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question about MLS and its operation

hi  ,

im trying to understand the MLS operation .

uptill now i understood that the MLS will work as a router if the port of the MLS configured as a router , and thwe MLS will work as switch if the port of the MLS configured as a switch .

this is a confusion i want to know  about .

my queston is ,  does the opearation of the MLS depend on the port configured or depend on another things ???

regrards

1 ACCEPTED SOLUTION

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Cisco Employee

question about MLS and its operation

Hi

The MLS operation depends on the way you have configured the port

1. If the port is configured with  no switchport", it will do only routing. It will not do any switching

2. if the port is configured as part of VLAN and there is no " interface VLAN" configured for that VLAN, it will do only Layer2 switching of packets

3. if the port is configured as part of an Access VLAN and you have routing enabled on that VLAN, it  can do switching and routing for the packets received on that port.

Thanks

Raju

View solution in original post

14 REPLIES 14
VIP Expert

question about MLS and its operation

Multi-Layer Switching (MLS) is part of the function of the switch.  Traditionally, most switches had layer-2 capabilities only.  MLS provide layer-2 and layer-3 functionality on the same device.  It used to be that only bigger switches like the 6500 series were capable of MLS, but now days most small switches i.e.. 3550, 3560, 3750, etc are capable of doing both.Of course, there are still some switches that can do layer-2 only.

HTH

Cisco Employee

question about MLS and its operation

Hi

The MLS operation depends on the way you have configured the port

1. If the port is configured with  no switchport", it will do only routing. It will not do any switching

2. if the port is configured as part of VLAN and there is no " interface VLAN" configured for that VLAN, it will do only Layer2 switching of packets

3. if the port is configured as part of an Access VLAN and you have routing enabled on that VLAN, it  can do switching and routing for the packets received on that port.

Thanks

Raju

View solution in original post

question about MLS and its operation

rajs2 wrote:

Hi

The MLS operation depends on the way you have configured the port

1. If the port is configured with  no switchport", it will do only routing. It will not do any switching

2. if the port is configured as part of VLAN and there is no " interface VLAN" configured for that VLAN, it will do only Layer2 switching of packets

3. if the port is configured as part of an Access VLAN and you have routing enabled on that VLAN, it  can do switching and routing for the packets received on that port.

Thanks

Raju

hi

Raju

thanks for ur reply ,

i think i was right when i said that the mls operation depend on the port config , but agian

the point 1 and 2 i understood them .

but the point 3 not yet !

can u give me explanation  or the sequence that will happen when packect reach a port configured for access and its vlan interface has an ip .

?

regards

Cisco Employee

question about MLS and its operation

1. The packet goes through layer2 lookup first. This is based on the destination MAC address. If the destination MAC is of the router's own MAC-address it will forward the packet to next layer for L3 lookup. if the destination MAC is belonging to any protocols. like CDP, STP,BGP,OSPF, those packets will be forwarded to the CPU. If the destination MAC is learned from some other port on same VLAN, it will be forwarded to that port. if the destination is not present in CAM table, packet will be flooded on all the ports belonging to that VLAN

2. During L3lookup, it will look at the destination IP address and forward the packet to the right port after adding the layer2 Encapsulation

Thanks

Raju

question about MLS and its operation

rajs2 wrote:

1. T If the destination MAC is of the router's own MAC-address it will forward the packet to next layer for L3 lookup.

Raju

about u said above ,

what do u mean with

router's own MAC-address

do u mean in this case the vlan mac address of the port that is access port and its vlan has an ip  , or  another thing ???

Cisco Employee

question about MLS and its operation

Hi,

do u mean in this case the vlan mac address of the port that is access port and its vlan has an ip  , or  another thing ???

- Access port doesnt need any MAC address to be assigned. Normally, on L3 port (physical or SVI) will have MAC address and if receive any frame with destination address as self MAC address (any MAC address assigned in local interface).

By, Router's MAC address, Raju meant the Layer2 address assigned in any L3 port of the router/switch.

