About Intervlan Routing Connections Computer Science Essay

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To transport packets between VLANs, you must use a Layer 3 device. Traditionally, this has been a router's function. The router must have a physical or logical connection to each VLAN so that it can forward packets between them. This is known as interVLAN routing.

InterVLAN routing can be performed by which of the following?

InterVLAN routing can be performed by an external router that connects to each of the VLANs on a switch. Separate physical connections can be used, or the router can access each of the VLANs through a single trunk link. The external router also can connect to the switch through a single trunk link, carrying all the necessary VLANs

Which of the following is true about InterVLAN routing connections?

InterVLAN routing can be performed by an external router that connects to each of the VLANs on a switch. Separate physical connections can be used, or the router can access each of the VLANs through a single trunk link. Part A of the figure illustrates this concept.

Multilayer switches can perform both Layer 2 switching and interVLAN routing, as appropriate. Layer 2 switching occurs between interfaces that are assigned to Layer 2

VLANs or Layer 2 trunks. Layer 3 switching can occur between any type of interface, as long as the interface can have a Layer 3 address assigned to it.

As with a router, a multilayer switch can assign a Layer 3 address to a physical interface. It also can assign a Layer 3 address to a logical interface that represents an entire VLAN. Which of the following reflects this statement?

A multilayer switch can assign a Layer 3 address to a physical interface. It also can assign a Layer 3 address to a logical interface that represents an entire VLAN.

This is known as a switched virtual interface (SVI). Keep in mind that the Layer 3 address you configure becomes the default gateway for any hosts that are connected to the interface or VLAN. The hosts will use the Layer 3 interface to communicate outside of their local broadcast domains.

Which of the following commands can display a port's current mode?

InterVLAN routing first requires that routing be enabled for the Layer 3 protocol. In the case of IP, you would enable IP routing. In addition, you must configure static routes or a dynamic routing protocol. By default, every switch port on most Catalyst switch platforms is a Layer 2 interface, whereas every switch port on a Catalyst 6500 is a Layer 3 interface. If an interface needs to operate in a different mode, you must explicitly configure it. An interface is either in Layer 2 or Layer 3 mode, depending on the use of the switchport interface configuration command. You can display a port's current mode with the following command:

Switch# show interface type mod/num switchport

What does the term 'Disabled' signify in the example configuration given below?

Switch# show interface gigabitethernet 0/1 switchport

Name: Gi0/1

Switchport: Disabled

Switch#

If the Switchport: line in the command output is shown as enabled, the port is in Layer 2 mode. If this line is shown as disabled, as in the following example, the port is in Layer 3 mode:

Switch# show interface gigabitethernet 0/1 switchport

Name: Gi0/1

Switchport: Disabled

Switch#

Tip: Whenever you see the word switchport, think Layer 2. So if switchport is disabled, it must be Layer 3.

If an interface is in Layer 3 mode and you need to reconfigure it for Layer 2 functionality instead, which of the following command sequences is used?

If an interface is in Layer 3 mode and you need to reconfigure it for Layer 2 functionality instead, use the following command sequence:

Switch(config)# interface type mod/num

Switch(config-if)# switchport

The switchport command puts the port in Layer 2 mode. Then you can use other switchport command keywords to configure trunking, access VLANs, and so on.

For Layer 3 functionality, you must explicitly configure switch ports with the following command sequence which of the following command sequences is used?

Physical switch ports also can operate as Layer 3 interfaces, where a Layer 3 network address is assigned and routing can occur, as shown previously in Figure 11-2. For Layer 3 functionality, you must explicitly configure switch ports with the following command sequence:

Switch(config)# interface type mod/num

Switch(config-if)# no switchport

Switch(config-if)# ip address ip-address mask [secondary]

The no switchport command takes the port out of Layer 2 operation. You then can assign a network address to the port, as you would to a router interface.

Note: Keep in mind that a Layer 3 port assigns a network address to one specific physical interface. If several interfaces are bundled as an EtherChannel, the EtherChannel can also become a Layer 3 port. In that case, the network address is assigned to the port-channel interface-not to the individual physical links within the channel.

For SVI port configuration, you need to first define or identify the VLAN interface; and then assign any Layer 3 functionality to it. Which of the following configuration commands will assign a Layer 3 functionality to the VLAN interface?

