Network Bulls
www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192
Switch performance issues can be tricky to troubleshoot, because the problem reported is
often subjective. For example, if a user reports that the network is running “slow,” the
user’s perception might mean that the network is slow compared to what he expects.
However, network performance might very well be operating at a level that is hampering
productivity and at a level that is indeed below its normal level of operation. At that point,
as part of the troubleshooting process, you need to determine what network component is
responsible for the poor performance. Rather than a switch or a router, the user’s client,
server, or application could be the cause of the performance issue.
If you do determine that the network performance is not meeting technical expectations
(as opposed to user expectations), you should isolate the source of the problem and diagnose
the problem on that device. This section assumes that you have isolated the device
causing the performance issue, and that device is a Cisco Catalyst switch.
Cisco Catalyst Switch Troubleshooting Targets
Cisco offers a variety of Catalyst switch platforms, with different port densities, different
levels of performance, and different hardware. Therefore, troubleshooting one of these
switches can be platform dependent. Many similarities do exist, however. For example, all
Cisco Catalyst switches include the following hardware components:
■ Ports: A switch’s ports physically connect the switch to other network devices.
These ports (also known as interfaces) allow a switch to receive and transmit traffic.
■ Forwarding logic: A switch contains hardware that makes forwarding decisions.
This hardware rewrites a frame’s headers.
■ Backplane: A switch’s backplane physically interconnects a switch’s ports. Therefore,
depending on the specific switch architecture, frames flowing through a switch
Key
Topic
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 123
enter via a port (that is, the ingress port), flow across the switch’s backplane, and are
forwarded out of another port (that is, an egress port).
■ Control plane: A switch’s CPU and memory reside in a control plane. This control
plane is responsible for running the switch’s operating system.
Figure 5-8 depicts these switch hardware components. Notice that the control plane does
not directly participate in frame forwarding. However, the forwarding logic contained in
the forwarding hardware comes from the control plane. Therefore, there is an indirect relationship
between frame forwarding and the control plane. As a result, a continuous load
on the control plane could, over time, impact the rate at which the switch forwards frames.
Also, if the forwarding hardware is operating at maximum capacity, the control plane begins
to provide the forwarding logic. So, although the control plane does not architecturally
appear to impact switch performance, it should be considered when
troubleshooting.
The following are two common troubleshooting targets to consider when diagnosing a
suspected switch issue:
■ Port errors
■ Mismatched duplex settings
The sections that follow evaluate these target areas in greater detail.
Port Errors
When troubleshooting a suspected Cisco Catalyst switch issue, a good first step is to
check port statistics. For example, examining port statistics can let a troubleshooter know
if an excessive number of frames are being dropped. If a TCP application is running slow,
the reason might be that TCP flows are going into TCP slow start, which causes the window
size, and therefore the bandwidth efficiency, of TCP flows to be reduced. A common
reason that a TCP flow enters slow start is packet drops. Similarly, packet drops for a UDP
flow used for voice or video could result in noticeable quality degradation, because
dropped UDP segments are not retransmitted.
Ingress
Port
Egress
Forwarding Hardware Port
Forwarding Logic
Control Plane
Memory
Backplane
CPU
Figure 5-8 Cisco Catalyst Switch Hardware Components
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
124 CCNP TSHOOT 642-832 Official Certification Guide
Table 5-5 Errors in the show interfaces interface_id counters errors Command
Error Counter Description
Align-Err An alignment error occurs when frames do not end with an even number of
octets, while simultaneously having a bad Cyclic Redundancy Check (CRC).
An alignment error normally suggests a Layer 1 issue, such as cabling or
port (either switch port or NIC port) issues.
FCS-Err A Frame Check Sequence (FCS) error occurs when a frame has an invalid
checksum, although the frame has no framing errors. Like the Align-Err
error, an FCS-Err often points to a Layer 1 issue.
Key
Topic
Although dropped frames are most often attributed to network congestion, another possibility
is that the cabling could be bad. To check port statistics, a troubleshooter could
leverage a show interfaces command. Consider Example 5-14, which shows the output of
the show interfaces gig 0/9 counters command on a Cisco Catalyst 3550 switch. Notice
that this output shows the number of inbound and outbound frames seen on the specified
port.