HTH,

Nagendra

Re: question about MLS and its operation

hi 

Nagendra

lets have an example :

assume i have MLS with vlan 1 & vlan 2

each vlan  svi interface has an ip and has routing enabled

                                               L2  -Switch 2960

                                                           |

                                                           |

                                                         Gi1/4

                                                           |

pc1 vlan 1--------------- Gi1/1---MLS-----Gi1/3-------------------router

                                                           |

                                                        Gi1/2

                                                           |

                                                           |

                                                           |

                                                   pc2 vlan 2

as u see now , Gi/1/1 & Gi1/2 are configure switch ports and acces ports to vlan 1 & vlan 2

Gi1/4 is connected to L2 switch  and working as L2 with trunk port

Gi1/3 is connected to Router and working as L3 port

######################################################################

vlan 1 ip is 192.168.1.1/24

pc1 ip is 192.168.1.2/24 and its ip gateway is 192.168.1.1

==================

vlan 2 ip is 192.168.2.1

pc2 ip os 192.168.2.2 and its gateway ip is 192.168.2.1

================

now when pc1 want to talk to pc2 i think that pc1 will send to the default gateway which is 192.168.1.1 and  send arp and get the mac of the vlan routed interface and in this case the MLS will look in the routing table ????

is my info correct ???

Cisco Employee

question about MLS and its operation

Look at my below config

interface TenGigabitEthernet10/1

ip address 192.168.12.1 255.255.255.0

7600-1#sh int ten10/1

TenGigabitEthernet10/1 is up, line protocol is up (connected)

  Hardware is X40G 10Gb 802.3, address is d0d0.fda5.6400 (bia d0d0.fda5.6400)

You can see the mac-address d0d0.fda5.6400 . If this router receives packets with destination mac-address d0d0.fda5.6400 , it will forward the packet to layer 3 for layer 3 processing

Thanks

Raju

question about MLS and its operation

rajs2 wrote:

Look at my below config

interface TenGigabitEthernet10/1

ip address 192.168.12.1 255.255.255.0

7600-1#sh int ten10/1

TenGigabitEthernet10/1 is up, line protocol is up (connected)

  Hardware is X40G 10Gb 802.3, address is d0d0.fda5.6400 (bia d0d0.fda5.6400)

You can see the mac-address d0d0.fda5.6400 . If this router receives packets with destination mac-address d0d0.fda5.6400 , it will forward the packet to layer 3 for layer 3 processing

Thanks

Raju

so , u mean that if a packed recived has destination mac of L3 interface if it was a svi or physical interface ,  am i right ??

Cisco Employee

question about MLS and its operation

Hi,

Yes. Any L3 interface (physical or SVI) will obviously have a L2 address. If a pure L3 interface receives a non self MAC address, it will ignore the packet. But if an access port (which is also enabled with SVI) receive a self destinated MAC address (address of SVI), it will decap and further look inot the L3 header. If it receives a non-sefl MAC address, it will treat it as L2 packet and behave as a normal L2 switch.

-Nagendra

Cisco Employee

question about MLS and its operation

Yes. if the received MAC-addres is of the routers L3 interface, the packet is handed over to L3 for routing the packet

Raju

Re: question about MLS and its operation

hi

Raju & 

-Nagendra

thank you very much ,

in summary , i can say that MLS has 3 cases that control it how to operate  as raju said :

1. If the port is configured with  no switchport", it will do only routing. It will not do any switching

2.  if the port is configured as part of VLAN and there is no " interface  VLAN" configured for that VLAN, it will do only Layer2 switching of  packets

3. if the port is configured as part of an Access VLAN and  you have routing enabled on that VLAN, it  can do switching and routing  for the packets received on that port.

uptill now  i feel i got good explanation about MLS operation , im wondering why the ccnp books didnt mention the point 3 above ???!!!!!

i read all the ccnp switching book and didnt have a good expalnation of the MLS ,

any way , thanks both for ur info u posted here .

any new question go in my mind i will ask it here

regards

VIP Expert

Re: question about MLS and its operation

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Posting

CSCO12006413 wrote:

hi  ,

im trying to understand the MLS operation .

uptill now i understood that the MLS will work as a router if the port of the MLS configured as a router , and thwe MLS will work as switch if the port of the MLS configured as a switch .

this is a confusion i want to know  about .

my queston is ,  does the opearation of the MLS depend on the port configured or depend on another things ???

regrards

MLS is basically L2 like switching of L3 packets.  It depends on both the capabilities of the hardware and that you're doing L3 forwarding.

Newer L3 switches hide the details of MLS, i.e. they do it auto-magically, but if you research, on Cisco's main web site, MLS on 5500s, especially using an external router, it may help clarify what MLS is all about.

Beginner

Re: question about MLS and its operation

MLS switches comprises of layer 2,3 and 4 to make it MLS there are some MLS switches that have there port enabled as a layer 2 so all you need to do is to check with the command show int fa0/1 switchport that will tell you what layer the switch belongs to.

and for your question if you use the no switchport comman on an interface it means you are hardcoding the port to work as a layer 3 switch and if u use the switchport command it will only work on layer mode. and routing is only performed on layer 3 mode for intervlan communication.

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