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Can I have solutions to the following questions?

If multilayer switch has routing capabilities then why we can go for dedicated router?

Why we cannot use a number for VLAN ID ohther than 1 to 4094 ?

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Accepted Solutions
Cisco Employee

Can I have solutions to the following questions?

Hi Ramesh,

If multilayer switch has routing capabilities then why we can go for dedicated router?

For several reasons - multilayer switch is usually equipped with a ports of the same type (i.e. Ethernet), while a router can accomodate various interface types (Ethernet, DSL, Serial, ISDN, WiFi, cable, ...). Next, some features may be available only on routers or high-end switches, for example, NAT, L2TP tunneling, sophisticated firewalling, deep packet inspection, complex queueing mechanisms, etc. This is all caused by the fact that routers implement these features in software so it is relatively easy to extend and improve these mechanisms at the expense of the router's throughput - the more features you turn on, the more work the router's processor has to do and thus the throughput decreases. Switches perform these functions in hardware for to achieve line-rate speeds, necessitating complicated and expensive hardware circuitry. Some of these features may be simply too complicated in order to be implemented in hardware at a reasonable price.

Why we cannot use a number for VLAN ID ohther than 1 to 4094 ?

Because the VLAN ID field carried in 802.1Q VLAN tags has 12 bits, yielding a total of 2^12=4096 possible values. VLAN ID 0 is reserved for priority-tagged frames without explicit VLAN membership indication, and the VLAN ID 4095 is an ID that is reserved for internal, proprietary switch use and should never be used as an ID of a real VLAN.

Best regards,

Peter

5 REPLIES
Cisco Employee

Can I have solutions to the following questions?

Hi Ramesh,

If multilayer switch has routing capabilities then why we can go for dedicated router?

For several reasons - multilayer switch is usually equipped with a ports of the same type (i.e. Ethernet), while a router can accomodate various interface types (Ethernet, DSL, Serial, ISDN, WiFi, cable, ...). Next, some features may be available only on routers or high-end switches, for example, NAT, L2TP tunneling, sophisticated firewalling, deep packet inspection, complex queueing mechanisms, etc. This is all caused by the fact that routers implement these features in software so it is relatively easy to extend and improve these mechanisms at the expense of the router's throughput - the more features you turn on, the more work the router's processor has to do and thus the throughput decreases. Switches perform these functions in hardware for to achieve line-rate speeds, necessitating complicated and expensive hardware circuitry. Some of these features may be simply too complicated in order to be implemented in hardware at a reasonable price.

Why we cannot use a number for VLAN ID ohther than 1 to 4094 ?

Because the VLAN ID field carried in 802.1Q VLAN tags has 12 bits, yielding a total of 2^12=4096 possible values. VLAN ID 0 is reserved for priority-tagged frames without explicit VLAN membership indication, and the VLAN ID 4095 is an ID that is reserved for internal, proprietary switch use and should never be used as an ID of a real VLAN.

Best regards,

Peter

New Member

Can I have solutions to the following questions?

Thank You Peter for your explanation. I faced this questions in my interviews. These days i am attending interviews.

I will not repeat the thing again by posting the same post under multiple categories.

Cisco Employee

Can I have solutions to the following questions?

Ramesh,

No problem. You are welcome.

Best regards,

Peter

Re: Can I have solutions to the following questions?

Just to add to peter impecable explanation. I would just like to a little bit to it. Generally speaking Routers are Feature rich but performance poor, MLS swithes are performance rich but feature poor.

for example if you compare a 3845 router and a 3750 switch. the router has features like MQC Qos, Netflow and other such features however its performance in terms of throughput is only (256Mbps-full duplex) although u have gig interfaces on the router. However the switch provides you line rate throughput but it doesnt support certains aspects of Qos like CBWFQ etc and also netflow.

so routers do have their own importance in networks.

HTH

Can I have solutions to the following questions?

The previous answers were real world and quite detailed and appreciated answers....oddly enough in my CCNP studies this morning I ran across this (below).  I always like real world answers better, but I know during test, sometimes they want rediculous text book answers.   Text below taken from howtonetwork.net btw...http://www.howtonetwork.net/members/LAN_Switching_Fundamentals.cfm.