Example 5-14 show interfaces gig 0/9 counters Command Output
To view errors that occurred on a port, you could add the keyword of errors after the
show interfaces interface_id counters command. Example 5-15 illustrates sample output
from the show interfaces gig 0/9 counters errors command.
Example 5-15 show interfaces gig 0/9 counters errors Command Output
Table 5-5 provides a reference for the specific errors that might show up in the output of
the show interfaces interface_id counters errors command.
SW1# show interfaces gig 0/9 counters
Port InOctets InUcastPkts InMcastPkts InBcastPkts
Gi0/9 31265148 20003 3179 1
Port OutOctets OutUcastPkts OutMcastPkts OutBcastPkts
Gi0/9 18744149 9126 96 6
SW1# show interfaces gig 0/9 counters errors
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize
Gi0/9 0 0 0 0 0
Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Gi0/9 5603 0 5373 0 0 0 0
continues
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 125
Table 5-5 Errors in the show interfaces interface_id counters errors Command
Error Counter Description
Xmit-Err A transmit error (that is, Xmit-Err) occurs when a port’s transmit buffer
overflows. A speed mismatch between inbound and outbound links often
results in a transmit error.
Rcv-Err A receive error (that is, Rcv-Err) occurs when a port’s receive buffer overflows.
Congestion on a switch’s backplane could cause the receive buffer on
a port to fill to capacity, as frames await access to the switch’s backplane.
However, most likely, a Rcv-Err is indicating a duplex mismatch.
UnderSize An undersize frame is a frame with a valid checksum but a size less than
64 bytes. This issue suggests that a connected host is sourcing invalid frame
sizes.
Single-Col A Single-Col error occurs when a single collisions occurs before a port
successfully transmits a frame. High bandwidth utilization on an attached
link or a duplex mismatch are common reasons for a Single-Col error.
Multi-Col A Multi-Col error occurs when more than one collision occurs before a port
successfully transmits a frame. Similar to the Single-Col error, high bandwidth
utilization on an attached link or a duplex mismatch are common
reasons for a Multi-Col error.
Late-Col A late collision is a collision that is not detected until well after the frame
has begun to be forwarded. While a Late-Col error could indicate that the
connected cable is too long, this is an extremely common error seen in mismatched
duplex conditions.
Excess-Col The Excess-Col error occurs when a frame experienced sixteen successive
collisions, after which the frame was dropped. This error could result from
high bandwidth utilization, a duplex mismatch, or too many devices on a
segment.
Carri-Sen The Carri-Sen counter is incremented when a port wants to send data on a
half-duplex link. This is normal and expected on a half-duplex port, because
the port is checking the wire, to make sure no traffic is present, prior to
sending a frame. This operation is the carrier sense procedure described by
the Carrier Sense Multiple Access with Collision Detect (CSMA/CD) operation
used on half-duplex connections. Full-duplex connections, however, do
not use CSMA/CD.
Runts A runt is a frame that is less than 64 bytes in size and has a bad CRC. A runt
could result from a duplex mismatch or a Layer 1 issue.
Giants A giant is a frame size greater than 1518 bytes (assuming the frame is not a
jumbo frame) that has a bad FCS. Typically, a giant is caused by a problem
with the NIC in an attached host.
(Continued)
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
126 CCNP TSHOOT 642-832 Official Certification Guide
SW1
Gig 0/9
Half-Duplex
Fa 5/47
Full-Duplex SW2
Figure 5-9 Topology with Duplex Mismatch
Mismatched Duplex Settings
As seen in Table 5-5, duplex mismatches can cause a wide variety of port errors. Keep in
mind that almost all network devices, other than shared media hubs, can run in full-duplex
mode. Therefore, if you have no hubs in your network, all devices should be running in
full-duplex mode.
A new recommendation from Cisco is that switch ports be configured to autonegotiate
both speed and duplex. Two justifications for this recommendation are as follows:
■ If a connected device only supported half-duplex, it would be better for a switch port
to negotiate down to half-duplex and run properly than being forced to run full-duplex
which would result in multiple errors.
