Networking-Blog

Tags: cisco 881w

by admin

Multilayer Switching, SVIs, Inter-VLAN Communication

The use of switched virtual interfaces (SVIs) is an important topic on the CCNP SWITCH exam,
and SVIs are becoming more and more common in today’s networks – so it’s a good idea for all of
us to know how they’re used and how to configure them.

Let’s take a look at how SVIs allow inter-VLAN communication without the use of a separate L3 device.

Multilayer switches allow us to create a logical interface that represents the VLAN. Remember
that the L2 switches you’ve worked with that have an “interface VLAN1” by default?
That’s actually a switched virtual interface (SVI).

An SVI exists for VLAN 1 by default, but that’s the only VLAN that has a “pre-created” SVI.
On a multilayer switch, such a logical interface can be configured for any VLAN.

MLS(config)#interface vlan 10
MLS(config-if)#ip address 10.1.1.1 255.255.255.0

Let’s put SVIs to work with a basic interVLAN routing configuration.

Switched Virtual Interface (SVI)

To allow these two hosts to communicate, you know that we’ve got to have an L3 device – and
now we have a different kind of L3 device than you’ve used before.
This L3 switch will allow interVLAN communication without involving a router.

Before we begin configuring, we’ll send pings between the two hosts. (In this example,
I’m using Cisco routers for hosts, but there are no routes of any kind on them.)

HOST_1#ping 30.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)

HOST_3#ping 20.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds:
…..
Success rate is 0 percent (0/5)

As expected, neither host can ping the other. Let’s fix that!

To get started, we’ll put the port leading to Host 1 into VLAN 11, and the port leading to Host 3 in VLAN 33.

SW1(config)#int fast 0/1
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 11

SW1(config-if)#int fast 0/3
SW1(config-if)#switchport mode access
SW1(config-if)#switchport access vlan 33

We’re going to create two SVIs on the switch, one representing VLAN 11 and the other representing
VLAN 33. Note that both SVIs show as up/up immediately after creation. Some Cisco and
non-Cisco documentation mentions that you should open the SVIs after creating them,
but that’s not necessarily the case in the real world. Couldn’t hurt, though. 🙂

Let’s create those two SVIs…

SW1(config)#int vlan11

01:30:04: %LINK-3-UPDOWN: Interface Vlan11, changed state to up01:30:05:
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan11, changed state to up

SW1(config-if)#ip address 20.1.1.11 255.255.255.0

SW1(config-if)#int vlan33

01:30:11: %LINK-3-UPDOWN: Interface Vlan33, changed state to up

01:30:12: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan33, changed state to up

SW1(config-if)#ip address 30.1.1.11 255.255.255.0

Verify the SVIs with show interface vlan. I’ll only show the top three rows of output for each SVI.

SW1#show int vlan11
Vlan11 is up, line protocol is up
Hardware is EtherSVI, address is 0012.7f02.4b41 (bia 0012.7f02.4b41)
Internet address is 20.1.1.11/24

SW1#show int vlan33
Vlan33 is up, line protocol is up
Hardware is EtherSVI, address is 0012.7f02.4b42 (bia 0012.7f02.4b42)
Internet address is 30.1.1.11/24

Note the hardware for those interfaces – “EtherSVI”.

Now let’s check that routing table…

SW1# show ip route
Default gateway is not set

Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty

Hmm, that’s not good. We don’t have one! There’s a simple reason, though – on L3 switches,
we need to enable IP routing, because it’s off by default.

Step One In L3 Switch Troubleshooting: Ensure IP Routing Is On!

SW1(config)#ip routing
SW1(config)#^Z

SW1#show ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

20.0.0.0/24 is subnetted, 1 subnets
C 20.1.1.0 is directly connected, Vlan11
30.0.0.0/24 is subnetted, 1 subnets
C 30.1.1.0 is directly connected, Vlan33

Now that looks like the routing table we’ve come to know and love! In this particular case,
there’s no need to configuring a routing protocol. You recall from your CCNA studies that when
router-on-a-stick is configured, the IP address assigned to the router’s subinterfaces should be
the default gateway setting on the hosts.