The logical Layer 3 interface is known as an SVI. However, when it is configured, it uses the much more intuitive interface name vlan vlan-id, as if the VLAN itself is a physical interface. First, define or identify the VLAN interface; then assign any Layer 3 functionality to it with the following configuration commands:

Switch(config)# interface vlan vlan-id

Switch(config-if)# ip address ip-address mask [secondary]

The VLAN must be defined and active on the switch before the SVI can be used. Make sure that the new VLAN interface also is enabled with the no shutdown interface configuration command.

Which of the following examples shows how VLAN 100 is created and then defined as a Layer 3 SVI?

The VLAN and the SVI are configured separately, even though they interoperate. Creating or configuring the SVI doesn't create or configure the VLAN; you still must

define each one independently. the following example shows how VLAN 100 is created and then defined as a Layer 3 SVI:

Switch(config)# vlan 100

Switch(config-vlan)# name Example_VLAN

Switch(config-vlan)# exit

Switch(config)# interface vlan 100

Switch(config-if)# ip address 192.168.100.1 255.255.255.0

Switch(config-if)# no shutdown

The Layer 3 engine (essentially a router) maintains routing information, whether from static routes or dynamic routing protocols. Basically, the routing table is reformatted into an ordered list with the most specific route first, for each IP destination subnet in the table. The new format is called a Forwarding Information Base (FIB) and contains routing or forwarding information that the network prefix can reference. Which of the following is true about the Forwarding Information Base (FIB)?

In the FIB, routes would be ordered with the most specific, or longest match, first, followed by less specific subnets. When the switch receives a packet, it easily can examine the destination address and find the longest-match destination route entry in the FIB. The FIB also contains the next-hop address for each entry. When a longest-match entry is found in the FIB, the Layer 3 next-hop address is found, too. The FIB also contains host route (subnet mask 255.255.255.255) entries. These normally are not found in the routing table unless they

are advertised or manually configured. Host routes are maintained in the FIB for the most efficient routing lookup to directly connected or adjacent hosts. And, as with a routing table, the FIB is dynamic in nature.

When the Layer 3 engine sees a change in the routing topology, it sends an update to the FIB, and the FIB receives the same change. Which of the following is being explained in this statement?

As with a routing table, the FIB is dynamic in nature. When the Layer 3 engine sees a change in the routing topology, it sends an update to the FIB. Anytime the routing table receives a change to a route prefix or the next-hop address, the FIB receives the same change. Also, if a next-hop address is changed or aged out of the Address Resolution Protocol (ARP) table, the FIB must reflect the same change.

Which of the following commands will display FIB table entries related to a specific interface or VLAN?

You can display FIB table entries related to a specific interface or VLAN with the following form of the show ip cef command:

Switch# show ip cef [type mod/num | vlan vlan-id] [detail]

Which of the following commands will help you view FIB entries by specifying an IP prefix address and mask?

You also can view FIB entries by specifying an IP prefix address and mask, using the following form of the show ip cef command:

Switch# show ip cef [prefix-ip prefix-mask] [longer-prefixes] [detail]. Normally, only an exact match of the IP

prefix and mask will be displayed if it exists in the CEF table. To see other longer match entries, you can add the longer-prefixes keyword.

CEF can also be optimized through the use of which of the following specialized forwarding techniques?

CEF operations can be handled on a single hardware platform, such as the Catalyst 3560 and 3750 switches. The FIB is generated and contained centrally in the switch. CEF can be optimized through the use of specialized forwarding hardware, using the following

techniques: â-  Accelerated CEF (aCEF) â-  Distributed CEF (dCEF)

CEF can be optimized through the use of specialized forwarding hardware, using the following techniques:

â-  Accelerated CEF (aCEF)

â-  Distributed CEF (dCEF)

Which of the following explains Accelerated CEF (aCEF)?

In Accelerated CEF (aCEF), CEF is distributed across multiple Layer 3 forwarding engines, typically located on Catalyst 6500 line cards. These engines do not have the capability to store and use the entire FIB, so only a portion of the FIB is downloaded to them at any time. This functions as an FIB "cache", containing entries that are likely to be used again. If FIB entries are not found in the cache, requests are sent to the Layer 3 engine for more FIB information. The net result is that CEF is accelerated on the line cards, but not necessarily at a sustained wire-speed rate.

CEF can be optimized through the use of specialized forwarding hardware, using the following techniques:

â-  Accelerated CEF (aCEF)

â-  Distributed CEF (dCEF)

Which of the following explains Distributed CEF (dCEF)?