■ The automatic medium-dependent interface crossover (auto-MDIX) feature can automatically
detect if a port needs a crossover or a straight-through cable to interconnect
with an attached device and adjust the port to work regardless of which cable
type is connected. You can enable this feature in interface configuration mode with
the mdix auto command on some models of Cisco Catalyst switches. However, the
auto-MDIX feature requires that the port autonegotiate both speed and duplex.
In a mismatched duplex configuration, a switch port at one end of a connection is configured
for full-duplex, whereas a switch port at the other end of a connection is configured
for half-duplex. Among the different errors previously listed in Table 5-5, two of the
biggest indicators of a duplex mismatch are a high Rcv-Err counter or a high Late-Col
counter. Specifically, a high Rcv-Err counter is common to find on the full-duplex end of a
connection with a mismatched duplex, while a high Late-Col counter is common on the
half-duplex end of the connection.
To illustrate, examine Examples 5-16 and 5-17, which display output based on the topology
depicted in Figure 5-9. Example 5-16 shows the half-duplex end of a connection, and
Example 5-17 shows the full-duplex end of a connection.
Example 5-16 Output from the show interfaces gig 0/9 counters errors and the show
interfaces gig 0/9 | include duplex Commands on a Half-Duplex Port
SW1# show interfaces gig 0/9 counters errors
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize
Gi0/9 0 0 0 0 0
Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Gi0/9 5603 0 5373 0 0 0 0
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 127
Example 5-17 Output from the show interfaces fa 5/47 counters errors and the show
interfaces fa 5/47 | include duplex Commands on a Full-Duplex Port
In your troubleshooting, even if you only have access to one of the switches, if you suspect
a duplex mismatch, you could change the duplex settings on the switch over which
you do have control. Then, you could clear the interface counters to see if the errors continue
to increment. You could also perform the same activity (for example, performing a
file transfer) the user was performing when he noticed the performance issue. By comparing
the current performance to the performance experienced by the user, you might be
able to conclude that the problem has been resolved by correcting a mismatched duplex
configuration.
TCAM Troubleshooting
As previously mentioned, the two primary components of forwarding hardware are forwarding
logic and backplane. A switch’s backplane, however, is rarely the cause of a
switch performance issue, because most Cisco Catalyst switches have high-capacity backplanes.
However, it is conceivable that in a modular switch chassis, the backplane will not
have the throughput to support a fully populated modular chassis, where each card in the
chassis supports the highest combination of port densities and port speeds.
SW1# show interfaces gig 0/9 include duplex
Half-duplex, 100Mb/s, link type is auto, media type is 10/100/1000BaseTX
SW1# show interfaces gig 0/9 counters errors
SW2# show interfaces fa 5/47 counters errors
Port Align-Err FCS-Err Xmit-Err Rcv-Err UnderSize OutDiscards
Fa5/47 0 5248 0 5603 27 0
Port Single-Col Multi-Col Late-Col Excess-Col Carri-Sen Runts Giants
Fa5/47 0 0 0 0 0 227 0
Port SQETest-Err Deferred-Tx IntMacTx-Err IntMacRx-Err Symbol-Err
Fa5/47 0 0 0 0 0
SW2# show interfaces fa 5/47 include duplex
Full-duplex, 100Mb/s
SW1# show interfaces gig 0/9 counters errors
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
128 CCNP TSHOOT 642-832 Official Certification Guide
Control Plane
Routing Processes
Traffic Policies
TCAM
Figure 5-10 Populating the TCAM
The architecture of some switches allows groups of switch ports to be handled by separated
hardware. Therefore, you might experience a performance gain by simply moving a
cable from one switch port to another. However, to strategically take advantage of this design
characteristic, you must be very familiar with the architecture of the switch with
which you are working.
A multilayer switch’s forwarding logic can impact switch performance. Recall that a
switch’s forwarding logic is compiled into a special type of memory called ternary content
addressable memory (TCAM), as illustrated in Figure 5-10. TCAM works with a switch’s
CEF feature to provide extremely fast forwarding decisions. However, if a switch’s TCAM
is unable, for whatever reason, to forward traffic, that traffic is forwarded by the switch’s
CPU, which has a limited forwarding capability.