When SVIs are in use, the default gateway set on the hosts should be the IP address assigned to the
SVI that represents that host’s VLAN. After setting this default gateway on the hosts, the hosts can
now successfully communicate.

Since we’re using routers for hosts, we’ll use the ip route command to set the default gateway.

HOST_1(config)#ip route 0.0.0.0 0.0.0.0 20.1.1.11

HOST_3(config)#ip route 0.0.0.0 0.0.0.0 30.1.1.11

Can the hosts now communicate, even though they’re in different VLANs?

HOST_1#ping 30.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds: !!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

HOST_3#ping 20.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds: !!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Yes, they can! We now have hosts in different VLANs communicating without a router involved –
all thanks to multilayer switches and SVIs.

Ports on multilayer switches can also be configured as routing ports, and have IP addresses assigned
directly to them.

April 13, 2011 8:57 pm

Tags: svi

by admin

Cisco HSRP Using the Track Statement

Assume you have the following hypothetical scenario, R2 and R7 on one site, connected
to a WAN infrastructure via multiple links (omitted here), where there are two gateways R5 & R6:

track

Client requests the following:

R2 should be the active HRSP router if R2 has connectivity to BOTH R5 and R6 loopbacks.
R7 should be the active HSRP router if R2 has connectivity to only ONE of the 2 gateways.
R2 should be the active HSRP router if R2 has NO connectivity to either R5 or R6.

The use of the track statements and a boolean expression, in conjunction with HSRP would suffice.

The first requirement is, if both 5.5.5.5/32 and 6.6.6.6/32 are in the routing table of R2,
then R2 should be Active. If one of these routes disappears, R7 should become Active, and
lastly if both routes disappear, R2 should become Active again.

The first two are easy, with two track statements, Track1 for 5.5.5.5/32 and Track2 for 6.6.6.6/32
and correct decrement amounts. But the last poses more of a challenge.

By using the boolean ‘AND’ statement for both Track1 and Track2, you’ll have the third track to
meet the requirements.

The config for these track statements are as follow :
!
R2# track 1 ip route 5.5.5.5 255.255.255.255 reachability track 2 ip route 6.6.6.6 255.255.255.255 reachability track 3 list boolean and object 1 not object 2 not

Track1 would be ‘TRUE’ if 5.5.5.5/32 is present in the routing table, ‘FALSE’ if non existent in the
routing table. Same for Track2.

Track3 says if (Object1 = Track1 is not true, ie false) AND (Object2 = Track2 is not true), thus if
both routes are not present in the routing table, then Track3 will be ‘TRUE’.
If one or both of these router are present in the routing-table, Track3 is ‘FALSE’.

With HSRP’s decrements statements, if the Track status is TRUE/UP, no decrement action is
necessary, but if the Track status is FALSE/DOWN, then the specified amount will be deducted from
the configured priority amount on that interface.

The configuration on the Ethernet interfaces to complete this :

R7# interface FastEthernet0/0 ip address 142.42.27.7 255.255.255.0 standby version 2 standby 27 ip 142.42.27.27 standby 27 priority 95 standby 27 preempt

R2# interface FastEthernet1/0 ip address 142.42.27.2 255.255.255.0 standby version 2 standby 27 ip 142.42.27.27 standby 27 priority 150 standby 27 preempt standby 27 track 1 decrement 10 standby 27 track 2 decrement 10 standby 27 track 3 decrement 50

R7 uses a priority of 95 and R2 uses a priority of 150. These values are based on the decrement
values used by the track statements. Let me explain:

Firstly what does Track3 do again?

Track3 is UP if both Track1 and Track2 are DOWN! If Track3 is UP, the priority is not decremented by 50.

