Describe how CEF-based MLS works.

CEF is a topology-based forwarding model in which all routing information is pre-populated into a forward information base (FIB) and L2 rewrite information is dynamically updated in an adjacency table.

What are the 2 main components of CEF?

- Forwarding information base (FIB) - CEF uses an FIB to make IP destination prefix-based switching decisions. The FIB maintains next-hop address information based on the information in the IP routing table.

 

- Adjacency table -The adjacency table maintains L2 addresses for all FIB entries.

Describe the innerworkings of centralized switching.

Centralized switching carries out forwarding decisions on a specialized ASIC that is central to all interfaces of a L3 switch. Routing, ACL, QoS, and forwarding decisions are made on the Supervisor Engine in a modular chassis.

Describe the innerworkings of distributed switching.

With distributed switching, the interfaces or like modules on L3 switches handle forwarding decisions independently. A centralized switching engine synchronizes L3 forwarding, routing, and rewrite tables to local tables on distributed switching-capable modules.

Explain how ARP throttling works on a CEF-based Catalyst switch.

During an ARP request, switches forward only the first several packets to the L3 engine for new destinations without rewrite information. The switch installs a throttling adjacency such that the switch drops subsequent packets to the specific destination address in hardware until an ARP response is received. The switch removes the throttling adjacency when an ARP reply is received from the L3 engine.

What type of tables does a multilayer switch build using high-speed memory tables?

MLSs build routing (CEF FIB and adjancency), bridging, QoS, and ACL tables in hardware using high-speed memory tables.

What command can be used to view the software CEF table as per the L3 switching engine?

What command can be used to display the adjacency table information on a Catalyst switch?

What command is used to debug CEF-related issues?

What are the 3 different network availability and stability problems that networks might experiance?

- Delay (or latency) - The amount of time it takes for a packet to reach a destination

 

- Delay variation (or jitter) - The change in interpacket latency within a stream over time

 

- Packet loss - The measure of lost packets between any given source and destination. 

What 2 types of delay comprise latency, or end-to-end delay?

- Fixed network delay - Includes encoding and decoding time and the latency required for the electrical and optical signals to travel the media en route to the receiver.

 

-  Variable-network delay - Refers to the network conditions, such as congestion, that affect the overall latency of a packet in transit from source to destination.

What are the different delay types that induce end-to-end latency?

- Packetization delay - Amount of time it takes to segment, sample, and encode signals, process data, and turn the data into packets.

 

- Serialization delay - Amount of time it takes to place the bits of a packet, encapsulated in a frame, onto the physical media

 

- Propagation delay - Amount of time it takes to transmit the bits of a frame across the physical wire

 

- Processing delay - Amount of time it takes for a network device to take the frame from an input interface, place it into a receive queue, and place it into the output queue of the output interface.

 

-  Queuing delay - Amount of time a packet resides in the output queue of an interface.

Out of the different types of delays that induce end-to-end latency, which is the only variable delay?

Queuing delay is the delay over which you have the most control with QoS features, and the only variable delay

What do switches use to combat jitter problems in traffic?

Cisco network devices use jitter buffers to smooth out changes in arrival times of data packets that contain data, voice, and video.

What are the 2 different QoS architectures used in IP networks when designing a QoS solution?

The 2 different models are IntServ and DiffServ

What are the 3 basic levels of service for QoS?

Best-effort service, Integrated services, and Differentiated services

Describe the best-effort service level for QoS.

The best-effort service level is the standard form of connectivity without guarantees. This type of service uses first-in, first-out (FIFO) queues, which simply transmit packets as they arrive in a queue with no preferential treatment.

Describe the integrated services level for QoS.

The integrated services level for QoS, also known as hard QoS, is an absolute reservation of services. In other words, the IntServ model implies that traffic flows are reserved explicitly by all intermediate systems and resources.

Describe the differentiated services level for QoS.

The differentiated services level for QoS, also known as soft QoS, is class-based, where some classes of traffic receive preferential handling over other traffic classes. They use statistical preferences, not a hard guarantee like integrated services, so it categorizes traffic and then sorts it into queues of various efficiencies.

Describe the use of Resource Reservation Protocol (RSVP)

IntServ is implemented through the use of RSVP, enabled at both the endpoints and the network devices between. The RSVP-enabled application requests a specific kind of service from the RSVP-enabled network before it sends data. Explicit signaling via RSVP makes the request, and the application informs the network of its traffic profile and requests a particular kind of service that can encompass its bandwidth and delay requirements.

How does Resource Reservation protocol (RSVP) provide admission control?

RSVP provides admission control using Common Open Policy Service (COPS), which centralizes admission control at a Policy Decision Point (PDP)

What does DiffServ use as the QoS priority descriptor value?

What are the 2 main per-hop behaviors (PHB) used to standardize the meanings for several codepoint values?

What does assured forwarding (AF) use to define classes?

Assured Frowarding (AF) defines classes by using DSCP values.

How many different classes does Assured Forwarding (AF) have?

AF has four AF classes, AF1x through AF4x, where the AF4x class is considered to have more importance than the AF1 class. Also, within each class, there are 3 drop probabilities.

What is the purpose of classifiation in regards to QoS?

Classification destinguishes a frame or packet with a specific priority or predetermined criteria. In the case of Catalyst switches, classification determines the internal DSCP value on frames. Catalyst switches use this internal DSCP value for QoS packet handling, including policing and scheduling as frames traverse the switch

How is traffic classified when QoS is enabled and with no other QoS configuration?

With QoS enabled and no other QoS configurations, traffic is treated with a default classification. With respect to DSCP values, the default classification for ingress frames is a DSCP value of 0

What are the available methods of packet classification on Catalyst switches?

- Per-interface trust modes

 

- Per-interface manual classification using specific DSCP, IP Precedence, or CoS values

 

- Per-packet based on ACLs

 

- Network-Based Application Recognition (NBAR) 

What QoS functions are generally preformed at the building access submodule?

- Congestion avoidance

What QoS functions are generally preformed on backbone L3 devices?

- Congestion avoidance

What QoS features are generally preforms on the Edge submodule?

At what submodule is it best to form trust boundaries?

The Building Access submodule is typically where the trust boundary is formed. In this submodule, the Catalyst switches set or trust the DSCP value of ingress packets for use through the entire network.

When is it necessary to preform classification and marking at the Building Distribution submodule?

Classification and marking other than trusting in the Building Distribution submodule is necessary only if the Building Access layer does not classify packets or if the Catalyst switches used in the Building Access submodule do not support adequate features necessary to apply QoS effectively.