The process of the TCAM sending packets to a switch’s CPU is called punting. Consider a
few reasons why a packet might be punted from a TCAM to its CPU:
■ Routing protocols, in addition to other control plane protocols such as STP, that send
multicast or broadcast traffic will have that traffic sent to the CPU.
■ Someone connecting to a switch administratively (for example, establishing a Telnet
session with the switch) will have their packets sent to the CPU.
■ Packets using a feature not supported in hardware (for example, packets traveling over
a GRE tunnel) are sent to the CPU.
■ If a switch’s TCAM has reached capacity, additional packets will be punted to the
CPU. A TCAM might reach capacity if it has too many installed routes or configured
access control lists.
From the events listed, the event most likely to cause a switch performance issue is a
TCAM filling to capacity. Therefore, when troubleshooting switch performance, you
might want to investigate the state of the switch’s TCAM. Please be sure to check documentation
for your switch model, because TCAM verification commands can vary between
platforms.
Key
Topic
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 129
As an example, the Cisco Catalyst 3550 Series switch supports a collection of show tcam
commands, whereas Cisco Catalyst 3560 and 3750 Series switches support a series of
show platform tcam commands. Consider the output from the show tcam inacl 1 statistics
command issued on a Cisco Catalyst 3550 switch, as shown in Example 5-18. The
number 1 indicates TCAM number one, because the Cisco Catalyst 3550 has three
TCAMs. The inacl refers to access control lists applied in the ingress direction. Notice that
fourteen masks are allocated, while 402 are available. Similarly, seventeen entries are currently
allocated, and 3311 are available. Therefore, you could conclude from this output
that TCAM number one is not approaching capacity.
Example 5-18 show tcam inacl 1 statistics Command Output on a Cisco Catalyst 3550
Series Switch
On some switch models (for example, a Cisco Catalyst 3750 platform), you can use the
show platform ip unicast counts command to see if a TCAM allocation has failed. Similarly,
you can use the show controllers cpu-interface command to display a count of
packets being forwarded to a switch’s CPU.
On most switch platforms, TCAMs cannot be upgraded. Therefore, if you conclude that a
switch’s TCAM is the source of the performance problems being reported, you could either
use a switch with higher-capacity TCAMs or reduce the number of entries in a
switch’s TCAM. For example, you could try to optimize your access control lists or leverage
route summarization to reduce the number of route entries maintained by a switch’s
TCAM. Also, some switches (for example, Cisco Catalyst 3560 or 3750 Series switches)
enable you to change the amount of TCAM memory allocated to different switch features.
For example, if your switch ports were configured as routing ports, you could reduce the
amount of TCAM space used for storing MAC addresses, and instead use that TCAM
space for Layer 3 processes.
High CPU Utilization Level Troubleshooting
The load on a switch’s CPU is often low, even under high utilization, thanks to the TCAM.
Because the TCAM maintains a switch’s forwarding logic, the CPU is rarely tasked to forward
traffic. The show processes cpu command that you earlier learned for use on a
router can also be used on a Cisco Catalyst switch to display CPU utilization levels, as
demonstrated in Example 5-19.
Example 5-19 show processes cpu Command Output on a Cisco Catalyst 3550 Series
Switch Key
Topic
Cat3550# show tcam inacl 1 statistics
Ingress ACL TCAM#1: Number of active labels: 3
Ingress ACL TCAM#1: Number of masks allocated: 14, available: 402
Ingress ACL TCAM#1: Number of entries allocated: 17, available: 3311
Cat3550# show processes cpu
CPU utilization for five seconds: 19%/15%; one minute: 20%; five minutes: 13%
PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process
1 0 4 0 0.00% 0.00% 0.00% 0 Chunk Manager
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
130 CCNP TSHOOT 642-832 Official Certification Guide
2 0 610 0 0.00% 0.00% 0.00% 0 Load Meter
3 128 5 25600 0.00% 0.00% 0.00% 0 crypto sw pk pro
4 2100 315 6666 0.00% 0.05% 0.05% 0 Check heaps
...OUTPUT OMITTED...