But Track3 will be DOWN if either, or both Track1 and/or Track2 are UP. If Track3 is DOWN the priority
is decremented by 50, to have a HSRP priority of 100, under normal circumstances.
With a priority of 100, R2 will be the Active HSRP router, since R7 has a priority of 95.
Biggest priority always wins.

But if Track1 goes to a DOWN state (ie 5.5.5.5/32 removed from the routing table), the priority of 100
is decremented by 10 to 90. This will make R7 the Active HSRP router since R7 now has the highest
priority with 95.

If in addition, 6.6.6.6/32 disappears from the routing table, Track2 will also go DOWN, along with Track1,
then only will Track3 kick in. (If Track1 & Track2 = DOWN, then Track3 is UP). The priority (100) on the
interface of R2 will be increased with 50, to 150, but be decremented by 2×10 for Track1 and Track2.
This will give Router2 a interface priority of 130, thus higher than Router7′s 95, and become Active again.

Lets see this on the CLI for better understanding :

Default Situation

Since both 5.5.5.5/32 and 6.6.6.6/32 are present in the routing table, Track1 and Track2 are up,
and as a result Track3 is down:

track1

The interface on R2 shows the status of the tracks, and the priority, currently a 100 due to
Track3 being down. (Configured priority 150 – Track3 (50) = 100). Notice R2 is in Active State.

track2

.

One Route Down :

Let see what happens when 5.5.5.5/32 is not reachable from R2 anymore.
That will cause Track1 to go DOWN, therefor the priority is decremented by an additional 10 to 90.
Notice R2 is not Active anymore, due to R2′s 90 priority being less than R7′s 95 priority.

track3

.

Both Routes Down

When both 5.5.5.5/32 and 6.6.6.6/32 are unreachable from R2, Track1 and Track2 will be DOWN.
Thus the boolean ‘and’ will change the status of Track3 to UP. Since Track3 is now UP,
only the amounts of the DOWN objects (Track1 and Track2) are used to decrement the priority
from 150 –10 –10 to 130.

Whilst the above IP SLA monitors the reachability, we actually want to act in the event that the
IP we’re monitoring isn’t available. We do this by adding the track statement below to reverse the
value using a boolean expression, affectively negating the value in track statement 1/2.

track 3 list boolean and object 1 not object 2 not

Notice that R2 is Active again.

track4

April 11, 2011 7:58 pm

Tags: hsrp track

by admin

Cisco Tracking Route Reachability

“DSL Backup,” let’s assume the T1 is for Internet, and we’re using DSL as a backup.
Won’t the router only use the higher-metric DSL route if the T1 line is down
(interface down, or line protocol down)?

Isn’t the more likely scenario that the T1 is up, but no traffic is routing? Maybe the ISP is
having routing problems or their Internet connection got cut or something. Put another way,
which is more likely to happen: your T1 going down, or the ISP having Internet trouble? I venture
to guess that the T1 line is pretty stable, but the ISP is far more likely to have “issues.” And in that
case, the router will see the T1 as up, and it will keep trying to push traffic out the T1 and will not fail
over to the DSL, correct?

I’ve heard of “object tracking” to help in these scenarios, so it can test for something (like ping Google)
and then make routing decisions based on the results of those tests.

Now, on to your example :

You have a T1 and a DSL link both capable of your Internet routing and reachability.
You want to use the T1 as your primary path and then switch over if it’s not available.

Well, you’re correct that if the whole line is down, that’s a simple issue. Are you running any
routing with the ISP? If you’re multi-homed, typically you’ll run BGP and perhaps receive some
customer routes from that ISP directly. If so, you may have another simple answer — just look
for route reachability.

Let’s assume that we receive 192.168.100.0/24 from this ISP. (By the way, I always try to use private
addresses in my examples to protect the innocent!)

