the identified secure MAC addresses, port security applies the configured violation mode.

* If traffic with a secure MAC address that is configured or learned on one secure port attempts to access another secure port in the same VLAN, applies the configured violation mode.

From the second statement we can figure out A is the correct answer. But for your information we will discuss other answers as well.

Answer B is not correct because we can’t configured which MAC address will be denied. We can only configure which MAC is allowed.

We can only configure the maximum MAC threshold, not the minimum threshold -> C is not correct.

The aging times are only configured for allowed MAC addresses, not for denied MAC -> D is correct.

For your information about aging time:

When the aging type is configured with the absolute keyword, all the dynamically learned secure addresses age out when the aging time expires

This is how to configure the secure MAC address aging type on the port:

Router(config-if)# switchport port-security aging type absolute

and configure the aging time (aging time = 120 minutes)

Router(config-if)# switchport port-security aging time 120

When this command is used, all the dynamically learned secure addresses age out when the aging time expires

(Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/port_sec.html)


What is the difference between OSPF and RIP?

RIP and OSPF are both routing protocols likely you have used without even knowing it. A routing protocol describes the way your data (or signals) get from your PC on a network to another PC or device on the network.

RIP = Hop count Algorithm

The only metric used to calculate the cost of a path (path is from point A to destination B) is the hop count from router A to router B, even if you have a path with more hops and more bandwidth available.

OSPF = Link State Algorithm

This is a more intelligent algorithm, tha build a topology of network and build the cost regarding some characteristics of path like bandwidth, load, reliability, etc.A real life example would be like when work is done and its time to drive home. You have multiple routes usually to choose from to get home. The shortest way home would obviously be a straight line from work to home. Unfortunately no magic road exists from where ever you are directly to where ever you need to go. So if you were using RIP to get home you would drive in a straight line (or take the roads that would more or less offer you a straight route home). Thing is, these roads may be BUSY, very busy in fact, you can sit in traffic for a half hour moving less than 1km sometimes. So technically the shortest way (straight line) may not be the most efficient way. Now if you were using OSPF, you would take into account traffic and other variables to decide how to get home. The quickest route home may not be the direct path, it may be a back road that has less traffic on it, through a sub division, and then to your house. However because you were using OSPF you got home quicker than if you were to use RIP as RIP didn’t really look at what the traffic was like. Now in the above paragraph turn the word “work” into “client” and the word “home” into “server” and replace the word “you” with “the packet” – magically we turn the above paragraph into tecnobabble


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