Notice in the output in Example 5-19 that the switch is reporting a 19 percent CPU load,
with 15 percent of the CPU load used for interrupt processing. The difference between
these two numbers is 4, suggesting that 4 percent of the CPU load is consumed with control
plane processing.
Although such load utilization values might not be unusual for a router, these values might
be of concern for a switch. Specifically, a typical CPU load percentage dedicated to interrupt
processing is no more than five percent. A value as high as ten percent is considered
acceptable. However, the output given in Example 5-19 shows a fifteen percent utilization.
Such a high level implies that the switch’s CPU is actively involved in forwarding packets
that should normally be handled by the switch’s TCAM. Of course, this value might only
be of major concern if it varies from baseline information. Therefore, your troubleshooting
efforts benefit from having good baseline information.
Periodic spikes in processor utilization are also not a major cause for concern if such
spikes can be explained. Consider the following reasons that might cause a switch’s CPU
utilization to spike:
■ The CPU processing routing updates
■ Issuing a debug command (or other processor-intensive commands)
■ Simple Network Management Protocol (SNMP) being used to poll network devices
If you determine that a switch’s high CPU load is primarily the result of interrupts, you
should examine the switch’s packet switching patterns and check the TCAM utilization. If,
however, the high CPU utilization is primarily the result of processes, you should investigate
those specific processes.
A high CPU utilization on a switch might be a result of STP. Recall that an STP failure
could lead to a broadcast storm, where Layer 2 broadcast frames endlessly circulate
through a network. Therefore, when troubleshooting a performance issue, realize that a
switch’s high CPU utilization might be a symptom of another issue.
Trouble Ticket: HSRP
This trouble ticket focuses on HSRP. HSRP was one of three first-hop redundancy protocols
discussed in this chapter’s “Router Redundancy Troubleshooting” section.
Trouble Ticket #2
You receive the following trouble ticket:
A new network technician configured HSRP on routers BB1 and BB2, where BB1 was
the active router. The configuration was initially working; however, now BB2 is acting
as the active router, even though BB1 seems to be operational.
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 131
S 1/0.2
.1
Lo 0
10.3.3.3/32
Fa 0/0
.1
S 1/0.2
.1
DLCI = 182
DLCI = 811
S 1/0.1
.1
Lo 0
10.1.1.1/32
Lo 0
10.2.2.2/32
172.16.1.0/30
Fa 0/0
FXS DLCI = 881
1/0/0
.11
FXS
1/0/1
R2
192.168.1.0/24
192.168.0.0/24
.11
Fa 0/1
172.16.2.0/30
S 1/0.1
.2
DLCI = 882
Fa 0/0
.22
10.1.3.0/30
10.1.2.0/24
Gig 0/8 Fa 5/46
Lo 0
10.4.4.4/32
S 1/0.2
.2
DLCI = 821 .2
Fa 0/0
Gig 0/9 Fa 5/47
Fa 5/45
x3333
Gig 0/10 Fa 5/48
100 Mbps
10 Mbps
R1
BB2
BB1
R2 FRSW
x1111 x2222
SW1 SW2
S 1/0.1
.2
DLCI = 181
Figure 5-11 Trouble Ticket #2 Topology
This trouble ticket references the topology shown in Figure 5-11.
As you investigate this issue, you examine baseline data collected after HSRP was initially
configured. Examples 5-20 and 5-21 provide show and debug command output collected
when HSRP was working properly. Notice that router BB1 was acting as the active HSRP
router, whereas router BB2 was acting as the standby HSRP router.
Example 5-20 Baseline Output for Router BB1
BB1# show standby brief
P indicates configured to preempt.
Interface Grp Prio P State Active Standby Virtual IP
Fa0/1 1 150 Active local 172.16.1.3 172.16.1.4
BB1# debug standby
HSRP debugging is on
*Mar 1 01:14:21.487: HSRP: Fa0/1 Grp 1 Hello in 172.16.1.3 Standby pri 100 vIP
172.16.1.4
*Mar 1 01:14:23.371: HSRP: Fa0/1 Grp 1 Hello out 172.16.1.1 Active pri 150 vIP
172.16.1.4
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
132 CCNP TSHOOT 642-832 Official Certification Guide
Example 5-21 Baseline Output for Router BB2
BB2# show standby brief
P indicates configured to preempt.