Track timer ip route 60
Track 1 ip route 192.168.100.0/24 reachability

This will simply look for the route in the routing table. Either it’s there or it’s not. In this case,
every 60 seconds your router will verify. The default is 15 seconds, but you can reconfigure it to
suit your own needs.

Now, even if the T1 goes down, you may still be able to reach this customer route
(a backup 0/0 route) but that specific route will not exist in your routing table.
So that’s the part we’re tracking.

Then on your route to the T1, we would simply put “track 1” at the end of it.

Ip route 0.0.0.0 0.0.0.0 (T1 intf) track 1
Ip route 0.0.0.0 0.0.0.0 (DSL intf) 100

And once we have a second route with higher AD value in place, when one goes away,
the other will come in.

If you aren’t learning any routes from your ISP, we may get into a little more complex scenario.
Check with your ISP (the T1 carrier) and see if there’s any particular IP address that you can reach
over the T1 that wouldn’t be reachable from non-customers (the rest of the Internet).
Something like a router’s loopback address would be good.

Now we’ll use the IP SLA features to test ICMP reachability to the address. When this ceases,
the route will go away. Now, this may take some working with the ISP because if they change
anything, suddenly it may appear as a failure when it really isn’t. But one thing at a time!

So if your ISP tells you that 10.234.100.15/32 is the loopback IP, we can set up ping tests.

ip sla monitor 6
type echo protocol ipIcmpEcho 10.234.100.15
frequency 300
ip sla monitor schedule 6 life forever start-time now
track 2 rtr 6 state
ip route 0.0.0.0 0.0.0.0 (T1 intf) track 2
ip route 0.0.0.0 0.0.0.0 (DSL intf) 100

The more complex our networks become, the more difficult it may be to design failover scenarios.
But the basic stuff we’ve had available for a while has been expanded to give us many more options
to deal with this complexity.

The tracking capability as an example was initially designed for HSRP/VRRP. But it’s been expanded
to use in static routes like we’re using here. The things we can track have also increased:

line-protocol on an interface
IP routing state of an interface
specific IP route reachability
route metric of a specific IP route
IP SLA (ping, udp echo, tcp signon, http status, etc.) metrics
logical combination in an and/or fashion of multiple single objects

Example :

The following example shows the configuration of the IP SLAs ICMP Echo operation number 6
that will start immediately and run indefinitely.
!

ip sla monitor 6

type echo protocol ipIcmpEcho 172.29.139.134 source-ipaddr 172.29.139.132

frequency 300

!

ip sla monitor schedule 6 life forever start-time now
!

What to Do Next

To view and interpret the results of an IP SLAs operation use the :

show ip sla monitor statistics command.

Checking the output for fields that correspond to criteria in your service level agreement will help you

determine whether the service metrics are acceptable.

April 11, 2011 10:59 am

Tags: route reachability, tracking

by admin

Cisco TCP Statistics

The following is sample output from the show tcp statistics command:

Router# show tcp statistics


Rcvd: 91995 Total, 3860 no port
      79 checksum error, 0 bad offset, 0 too short
      7234 packets (549608 bytes) in sequence
      36 dup packets (3621 bytes)
      0 partially dup packets (0 bytes)
      113 out-of-order packets (0 bytes)
      0 packets (0 bytes) with data after window
      0 packets after close
      0 window probe packets, 0 window update packets
      206 dup ack packets, 0 ack packets with unsend data
      7705 ack packets (1025558 bytes)
Sent: 88895 Total, 0 urgent packets
      77647 control packets (including 121 retransmitted)
      6352 data packets (1035719 bytes)
      99 data packets (18018 bytes) retransmitted
      0 data packets (0 bytes) fastretransmitted
      4797 ack only packets (3093 delayed)
      0 window probe packets, 0 window update packets
0 Connections initiated, 1374 connections accepted,
1296 connections established
148134 Connections closed (including 137 dropped,
73383 embryonic dropped)
220 Total rxmt timeout,
0 connections dropped in rxmt timeout
32 Keepalive timeout, 32 keepalive probe,
6 Connections dropped in keepalive

