Ethernet

Ethernet refers to the family of LAN products covered by the IEEE 802.3 standard. This standard defines the carrier sense multiple access collision detect (CSMA/CD) protocol. Four data rates are currently defined for operation over optical fiber and twisted-pair cables:

■ 10 Mbps—10BASE-T Ethernet
■ 100 Mbps—Fast Ethernet
■ 1000 Mbps—Gigabit Ethernet
■ 10,000 Mbps—10 Gigabit Ethernet

Ethernet has replaced just about every other LAN technology because of the following reasons:

■ It is easy to understand, implement, manage, and maintain.
■ It has a relatively low cost.
■ It provides extensive topological flexibility.
■ It is a standards-compliant technology.

802.3

802.3 defines the original shared media LAN technology. This early Ethernet specification runs at 10 Mbps. Ethernet can run over various media such as twisted pair and coaxial. We often see 802.3 Ethernet referred to as different terms because of the differences in the underlying media.

Here are examples:

■ 10BASE-T—Ethernet over Twisted Pair Media
■ 10BASE-F—Ethernet over Fiber Media
■ 10BASE2—Ethernet over Thin Coaxial Media
■ 10BASE5—Ethernet over Thick Coaxial Media

802.3U (Fast Ethernet)

Fast Ethernet refers to any one of a number of 100-Mbps Ethernet specifications. As its name implies, Fast Ethernet offers speeds 10 times that of the 10BASE-T Ethernet specification. Although Fast Ethernet is a much faster technology, it still preserves such qualities as frame format, MAC mechanisms, and maximum transmission unit (MTU). These similarities permit you to use existing 10BASE-T applications and network management tools on Fast Ethernet networks.

802.3Z (Gigabit Ethernet)

Once again, this Ethernet technology builds on the foundations of the old, but it increases speeds tenfold over Fast Ethernet to 1000 Mbps, or 1 gigabit per second (Gbps).
802.3AB (Gigabit Ethernet over Copper)

Gigabit Ethernet over Copper (also known as 1000BASE-T) is yet another extension of the existing Fast Ethernet standard. 802.3AB specifiesGigabit Ethernet operation over the Category 5e/6 cabling systems already installed. This reuse of the existing infrastructure helps make 802.3AB a highly cost-effective solution.

10 Gigabit Ethernet

The latest in Ethernet technologies, 10 Gigabit Ethernet, provides the following features:

■ High bandwidth
■ Low cost of ownership
■ Scalability from 10 Mbps to 10,000 Mbps
Long Reach Ethernet
The Cisco Long Reach Ethernet (LRE) networking solution delivers 5 to 15-Mbps speeds over existing Category 1/2/3 wiring. As the name conveys, this Ethernet-like performance extends 3500 to 5000 feet.

EtherChannel

EtherChannel allows you to bundle redundant links and treat them as a single link, thus achieving substantial bandwidth and redundancy benefits. It is often advisable to use an EtherChannel for key trunks in your campus design. Notice that EtherChannel affects STP, because ordinarily one or more of the links would be disabled to prevent a loop.

Following are guidelines for EtherChannel:

■ All Ethernet interfaces on all modules must support EtherChannel.

■ You have a maximum of eight interfaces per EtherChannel.

■ The ports do not need to be contiguous or on the same module.

■ All ports in the EtherChannel must be set for the same speed and duplex.

■ Enable all interfaces in the EtherChannel.

■ An EtherChannel will not form if one of the ports is a Switched Port Analyzer (SPAN) destination.

■ For Layer 3 EtherChannels, assign a Layer 3 address to the portchannel logical interface, not the physical interfaces.

■ Assign all EtherChannel ports to the same VLAN or ensure they are all set to the same trunk encapsulation and trunk mode.

■ The same allowed range of VLANs must be configured on all ports in an EtherChannel.

■ Interfaces with different STP port path costs can form anEtherChannel.

■ After an EtherChannel has been configured, a configuration made to the physical interfaces affects the physical interfaces only.

EtherChannel load balancing can use MAC addresses, IP addresses, or Layer 4 port numbers—either source, destination, or both source and destination addresses.