BB1# u all
All possible debugging has been turned off
BB1# show standby fa 0/1 1
FastEthernet0/1 - Group 1
State is Active
10 state changes, last state change 00:12:40
Virtual IP address is 172.16.1.4
Active virtual MAC address is 0000.0c07.ac01
Local virtual MAC address is 0000.0c07.ac01 (v1 default)
Hello time 3 sec, hold time 10 sec
Next hello sent in 1.536 secs
Preemption disabled
Active router is local
Standby router is 172.16.1.3, priority 100 (expires in 9.684 sec)
Priority 150 (configured 150)
IP redundancy name is “hsrp-Fa0/1-1” (default)
BB1# show run
...OUTPUT OMITTED...
hostname BB1
!
interface Loopback0
ip address 10.3.3.3 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.2.1 255.255.255.0
!
interface FastEthernet0/1
ip address 172.16.1.1 255.255.255.0
standby 1 ip 172.16.1.4
standby 1 priority 150
!
router ospf 1
network 0.0.0.0 255.255.255.255 area 0
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
Chapter 5: Advanced Cisco Catalyst Switch Troubleshooting 133
As part of testing the initial configuration, a ping was sent to the virtual IP address of
172.16.1.4 from router R2 in order to confirm that HSRP was servicing requests for that IP
address. Example 5-22 shows the output from the ping command.
Example 5-22 PINGing the Virtual IP Address from Router R2
As you begin to gather information about the reported problem, you reissue the show
standby brief command on routers BB1 and BB2. As seen in Examples 5-23 and 5-24,
router BB1 is administratively up with an HSRP priority of 150, whereas router BB2 is administratively
up with a priority of 100.
Example 5-23 Examining the HSRP State of Router BB1’s FastEthernet 0/1 Interface
Interface Grp Prio P State Active Standby Virtual IP
Fa0/1 1 100 Standby 172.16.1.1 local 172.16.1.4
BB2# show run
...OUTPUT OMITTED...
hostname BB2
!
interface Loopback0
ip address 10.4.4.4 255.255.255.255
!
interface FastEthernet0/0
ip address 10.1.2.2 255.255.255.0
!
interface FastEthernet0/1
ip address 172.16.1.3 255.255.255.0
standby 1 ip 172.16.1.4
!
router ospf 1
network 0.0.0.0 255.255.255.255 area 0
R2# ping 172.16.1.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.4, timeout is 2 seconds:
!!!!!
BB1# show standby brief
P indicates configured to preempt.
Interface Grp Prio P State Active Standby Virtual IP
Fa0/1 1 150 Standby 172.16.1.3 local 172.16.1.4
www.CareerCert.info
www.CareerCert.info
www - CareerCert - info
134 CCNP TSHOOT 642-832 Official Certification Guide
Example 5-24 Examining the HSRP State of Router BB2’s FastEthernet 0/1 Interface
Take a moment to look through the baseline information, the topology, and the show
command output. Then, hypothesize the underlying cause, explaining why router BB2 is
currently the active HSRP router, even thought router BB1 has a higher priority. Finally,
on a separate sheet of paper, write out a proposed action plan for resolving the reported
issue.
Suggested Solution
Upon examination of BB1’s output, it becomes clear that the preempt feature is not enabled
for the Fast Ethernet 0/1 interface on BB1. The absence of the preempt feature explains
the reported symptom. Specifically, if BB1 had at one point been the active HSRP
router for HSRP group 1, and either router BB1 or its Fast Ethernet 0/1 interface became
unavailable, BB2 would have become the active router. Then, if BB1 or its Fast Ethernet 0/1
interface once again became available, BB1 would assume a standby HSRP role, because
BB1’s FastEthernet 0/1 interface was not configured for the preempt feature.
To resolve this configuration issue, the preempt feature is added to BB1’s Fast Ethernet 0/1
interface, as shown in Example 5-25. After enabling the preempt feature, notice that router
BB1 regains its active HSRP role.
No comments:
Post a Comment