show tcp statistics Field Descriptions
Field	Description
Rcvd:	Statistics in this section refer to packets received by the router.
Total	Total packets received.
no port	number of packets received with no port.
checksum error	Number of packets received with checksum error.
bad offset	Number of packets received with bad offset to data.
too short	Number of packets received that were too short.
packets in sequence	Number of data packets received in sequence.
dup packets	Number of duplicate packets received.
partially dup packets	Number of packets received with partially duplicated data.
out-of-order packets	Number of packets received out of order.
packets with data after window	Number of packets received with data that exceeded the receiver’s window size.
packets after close	Number of packets received after the connection has been closed.
window probe packets	Number of window probe packets received.
window update packets	Number of window update packets received.
dup ack packets	Number of duplicate acknowledgment packets received.
ack packets with unsent data	Number of acknowledgment packets with unsent data received.
ack packets	Number of acknowledgment packets received.
Sent:	Statistics in this section refer to packets sent by the router.
Total	Total number of packets sent.
urgent packets	Number of urgent packets sent.
control packets	Number of control packets (SYN, FIN, or RST) sent.
data packets	Number of data packets sent.
data packets retransmitted	Number of data packets re-sent.
ack only packets	Number of packets sent that are acknowledgments only.
window probe packets	Number of window probe packets sent.
window update packets	Number of window update packets sent.
Connections initiated	Number of connections initiated.
connections accepted	Number of connections accepted.
connections established	Number of connections established.
Connections closed	Number of connections closed.
Total rxmt timeout	Number of times the router tried to re-send, but timed out.
Connections dropped in rxmit timeout	Number of connections dropped in re-send timeout.
Keepalive timeout	Number of keepalive packets in timeout.
keepalive probe	Number of keepalive probes.
Connections dropped in keepalive	Number of connections dropped in keepalive.

Related Commands


Command	Description
clear tcp statistics	Clears TCP statistics.

April 4, 2011 6:14 pm

Tags: tcp statistics

by admin

Cisco IP Helper-Address DHCP

A UDP Helper Address is a special router configuration used to forward broadcast network traffic from
a client machine on one subnet to a server in another subnet. This is used primarily in business
environments as way to save money by not having to buy a server for each subnet.

A helper address can also be used to forward certain other UDP network configuration messages.
As a result a UDP helper address is sometimes created to allow two server machines to communicate
across different subnets.
!

To resolve this a UDP Helper Address is established in the router configuration to forward broadcast
network traffic from the PCs to the DHCP server. The DHCP server responds with a unicast IP address
from the given range. It reserves the IP address for the time it can calculate that it can get a response.

If the client responds again with a broadcast it distributes the IP address.

Example :

Configuration on Router A

RouterA# conf t
RouterA(config)# interface fastethernet0/0
RouterA(config-if)# ip address 192.168.1.1 255.255.255.0
RouterA(config-if)# ip helper-address 10.10.10.1

Some other important considerations for ip helper-address.

By default, the ip helper-address command forwards also some other broadcast protocols
in addition to the DHCP (BOOTP) protocol.

It forwards by default the following eight UDP broadcast protocols:

UDP 37 (Time protocol)
UDP 49 (TACACS)
UDP 53 (DNS)
UDP 67 (DHCP Server)
UDP 68 (DHCP Client)
UDP 69 (TFTP)
UDP 137 (NetBios)
UDP 138 (NetBios Datagram service)

If you want to add more broadcast protocols to be forwarded, or even remove some of the default
forwarded protocols, you can use the “ip forward-protocol” command under global config mode.

Example:

Remove the NetBios protocols (137,138) from being forwarded by default,
and add NTP protocol 123 to be forwarded by ip helper-address.

RouterA(config)# no ip forward-protocol udp 137
RouterA(config)# no ip forward-protocol udp 138
!