Here is an example:

Router# configure terminal
Router(config)# interface range fastethernet 2/2 -8
Router(config-if)# channel-group 2 mode desirable
Router(config-if)# end

VTP pruning

VTP pruning enables you to limit the amount of traffic sent on trunk ports. It limits the distribution of flooded frames to only switches that have members of the particular VLAN. You can enable VTP pruning with this command:

vtp pruning

When we enable pruning on the switch, all VLANs are pruned by default (with the exception of VLAN 1). You need to configure pruning on only one VTP server, and the setting automatically propagates. You can change this behavior by making select VLANs you choose pruneineligible. This is done with the following command:

switchport trunk pruning vlan {none {{add except remove} vlan[,vlan[,vlan[,...]]}}
The Cisco IOS command is as follows:

vtp pruning

VLAN trunking

802.1Q

The IEEE 802.1Q standard trunking protocol uses an extra tag in the MAC header to identify the VLAN membership of a frame across bridges. This tag is used for VLAN and quality of service (QoS) priority identification.

The VLAN ID (VID) associates a frame with a specific VLAN and provides the information that switches need to process the frame across the network. Notice that a tagged frame is 4 bytes longer than an untagged frame and contains 2 bytes of Tag Protocol Identifier (TPID) and 2 bytes of Tag Control Information (TCI). These components of an 802.1Q tagged frame are described in more detail here:

■ TPID—The Tag Protocol Identifier has a defined value of 8100 in hex; with the EtherType set at 8100, this frame is identified as carrying the IEEE 802.1Q/802.1P tag.

■ Priority—The first 3 bits of the Tag Control Information define user priority; notice the eight (23) possible priority levels. IEEE 802.1P defines the operation for these 3 user-priority bits.

■ CFI—The Canonical Format Indicator is a single-bit flag, always set to 0 for Ethernet switches. CFI is used for compatibility reasons between Ethernet networks and Token Ring.

■ VID—VLAN ID identifies the VLAN; notice it allows the identification of 4096 (212) VLANs. Two of these identifications are reserved, permitting the creation of 4094 VLANs.

802.1Q trunks feature a concept called the native VLAN. The native VLAN is a VLAN for which frames are not tagged. Here are the aspects of the native VLAN:

■ The VLAN a port is in when not trunking.

■ The VLAN from which frames are sent untagged on an 802.1Q port.

■ The VLAN to which frames are forwarded if received untagged on an 802.1Q port.

Cisco switches produce errors if the native VLAN does not match at each end of the link. The default native VLAN in Cisco devices is VLAN 1.

We can control the 802.1Q VLAN traffic that is sent over a trunk; this is possible for security purposes or load balancing.

The command used to create and control trunks on Cisco IOS-based switches is the interface command:

switchport trunk {allowed vlan vlan-list} {encapsulation {dot1q isl negotiate}} {native vlan vlan-id} {pruning vlan vlan-list}

VLAN Trunking Protocol (VTP) is a Cisco-proprietary Layer 2 multicast messaging protocol that synchronizes VLAN information across all media types and tagging methods on your switches. To enjoy the benefits of VTP, your switches must meet the following requirements:

■ We must configure the VTP domain name identically on each device; domain names are case-sensitive.
■ The switches must be adjacent.
■ The switches must be connected with trunk links.
■ The same VTP password must be configured if used in the domain.

Generally, you find four items in all VTP messages:

■ VTP protocol version (either 1 or 2)
■ VTP message type
■ Management domain name length
■ Management domain name

VTP has four possible message types:

■ Summary advertisements
■ Subset advertisements
■ Advertisement requests
■ VTP Join messages (used for pruning)

The VTP configuration revision number is extremely important. This value is used to determine whether a switch has stale information about VLANs and ultimately controls whether the switch overwrites its VLAN database with new information. The revision number increments each time a change is made to the VLAN database on a Server mode VTP system. The number is one from 0 to 4,294,967,295. We must ensure when introducing new Server mode switches that you do not inadvertently overwrite the VLAN database because of a higher configuration revision number on the new switch. Introducing new switches in Transparent mode helps ensure that this problem never results.

You have three possible modes for your VTP servers:

■ Server—This mode enables you to create, modify, and delete VLANs; these changes are advertised to VTP Client mode systems; Catalyst switches default to this mode.

■ Client—This mode does not allow for the creation, modification, or deletion of VLANs on the local device; VLAN configurations are synchronized from Server mode system(s).

■ Transparent—This mode permits the addition, deletion, and modification of VLAN information, but the information resides only locally on the Transparent device; these systems forward advertisements from servers but do not process them.