The Differences between Switches and Routers

In modern-day networks, Multilayer switches, such as the Cisco Catalyst 6500 series switches, merge router and switch functionality. Because of this blurred line, it becomes even more important for network engineers to have a solid understanding of the differences between LAN switches and network routers when it comes to addressing the following design concerns:

  • Network loops
  • Network convergence
  • Broadcast traffic
  • Inter-subnet communication
  • Network security
  • Media dependence

LAN switches use the Spanning Tree Protocol (STP) to prevent Layer 2 loops. This is performed by the Spanning Tree Algorithm (STA), which places redundant links in a blocked state. Although this does prevent network loops, it also means that only a subset of the network topology is used for forwarding data. Routers, on the other hand, do not block redundant network paths; instead, they rely on routing protocols in order to use the optimum path and to prevent loops.

A switched network is said to be converged when all ports are in a forwarding or blocking state, while a routed network is said to be converged when all routers have the same view of the network. Depending on the size of the switched network, convergence might take a very long time. Routers have the advantage of using advanced routing protocols, such as OSPF, that maintain a topology of the entire network, allowing for rapid convergence.

By default, LAN switches will forward Broadcast Multicast and unknown Unicast frames. In large networks with many of these types of packets, the LAN can become saturated quickly, resulting in poor performance, packet loss, and an unpleasant user experience. Because routers do not forward Broadcasts by default, they can be used to break up Broadcast domains.

Although multiple physical switches can exist on the same LAN, they provide connectivity to hosts on the assumption that they are all on the same logical network. In other words, Layer 2 addressing assumes a flat address space with universally unique addresses. Routers can use a hierarchical addressing structure, which allows them to associate a logical addressing structure to a physical infrastructure so that each network segment has an IP subnet. This provides a routed network a more flexible traffic flow because routers can use the hierarchy to determine optimal paths depending on dynamic factors, such as bandwidth, delay, etc.

Both LAN switches and routers can provide network security, but this is based on different information. Switches can be configured to filter based on many variables pertaining to Data Link Layer frames. Routers can use Network and Transport Layer information. Multilayer switches have the capability of providing both types of filtering.

When designing switched internetworks, it is imperative to ensure that network hosts use the MTU representing the lowest common denominator of all the switched LANs that make up the internetwork. When using switches, however, this results in poor performance and limits throughput, even on fast links. Unlike LAN switches, however, most Layer 3 protocols can fragment packets that are too large for a particular media type, so routed networks can accommodate different MTUs, which allow them to maximize throughput in internetworks.

Table 1-1 below lists the minimum and maximum frame size for common types of media that may be found within internetworks:

Table 1-1. Frame Size for Common Media Types

Media Type

Minimum Valid Frame Size

Maximum Valid Frame Size

Ethernet

46 bytes

1500 bytes

Token Ring

32 bytes

16 KB theoretical, 4 KB normal

Fast Ethernet

46 bytes

1500 bytes

FDDI

32 bytes

4468 bytes

Serial HDLC

14 bytes

No limit, 4.5 KB normal

The Advantages of Using LAN Switches

LAN switches provide several advantages over bridges. These advantages include increased bandwidth to users via microsegmentation and supporting VLANs, which increase the number of Broadcast domains while reducing their overall size. In addition to these advantages, Cisco Catalyst switches also support Automatic Packet Recognition and Translation (APaRT).
Cisco's APaRT technology recognizes and converts a variety of Ethernet protocol formats into industry-standard CDDI and FDDI formats. Not all switches can provide these functions. 

The Advantages of Using Routers

Even within switched LANs, the importance of routers cannot be ignored. Routers, or Multilayer switches, provide the following critical functions in switched LANs:

  • Broadcast and Multicast control
  • Media transition
  • Network segment services

By default, routers do not forward Broadcast or Multicast packets. Instead, routers control Broadcast and Multicast packets via the following three methods:

  1. By caching the addresses of remote hosts and responding on behalf of remote hosts
  2. By caching advertised network services and responding on behalf of those services
  3. By providing special protocols, such as IGMP and PIM

Both routers and Multilayer switches can be used to connect networks of different media types, such as Fiber, Ethernet, and Token Ring, for example. Therefore, if a requirement for a switched campus network design is to provide high-speed connectivity between different media, these devices play a significant part in the design.

Routers are also responsible for providing Broadcast services, such as Proxy ARP, to a local network segment. When designing the switched LAN, it is important to consider the number of routers that can provide reliable services to a given network segment or segments.

As far as the VLAN answer, thats a good question....

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