RouterA(config)#ip forward-protocol udp 123

!
!
Notes :

bootpc : Bootstrap Protocol (BOOTP) client (68)
bootps : Bootstrap Protocol (BOOTP) server (67)

April 4, 2011 12:15 pm

Tags: IP Helper-Address

by admin

Cisco Backup delay for Gigabit interface

Command Description:

To define how much time should elapse before a secondary line status changes after a primary line status has changed, use the backup delay interface configuration command. Use the no form of this command to return to the default, so that as soon as the primary fails, the secondary is immediately brought up without delay.

We recommend that you enable some delay before activating and deactivating a secondary line.

Example :

The following example sets a 10-second delay on deactivating the secondary line. However, interface gigabitEthernet0/1 is activated immediately.

The enable and disable backup delay must be 0.
!

Router(config)#interface gigabitEthernet0/1
Router(config-if)#backup interface gigabitEthernet0/2

Router(config-if)#backup delay 0 10

which means if the primary link is back up again, the secondary should wait for 10 seconds before switching the traffic to the primary.

Once you activate the command , immediately the Back up interface will be in Standby mode and once the primary interface is back up, it should wait for whatever seconds configured as it will preempt to check if the interface is alive and waits for the specified amount before the traffic is back to the primary interface.

April 3, 2011 7:01 pm

Tags: backup delay

by admin

Cisco EtherChannel Trunk

Cisco Etherchannel Trunking with Multi Vlan Routing on a Multilayer Switch 2900 :
!
track 1 interface Port-channel1 line-protocol
!
hostname HQ1
!
interface Port-channel1
no ip address
hold-queue 150 in
!
interface Port-channel1.10
description I.T_vlan
encapsulation dot1Q 10
ip address 10.1.1.7 255.255.0.0
standby 1 ip 10.1.1.6
standby 1 priority 120
standby 1 preempt
standby 1 track 1 decrement 10
!
interface Port-channel1.19
description office_vlan
encapsulation dot1Q 19
ip address 10.99.1.7 255.255.0.0
  standby 2 ip 10.99.1.6
standby 2 priority 120
standby 2 preempt
standby 2 track 1 decrement 10
!
interface Port-channel1.20
description finance_vlan
encapsulation dot1Q 20
ip address 10.33.1.7 255.255.0.0
standby 3 ip 10.33.1.6
standby 3 priority 120
standby 3 preempt
standby 3 track 1 decrement 10
!
interface Port-channel1.99
description mgmt_vlan
encapsulation dot1Q 99
ip address 10.214.1.7 255.255.0.0
standby 4 ip 10.214.1.6
standby 4 priority 120
standby 4 preempt
standby 4 track 1 decrement 10
!
!
interface GigabitEthernet0/1
description etherchannel_channel-port1_member
no ip address
channel-group 1
!
!
interface GigabitEthernet0/2
description etherchannel_channel-port1_member
no ip address
channel-group 1
!
!
!
!
Multilayer Switch 2960 :

interface Port-channel1
switchport trunk allowed vlan 10,19,20,99,200
switchport mode trunk
!
!
interface GigabitEthernet0/21
description PortChan_connected_HQ1
switchport trunk allowed vlan 10,19,20,99,200
switchport mode trunk
channel-group 1 mode on
!
interface GigabitEthernet0/22
description PortChan_connected_HQ1
switchport trunk allowed vlan 10,19,20,99,200
switchport mode trunk
channel-group 1 mode on

April 3, 2011 5:38 pm

Tags: etherchannel

by admin

Cisco PPPOE

Here is a sample config for PPPoE configuration. Replace interface names, usernames and passwords with the right ones for you network.
Authentication details should come from your ISP.