Here is a sample configuration of VTP for a Server mode system in Cisco IOS mode. Note that changing the VTP domain on this system resets the configuration revision number to 0:

Switch# configure terminal
Switch(config)# vtp mode server
Setting device to VTP SERVER mode.
Switch(config)# vtp domain Lab_Network
Setting VTP domain name to Lab_Network
Switch(config)# end
Switch#

Unicast flooding

If a destination MAC address is not in the MAC address table of the switch, the frame is flooded out all ports for that respective VLAN. Although some flooding is unavoidable and expected, excessive flooding might be caused by asymmetric routing, STP topology changes, or forwarding table overflow. Also, flooding can result from attacks on the network, especially in the case of denial-of-service (DoS) attacks.

Switches can now implement a unicast flood-prevention feature. This is implemented through the following global configuration command:

mac-address-table unicast-flood {limit kfps} {vlan vlan} {filter timeout alert shutdown}

An alternative configuration approach found on some Catalyst model devices (such as the 6500 series) is to use what is known as Unknown Unicast Flood Blocking (UUFB). This is configured with the following simple interface command:

switchport block unicast

Storm Control

The Storm Control feature protects a LAN from being affected by unicast, broadcast, or multicast storms that might develop. The switch implements storm control by counting the number of packets of a specified type received within the one-second time interval and compares the measurement with a predefined suppression-level threshold. Storm Control can typically enable the administrator to control traffic by a percentage of total bandwidth or the traffic rate at which packets are received. It is important to note that when the rate of multicast traffic exceeds a set threshold, all incoming traffic (broadcast, multicast, and unicast) is dropped until the level drops below the specified threshold level. Only spanning-tree packets are forwarded in this situation. When broadcast and unicast thresholds are exceeded, traffic is blocked for only the type of traffic that exceeded the threshold.

Storm Control is configured at the interface level with the following command:

storm-control {broadcast multicast unicast} level {level [level-low] pps pps [pps-low]}

BPDU Guard

This Cisco STP feature protects the network from loops that could occur if BPDUs were received on a PortFast port. Because BPDUs should never arrive at these ports, their reception indicates a misconfiguration or a security breach. BPDU Guard causes the port to errordisable upon the reception of these frames.

You can configure BPDU Guard globally to have the feature enabled for all PortFast ports on the system. The command to do this is as follows:

spanning-tree portfast bpduguard

You can also enable the feature at the interface level. Use this command:

spanning-tree bpduguard enable

You can enable this feature at the interface level even if PortFast is not enabled on the port. Once again, the receipt of a BPDU causes the port to error-disable.

Root Guard

Root Guard enables an administrator to enforce the root bridge placement in the network. Service providers that connect switches to customer networks are often interested in this technology because they want to ensure that no customer device inadvertently or otherwise becomes the root of the spanning tree. Root Guard ensures that the port on which Root Guard is enabled is the DP. If the switch receives superior STP BPDUs on a Root Guard–enabled port, the port is moved to a root-inconsistent STP state. This root-inconsistent state is effectively equal to the Listening port state. No traffic is forwarded across this port. This protects the current placement of the root bridge in the infrastructure.
We can enable this feature on a port with the following interface configuration command:

spanning-tree guard root

Unidirectional Link Detection

Unidirectional Link Detection (UDLD), as in the below diagram, detects and disables unidirectional links. A unidirectional link occurs when traffic transmitted from the local switch is received by the neighbor, but traffic sent from the neighbor is not. Unidirectional links can cause a variety of problems, including spanning-tree loops. UDLD performs tasks that autonegotiation cannot perform.

To perform UDLD, packets are sent to neighbor devices on interfaces with UDLD enabled. Therefore, both sides of the link must support UDLD. By default, UDLD is locally disabled on copper interfaces and is locally enabled on all Ethernet fiber-optic interfaces. The Cisco IOScommand to enable UDLD on an interface is simply this:
udld enable

Loop Guard

As its name implies, Loop Guard is a method for ensuring that STP loops never occur in a particular topology. Even though STP guards against such loops as best it can, they could still occur because of things like unidirectional link failures or switch congestion issues. Loop Guard prevents loops conservatively by preventing alternate or root ports from becoming DPs in the topology. If BPDUs are not received on a non-DP, and Loop Guard is enabled, that port is moved into the STP loop-inconsistent Blocking state, instead of the Listening / Learning / Forwarding state. Loop Guard operates only on ports that are considered point-to-point by the spanning tree, and it cannot be run in conjunction with Root Guard on an interface.