In Distributed CEF (dCEF), CEF can be distributed completely among multiple Layer 3 forwarding engines for even greater performance. Because the FIB is selfcontained for complete Layer 3 forwarding, it can be replicated across any number of independent Layer 3 forwarding engines. The Catalyst 6500 has line cards that

support dCEF, each with its own FIB table and forwarding engine. A central Layer 3 engine (the MSFC3, for example) maintains the routing table and generates the FIB,

which is then dynamically downloaded in full to each of the line cards.

What is the adjacency table?

A router normally maintains a routing table containing Layer 3 network and next-hop information, and an ARP table containing Layer 3 to Layer 2 address mapping. These tables are kept independently. Recall that the FIB keeps the Layer 3 next-hop address for each entry. To streamline packet forwarding even more, the FIB has corresponding Layer 2 information for every next-hop entry. This portion of the FIB is called the adjacency table, consisting of the MAC addresses of nodes that can be reached in a single Layer 2 hop.

Which of the following commands will display the adjacency table's contents?

The FIB has corresponding Layer 2 information for every Layer 3 next-hop entry. This portion of the FIB is called the adjacency table, consisting of the MAC addresses of nodes that can be reached in a single Layer 2 hop. You can display the adjacency table's contents with the following command:

Switch# show adjacency [type mod/num | vlan vlan-id] [summary | detail]

Adjacencies are kept for each next-hop router and each host that is connected directly to the local switch. You can see more detailed information about the adjacencies by using the detail keyword.

As a next-hop address receives a valid ARP entry, the adjacency table is updated. If an ARP entry does not exist, the FIB entry is marked as "CEF glean". What does CEF Glean signify?

The adjacency table information is built from the ARP table. As a next-hop address receives a valid ARP entry, the adjacency table is updated. If an ARP entry does not exist, the FIB entry is marked as "CEF glean". This means that the Layer 3 forwarding engine can't forward the packet in hardware because of the missing Layer 2 next-hop address. The packet is sent to the Layer 3 engine so that it can generate an ARP request and receive an ARP reply. This is known as the CEF glean state, in which the Layer 3 engine must glean the next-hop destination's

MAC address.

What is meant by ARP throttling or throttling adjacency?

During the time that an FIB entry is in the CEF glean state waiting for the ARP resolution, subsequent packets to that host are immediately dropped so that the input queues do not fill and the Layer 3 engine does not become too busy worrying about the need for duplicate

ARP requests. This is called ARP throttling or throttling adjacency. If an ARP reply is not received in 2 seconds, the throttling is released so that another ARP request can be triggered. Otherwise, after an ARP reply is received, the throttling is released, the FIB entry can be completed, and packets can be forwarded completely in hardware.

Which of the following is a type of adjacency listing?

The adjacency table also can contain other types of entries so that packets can be handled efficiently. For example, you might see the following adjacency types listed:

â-  Null adjacency - Used to switch packets destined for the null interface

â-  Drop adjacency - Used to switch packets that can't be forwarded normally

â-  Discard adjacency - Used when packets must be discarded because of an access list or other policy action.

â-  Punt adjacency - Used when packets must be sent to the Layer 3 engine for further processing.

Which of the following commands will help you gauge drop adjacency activity?

Drop adjacency is used to switch packets that can't be forwarded normally. In effect, these packets are dropped without being forwarded. Packets can be dropped because of an encapsulation failure, an unresolved address, an unsupported protocol, no valid route present, no valid adjacency, or a checksum error. You can gauge drop adjacency activity with the following command:

Switch# show cef drop

CEF Drop Statistics

Slot Encap_fail Unresolved Unsupported No_route No_adj ChkSum_Err

RP 8799327 1 45827 5089667 32 0

Switch#

You can gauge the CEF punt activity by looking at the various punt adjacency reasons listed by which of the following commands?

Punt adjacency is used when packets must be sent to the Layer 3 engine for further processing. You can gauge the CEF punt activity by looking at the various punt adjacency

reasons listed by the show cef not-cef-switched command:

Switch# show cef not-cef-switched

CEF Packets passed on to next switching layer

Slot No_adj No_encap Unsupp'ted Redirect Receive Options Access Frag

RP 3579706 0 0 0 41258564 0 0 0

Switch#

Which of the following are the various punt adjacency reasons listed by the show cef not-cef-switched command?