vpdn enable
no vpdn logging
!
vpdn-group 1
request-dialin
protocol pppoe
!
!
!
!
interface FastEthernet0/0
description My LAN
ip address 10.0.0.0 255.255.255.0
ip nat inside
!
interface FastEthernet0/1
description Connection To PPPoE Supplier
no ip address
pppoe enable
pppoe-client dial-pool-number 1
!
interface Dialer1
ip address negotiated
ip nat outside
ip mtu 1492
encapsulation ppp
no ip mroute-cache
dialer pool 1
dialer-group 1
ppp authentication pap
ppp pap sent-username “username” password “password”
!
dialer-list 1 protocol ip permit
ip nat inside source list 1 interface Dialer1 overload
ip route 0.0.0.0 0.0.0.0 dialer1
access-list 1 permit 10.0.0.0 0.0.0.255

April 2, 2011 6:26 pm

Tags: pppoe

by admin

Cisco EtherChannel

In EtherChannel, you can combine multiple physical links between switches into a single logical link
to increase the bandwidth size.

You can combine maximum eight physical links into one etherchannel link for the purpose of
high speed communication and load balancing between switches.

Etherchannel can be configured using one of these modes :

Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP)

LACP allows a network device to negotiate an automatic bundling of links by sending LACP packets
to the peer (directly connected device that also implements LACP).

While STP is helping us by preventing switching loops, STP is also hurting us by preventing us
from using a perfectly valid path between SW1 and SW2.
We could literally double the bandwidth available between the two switches if we could use that
path that is currently being blocked.

The secret to using the currently blocked path is configuring an Etherchannel.
An Etherchannel is simply a logical bundling of 2 – 8 physical connections between two Cisco switches.

Configuring an Etherchannel is actually quite simple. Use the command “channel-group 1 mode on“ on
every port you want to be placed into the Etherchannel. Of course, this must be done on both switches
if you configure an Etherchannel on one switch and don’t do so on the correct ports on the other switch,
the line protocol will go down and stay there.

The beauty of an Etherchannel is that STP sees the Etherchannel as one connection.
If any of the physical connections inside the Etherchannel go down, STP does not see this,
and STP will not recalculate. While traffic flow between the two switches will obviously be slowed,
the delay in transmission caused by an STP recalculation is avoided.
An Etherchannel also allows us to use multiple physical connections at one time.

Here’s how to put these ports into an Etherchannel:

SW1 :

config t
interface fa0/45
channel-group 1 mode on
!
interface fa0/46
channel-group 1 mode on
!
interface fa0/47
channel-group 1 mode on
!
interface fa0/48
channel-group 1 mode on
!

Creating a Port-Channel Interface Port-Channel 1
!

SW2 :
!

config t
interface fa0/1
channel-group 1 mode on
!
interface fa0/2
channel-group 1 mode on
!
interface fa0/3
channel-group 1 mode on
!
interface fa0/4
channel-group 1 mode on
!

show etherchannel – command to display port-channel information after configuration
!

Configuration in the Cisco 3560 Catalyst Switch 1 :

config t
interface port-channel 1
no switchport
no shutdown
ip address 192.168.1.1 255.255.255.0

!

config t

interface range gigabitEthernet 0/45 – 48

(config-if-range)no switchport

(config-if-range)no ip address

(config-if-range)channel-group 1 mode desirable
!
(config-if-range)channel-protocol lacp
Note: LACP protocol is configured to be used in this channel

Note: This example shows how to configure Gigabit Ethernet ports 0/45- 0/48 into port channel 1
with PAgP mode desirable

Configuration in the Cisco 3560 Catalyst Switch 2 :

config t
interface port-channel 1
no switchport
no shutdown
ip address 192.168.1.2 255.255.255.0

!
config t

interface range gigabitEthernet 0/1 – 4

(config-if-range)# no switchport

(config-if-range)# no ip address

(config-if-range)# channel-group 1 mode auto
!
(config-if-range)channel-protocol lacp
Note: LACP protocol is configured to be used in this channel

March 29, 2011 1:40 pm