To enable Loop Guard, you can use the following global configuration mode command:

spanning-tree loopguard default

802.1s Multiple Spanning Tree

MSTP (IEEE 802.1s) is an IEEE standard that allows several VLANs to be mapped to a reduced number of spanning-tree instances. This provides advantages over PVST+ because typical topologies need only a few spanning-tree topologies to be optimized.
We configure a set of switches with the same MISTP parameters, and this becomes an MST region. With MISTP, you have an internal spanning tree capable of representing the entire MST region as a common spanning tree for backward compatibility with earlier IEEE implementations.

Follow are the steps to configure MISTP:

Step 1. Globally enable MISTP (MSTP) on your switches:

spanning-tree mode mst

Step 2. Enter MST configuration submode:

spanning-tree mst configuration

Step 3. Set the MST region name:

name name

Step 4. Set a configuration revision number:

revision rev_num

Step 5. Map your VLANs to MST instances:

instance int vlan range


We can verify an MSTP configuration using the following commands:

show spanning-tree mst configuration
show spanning-tree mst vlan_id

802.1w Rapid Spanning Tree Protocol

Rapid Spanning Tree Protocol (RSTP or IEEE 802.1w) improves on 802.1D. The protocol incorporates many new features to speed convergence, including incorporation of the ideas presented by Cisco in its enhancements to 802.1D. Although there are many, many improvements with the new technology, the configuration remains almost identical - and the two technologies can coexist. Full benefits are not realized until all systems are running RSTP, however. RSTP requires full-duplex, point-to-point connections between adjacent switches to achieve fast convergence RSTP defines edge ports as those not participating in STP. Edge ports can be statically configured or will be recognized by the PortFast configuration command.

RSTP port states

RSTP port states are simplified from 802.1D and consist of the following:

■ Discarding
■ Learning
■ Forwarding

Also, the port states are no longer tied directly to port roles. For example, a DP could be Discarding, even though it is destined to transition to the Forwarding state.

RSTP port roles

■ Root port—This port role exists in 802.1D, too, and is the "best" path back to the root bridge; it must exist on all nonroot bridges.
■ Designated port—This port role exists in 802.1D, too, and there must be a DP on all segments in the topology. By default, all ports on the root bridge are DPs.
■ Alternative port—This port role is new to 802.1w. This port is a quickly converging backup port to the current DP on a segment.
■ Backup port—This port role is new to 802.1w. This port is a quickly converging backup to the root port for a system.


RSTP BPDUs

All bridges now send BPDUs every hello time period (2 seconds by default). The BPDUs now act as a keepalive—protocol information is aged if no BPDUs are heard for three consecutive hello times.

RSTP proposal and agreement process/topology change mechanism

Convergence occurs on a link-by-link basis in 802.1w. No longer is there a reliance on timers for convergence as there is in 802.1D. A proposal and agreement process replaces the timer methodology of STP and flows downstream from the root device.
In RSTP, only nonedge ports moving to the Forwarding state cause a topology change (TC). The originator of a TC is now responsible for flooding it through the network.

Implementing RSTP

On most Cisco switches, configuring 802.1s (Multiple Spanning Tree, MST) automatically enables RSTP. Cisco did invent a mode of operation that allows you to use RSTP without the implementation of MST. It is called PVST+ mode. You can enable it on a switch with the following command:

spanning-tree mode rapid-pvst

Cisco-proprietary enhancements to 802.1D

PortFast

PortFast, shown in Figure 2-3, is a Cisco-proprietary enhancement to the 802.1D STP implementation. You apply the command to specific ports, and that application has two effects:

■ Ports coming up are put directly into the forwarding STP mode.
■ The switch does not generate a TCN when a port configured for

PortFast is going up or down—for example, when a workstation power-cycles. Therefore, consider enabling PortFast on ports that are connected to end-user workstations. Caution must be used with PortFast ports to ensure that hubs, switches, bridges, or any other device that could cause a loop are not connected to these ports.

UplinkFast

Configure UplinkFast on wiring closet switches. It detects a directly
connected failure and allows a new root port to come up almost immediately.
When you are configuring UplinkFast, the local switch has a priority
set to 49,152, and it adds 3000 to the cost of all links. Finally, a mechanism
is included that causes the manipulation of MAC address tables
for other bridges.

BackboneFast

Configure BackboneFast on all switches. It speeds convergence when
the failure occurs and is indirectly located, such as in the core of the
backbone. It reduces convergence from about 50 seconds to about 30
seconds.