You can gauge the CEF punt activity by looking at the various punt adjacency reasons listed by the show cef not-cef-switched command:

Switch# show cef not-cef-switched

CEF Packets passed on to next switching layer

Slot No_adj No_encap Unsupp'ted Redirect Receive Options Access Frag

RP 3579706 0 0 0 41258564 0 0 0

Switch#

The reasons shown are as follows:

â-  No_adj-An incomplete adjacency

â-  No_encap-An incomplete ARP resolution

â-  Unsupp'ted-Unsupported packet features

â-  Redirect-ICMP redirect

â-  Receive-Layer 3 engine interfaces; includes packets destined for IP addresses that are assigned to interfaces on the Layer 3 engine, IP network addresses, and IP broadcast addresses

â-  Options-IP options present

â-  Access-Access list evaluation failure

â-  Frag-Fragmentation failure

When a multilayer switch finds valid entries in the FIB and adjacency tables, a packet is almost ready to be forwarded. But one step remains: The packet header information must be rewritten. Which of the following changes to the packet does the packet rewrite engine make just before forwarding?

When a multilayer switch finds valid entries in the FIB and adjacency tables, a packet is almost ready to be forwarded. One step remains: The packet header information must be rewritten.

The packet is untouched and still has the original destination MAC address of the switch itself. The IP header also must be adjusted, as if a traditional router had done the forwarding.

The switch has an additional functional block that performs a packet rewrite in real time. The packet rewrite engine makes the following changes to the packet just before forwarding:

â-  Layer 2 destination address - Changed to the next-hop device's MAC address

â-  Layer 2 source address - Changed to the outbound Layer 3 switch interface's MAC address

â-  Layer 3 IP TTL - Decremented by one because one router hop has just occurred

â-  Layer 3 IP checksum - Recalculated to include changes to the IP header

â-  Layer 2 frame checksum - Recalculated to include changes to the Layer 2 and Layer 3 headers

A traditional router normally would make the same changes to each packet. The multilayer switch must act as if a traditional router were being used, making identical changes. However, the multilayer switch can do this very efficiently with dedicated packet-rewrite hardware and address information obtained from table lookups.

Which of the following interface configuration commands is used to disable CEF on the Catalyst 3750 and 4500, respectively?

CEF is enabled on all CEF-capable Catalyst switches by default. In fact, the Catalyst 6500 (with a Supervisor 720 and its integrated MSFC3, or a Supervisor 2 and MSFC2 combination) runs CEF inherently, so CEF never can be disabled. Switches such as the Catalyst 3750 and 4500 run CEF by default, but you can disable CEF on a per-interface basis. You can use the no ip route-cache cef and no ip cef

interface configuration commands to disable CEF on the Catalyst 3750 and 4500, respectively. You should always keep CEF enabled whenever possible, except when you need to disable it for debugging purposes.

Which of the following EXEC commands verifies the configuration of a Layer 2 port?

To verify the configuration of a Layer 2 port, you can use the following EXEC command:

Switch# show interface type mod/num switchport

The output from this command displays the access VLAN or the trunking mode and native VLAN. The administrative modes reflect what has been configured for the port,

whereas the operational modes show the port's active status.

Which of the following EXEC commands will verify the configuration of a Layer 3 or routed port?

To verify the configuration of a Layer 3 or routed port, you can use the same command as you would to verify the configuration of a Layer 2 port. In this case, you should see the switchport (Layer 2) mode disabled.

Which of the following EXEC commands will verify the configuration of an SVI?

To verify the configuration of an SVI, you can use the following EXEC command:

Switch# show interface vlan vlan-id

The VLAN interface should be up, with the line protocol also up. If this is not true, either the interface is disabled with the shutdown command or the VLAN itself has not been defined on the switch. Use the show vlan command to see a list of configured VLANs.

Which of the following EXEC commands can you use to view the entire FIB?

CEF operation depends on the correct routing information being generated and downloaded to the Layer 3 forwarding engine hardware. This information is contained in the FIB and is maintained dynamically. To view the entire FIB, use the following EXEC command:

Switch# show ip cef

To see complete FIB table information for a specific interface, use the following EXEC command:

Switch# show ip cef type mod/num [detail]

Suppose a host connects to the network, but doesn't yet have an IP address. It needs to request an address via DHCP. In which of the following ways will it negotiate so it can send a packet to a DHCP server without having a valid IP address to use as a source address?