Topology changes Notification (TCN)

STP uses a Topology Change Notification (TCN) BPDU to alert the root bridge that a topology change to the spanning tree might need to occur. The Type field of the BPDU signifies the TCN BPDU: 0x80. TCN BPDUs improve convergence time when failures in the network occur—primarily because they help in a rapid updating of the MAC address tables.
The TCN process of 802.1D is as follows:
1. A bridge sends a TCN BPDU in two cases:
a. It takes a port into forwarding, and it has at least one designated port (DP).
b. A port goes from Forwarding/Learning to Blocking. TCNs are sent out the root port of nonroot devices; they are sent each hello interval until they are acknowledged by the upstream device.
2. Upstream bridges process TCN on DPs.
3. The upstream switch sets the Topology Change Acknowledgement (TCA) field of the next configuration BPDU received and sends this downstream. This causes the downstream switch to stop sending TCN BPDUs.
4. The upstream switch then sends the TCN further upstream.
5. This continues until the root bridge receives the TCN.
6. The root bridge then sets the TCA and Topology Change flags in the next configuration BPDU sent out downstream.
7. The root bridge sets the TC flag in all BPDUs sent for Forward Delay + Max Age. This instructs all switches to age MAC table address entries faster.

Spanning Tree Protocol

802.1D

802.1D Spanning Tree Protocol (STP) is a Layer 2 loop-prevention mechanism. It is an IEEE standards-based protocol. Over the years, Cisco has enhanced this protocol with new features to make muchneeded improvements. This chapter discusses those improvements and new IEEE versions of the protocol that dramatically improve the technology.

Layer 2 loops are terrible because of no Time To Live (TTL) value in frame. Loops can cause broadcast storms, MAC table corruption, and multiple-frame copies.

STP process

The bridge ID is a critical element for the creation of the spanning-tree, loop-free topology. The bridge ID consists of a 2-byte bridge priority and a 6-byte MAC address. The default priority is 32,768. Newer switch operating systems feature a third component for the bridge ID: the extended system ID. This value is just the VLAN ID. Use of the three-part bridge ID allows each VLAN to have a unique bridge ID while still using the same MAC address and priority value. Previously, multiple MAC addresses were needed for each VLAN to ensure uniqueness.

Path cost is the measure of distance from one bridge to another. Links are assigned a cost value by STP. This cost value is based on bandwidth. Higher-bandwidth links receive a lower-cost value, and STP deems a lower-cost path as preferred to a higher-cost path. Initially with STP operations, a root bridge must be selected. This root bridge will have all of its ports in the forwarding state (designated ports) and will be the central reference point for the creation of a loopfree Layer 2 topology. For the “election” of this device, configuration bridge protocol data units (BPDU) are sent between switches for each port. Switches use a four-step process to save a copy of the “best” BPDU seen on every port. When a port receives a better BPDU, it stops sending them. If the BPDUs stop arriving for 20 seconds (the default), the port begins sending them again. The process for selecting the best
BPDU is as follows:

1. Lowest root bridge ID (BID)
2. Lowest path cost to root bridge
3. Lowest sender BID
4. Lowest port ID (for example, Fa0/10 versus Fa0/20)

After the root bridge for the network has been determined, this reference point can be used to create the loop-free topology. This initial creation of the loop-free topology takes place in three steps:

Step 1. Elect a root bridge. The lowest BID wins.
Step 2. Elect root ports. Every nonroot bridge selects one root port.
Step 3. Elect designated ports. Each segment has one designated port (the bridge with the designated port is the designated bridge for that segment); all active ports on the root bridge are designated (unless you connect two ports to each other).

When convergence occurs, BPDUs radiate out from the root bridge over loop-free paths. Figure below shows an example of STP in action.

Ports have a port state under 802.1D STP. Ports begin life on the switch as disabled and gradually transition to a forwarding state as long as STP deems it is safe to do so. The possible states are listed here along with the timers that control the transition times. Note that the states are carefully ordered here to demonstrate the order of transition:

1. Disabled—Administratively down
2. Blocking—BPDUs received only (20 sec)
3. Listening—BPDUs sent and received (15 sec)
4. Learning—Bridging table is built (15 sec)
5. Forwarding—Sending/receiving data

STP timers are used in the process to control convergence:

■ Hello—2 sec (time between each configuration BPDU)
■ Forward Delay—15 sec (controls durations of listening/learning states)
■ Max Age—20 sec (controls the duration of the blocking state)

Default convergence time is 30 to 50 seconds. Timer modification is possible from the root bridge. See Below figure:- Although the timers can be manipulated, Cisco does not recommend this. Instead, there are Cisco mechanisms that can be used to improve convergence times without direct manipulation of the timers by the administrator. Convergence time is a recognized issue with STP and the exact reason for IEEE’s creation of new versions of the protocol.

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Virtual Router Redundancy Protocol

Virtual Router Redundancy Protocol (VRRP) is so similar to HSRP that it can be basically thought of as the standards-based version of the protocol. Like HSRP, it lacks the inherent load-balancing capabilities that GLBP provides. Although there are many customization commands, the command to enable the protocol is just like that of the other redundancy protocols in structure:

vrrp group ip ip-address [secondary]

Hot Standby Router Protocol

The Hot Standby Router Protocol (HSRP) provides high network availability by routing IP traffic from hosts without relying on the availability of any single router. HSRP is used in a group of routers to select an active router and a standby router. The active router is the router of choice for routing packets; a standby router is a router that takes over the routing duties when an active router fails, or when other preset conditions are met.

HSRP is useful for hosts that do not support a router discovery protocol (such as Internet Control Message Protocol [ICMP] Router Discovery Protocol [IRDP]) and that cannot switch to a new router when their selected router reloads or loses power.
When the HSRP is configured on a network segment, it provides a virtual MAC address and an IP address that is shared among a group of routers running HSRP. The address of this HSRP group is referred to as the virtual IP address. One of these devices is selected by the protocol to be the active router.

HSRP detects when the designated active router fails, at which point a selected standby router assumes control of the MAC and IP addresses of the Hot Standby group. A new standby router is also selected at that time. Devices that are running HSRP send and receive multicast User Datagram Protocol (UDP)-based hello packets to detect router failure and to designate active and standby routers. Below is an example of an HSRP topology.

Devices that are running HSRP send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers. You can configure multiple Hot Standby groups on an interface, thereby making fuller use of redundant routers and load sharing. To do so, specify a group number for each Hot Standby command you configure for the interface.

To enable the HSRP on an interface, we can use the following command:
Router(config-if)# standby [group-number] ip [ip-address
[secondary]]

To configure the time between hello packets and the hold time before other routers declare the active router to be down, use the following command:

Router(config-if)# standby [group-number] timers [msec]
hellotime [msec] holdtime

To set the Hot Standby priority used in choosing the active router. The priority value range is from 1 to 255, where 1 denotes the lowest priority and 255 denotes the highest priority:

Router(config-if)# standby [group-number] priority priority

Internet Control Message Protocol

Internet Control Message Protocol (ICMP) assists the operation of the IP network by delivering messages about the network’s functionality—or lack thereof. ICMP includes functions for the following:
...Communicating network errors—Such as host or network unreachable.
...Announcing network congestion—An example is the ICMP Source Quench messages used to cause a sender to slow down transmission because of a router buffering too many packets.
...Provide troubleshooting tools—The Echo function is used by the ping utility to test connectivity between two systems.
...Communicate timeouts in the network—If a packet’s TTL reaches 0, an ICMP message can be sent announcing this fact.

ICMP protocol unreachable messages

If the Cisco device receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the device receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP host unreachable message to the source. This feature is enabled by default. To enable it if it’s disabled, use the following command:

Router(config-if)# ip unreachables


ICMP redirects
If the router resends a packet through the same interface on which it was received, the Cisco IOS Software sends an ICMP redirect message to the originator of the packet, telling the originator that the router is on a subnet directly connected to the receiving device and that it must forward the packet to another system on the same subnet. To enable the sending of ICMP redirect messages if this feature was disabled, use the following command:

Router(config-if)# ip redirects

Address Resolution Protocol

Address Resolution Protocol (ARP) is used to resolve IP addresses to MAC addresses in an Ethernet network. A host wanting to obtain a physical address broadcasts an ARP request onto the TCP/IP network. The host on the network that has the IP address in the request then replies with its physical hardware address. When a MAC address is determined, the IP address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP). Reverse Address Resolution Protocol (RARP) works the same way as ARP, except that the RARP request packet requests an IP address rather than a MAC address. Use of RARP requires a RARP server on the same network segment as the router interface. RARP often is used by diskless nodes that do not know their IP addresses when they boot. The Cisco IOS Software attempts to use RARP if it does not know the IP address of an interface at startup. Also, Cisco routers can act as RARP servers by responding to RARP requests that they can answer.