The Dynamic Host Configuration Protocol (DHCP) is usually leveraged to provide a means for dynamic address assignment to any host that can use the protocol. DHCP is defined in RFC 2131 and is built around a client/server model - hosts requesting IP addresses use a DHCP client, whereas address assignment is handled by a DHCP server. Suppose a host connects to the network, but doesn't yet have an IP address. It needs to request an address via DHCP. It can do so through the following negotiation steps:

1. The client sends a "DHCP Discover" message as a broadcast - Even without a valid source address, the client can send to the broadcast address to find any DHCP server that might be listening. The client's MAC address is included in the broadcast message.

2. A DHCP server replies with a "DHCP Offer" message - The offer contains an offer for the use of an IP address, subnet mask, default gateway, and some parameters for using the IP address.

The server also includes its own IP address to identify who is making the offer. (There could be multiple addresses offered, if more than one DHCP server received the broadcast DHCP Discover message.) Because the client doesn't yet have a valid IP address, the server must broadcast the offer so the client can receive it.

3. The client sends a "DHCP Request" message - When it is satisfied with a DHCP offer, the client formally requests use of the offered address. A record of the offer is included so that only the server that sent the offer will set aside the requested IP address. Again, the request is sent as a broadcast because the client hasn't officially started using a valid address.

4. The DHCP server replies with a "DHCP ACK" message - The IP address and all parameters for its use are returned to the client as formal approval to begin using the address. The ACK message is sent as a broadcast.

After you have configured a Layer 3 address on a switch interface, you can configure a DHCP server that runs natively on the switch itself. Which of the following command sequences to configure a DHCP server?

After you have configured a Layer 3 address on a switch interface, you can configure a DHCP server that runs natively on the switch itself. The switch will intercept DHCP broadcast packets from client machines within a VLAN. Use the following command sequence to configure a DHCP server:

Switch(config)# ip dhcp excluded-address start-ip end-ip

Switch(config)# ip dhcp pool pool-name

Switch(config-dhcp)# network ip-address subnet-mask

Switch(config-dhcp)# default-router ip-address [ip-address2] [ip-address3] ...

Switch(config-dhcp)# lease {infinite | {days [hours [minutes]]}}

Switch(config-dhcp)# exit

If there are addresses within the IP subnet that should be reserved and not offered to clients, use the ip dhcp excluded-address command. You can define a range of addresses or a single address to be excluded.

The ip dhcp pool command uses a text string pool-name to define the pool or scope of addresses that will be offered. The network command identifies the IP subnet and subnet mask of the address range. The subnet should be identical to the one configured on the Layer 3 interface. In fact, the switch uses the network command to bind its DHCP server to the matching Layer 3 interface. By definition, the network and broadcast addresses for the subnet won't be offered to any client. The default-router command identifies the default router address that will be offered to clients. Generally, the default router should be the IP address of the corresponding Layer 3 interface on the switch.

Finally, you can set the IP address lease duration with the lease command. By default, leases are offered with a 1 day limit. You can monitor the DHCP server address leases with the show ip dhcp binding command.

In which of the following ways can you configure the multilayer switch to relay DHCP messages between clients and the server, even if they are located on different VLANs or subnets?

If a DHCP server is centrally located in the network, you can configure the multilayer switch to relay DHCP messages between clients and the server, even if they are located on different VLANs or subnets. First, configure a Layer 3 interface that joins the same VLAN as the client machines. This interface can be the default gateway for the clients and can act as a DHCP relay. Next, use the ip helper-address command to identify the IP address of the actual DHCP server, as in the following example:

Switch(config)# interface vlan5

Switch(config-if)# ip address 192.168.1.1 255.255.255.0

Switch(config-if)# ip helper-address 192.168.199.4

Switch(config-if)# exit

As a DHCP relay, the switch will intercept the broadcast DHCP messages from the client and will forward them on to the server address as unicast messages. The switch keeps track of the subnet where the client messages arrived so that it can relay the DHCP server responses back appropriately.

While configuring a DHCP relay, you can use the ip helper-address command to identify the IP address of the actual DHCP server. Which of the following methods will configure more than one helper address?

You can configure more than one helper address by repeating the ip helper-address command with different addresses. In this case, the switch will relay each DHCP request from a client to each of the helper addresses simultaneously. If more than one server replies, each reply will be relayed back to the client and the client will have to choose one acceptable response.

Which of the following examples displays the FIB contents for a switch?

To view the entire FIB, use the following EXEC command:

Switch# show ip cef

The following example shows sample output from this command.

Switch# show ip cef

Prefix Next Hop Interface

0.0.0.0/32 receive

192.168.199.0/24 attached Vlan1

192.168.199.0/32 receive

192.168.199.1/32 receive

192.168.199.2/32 192.168.199.2 Vlan1

192.168.199.255/32 receive

Switch#

On this switch, only VLAN 1 has been configured with the IP address 192.168.199.1 255.255.255.0. Notice several things about the FIB for such a small configuration:

â-  0.0.0.0/32 - An FIB entry has been reserved for the default route. No next hop is defined, so the entry is marked "receive" so that packets will be sent to the Layer 3 engine for further processing.

â-  192.168.199.0/24 - The subnet assigned to the VLAN 1 interface is given its own entry. This is marked "attached" because it is connected directly to an SVI, VLAN 1.

â-  192.168.199.0/32 - An FIB entry has been reserved for the exact network address. This is used to contain an adjacency for packets sent to the network address, if the network is not directly connected. In this case, there is no adjacency, and the entry is marked "receive"

â-  192.168.199.1/32 - An entry has been reserved for the VLAN 1 SVI's IP address. Notice that this is a host route (/32). Packets destined for the VLAN 1 interface must be dealt with internally, so the entry is marked "receive".

â-  192.168.199.2/32 - This is an entry for a neighboring multilayer switch, found on the VLAN 1 interface. The next-hop field has been filled in with the same IP address, denoting that an adjacency is available.

â-  192.168.199.255/32 - An FIB entry has been reserved for the 192.168.199.0 subnet's broadcast address. The route processor (Layer 3 engine) handles all directed broadcasts, so the entry is marked "receive".

To see complete FIB table information for a specific interface, use the following EXEC command:

Switch# show ip cef type mod/num [detail]

Why are IP addresses are offered on a leased basis after a DHCP negotiation?

Because DHCP is a dynamic mechanism, IP addresses are offered on a leased basis. Before the offered lease time expires, the client must try to renew its address; otherwise, that address may be offered up to a different client.

Which of the following commands can you use if there are addresses within the IP subnet that should be reserved and not offered to

clients?

If there are addresses within the IP subnet that should be reserved and not offered to clients, use the ip dhcp excluded-address command. You can define a range of addresses or a single address to be excluded.

The following command sequence to configure a DHCP server:

Switch(config)# ip dhcp excluded-address start-ip end-ip

Switch(config)# ip dhcp pool pool-name

Switch(config-dhcp)# network ip-address subnet-mask

Switch(config-dhcp)# default-router ip-address [ip-address2] [ip-address3] ...

Switch(config-dhcp)# lease {infinite | {days [hours [minutes]]}}

Switch(config-dhcp)# exit

Which of the following is true about the above configuration?

The ip dhcp pool command uses a text string pool-name to define the pool or scope of addresses that will be offered. The network command identifies the IP subnet and subnet mask of the address range. The subnet should be identical to the one configured on the Layer 3 interface. In fact, the switch uses the network command to bind its DHCP server to the matching Layer 3 interface. By definition, the network and broadcast addresses for the subnet won't be offered to any client. The default-router command identifies the default router address that will be offered to clients. Generally, the default router should be the IP address of the corresponding Layer 3 interface on the switch. Finally, you can set the IP address lease duration with the lease command. By default, leases are offered with a 1 day limit. You can monitor the DHCP server address leases with the show ip dhcp binding command.

If a DHCP server is centrally located in the network, you can configure the multilayer

switch to relay DHCP messages between clients and the server, even if they are located on different VLANs or subnets. To configure a DHCP relay, which of the following steps comes first?

First, configure a Layer 3 interface that joins the same VLAN as the client machines. This interface can be the default gateway for the clients and can act as a DHCP relay. Next, use the ip helper-address command to identify the IP address of the actual DHCP server, as in the following example:

Switch(config)# interface vlan5

Switch(config-if)# ip address 192.168.1.1 255.255.255.0

Switch(config-if)# ip helper-address 192.168.199.4

Switch(config-if)# exit

As a DHCP relay, the switch will intercept the broadcast DHCP messages from the client and will forward them on to the server address as unicast messages. The switch keeps track of the subnet where the client messages arrived so that it can relay the DHCP server responses back appropriately.

You can configure more than one helper address by repeating the ip helper-address command with different addresses. In this case, the switch will relay each DHCP request from a client to each of the helper addresses simultaneously. If more than one server replies, each reply will be relayed back to the client and the client will have to choose one acceptable response.

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