What is a Service Instance?

Unanswered Question
Mar 18th, 2010


What is a service instance?

What does this configuration do?

service instance 1110 ethernet
  encapsulation dot1q 421
  rewrite ingress tag pop 1 symmetric
  service-policy output samsung-downlink-10M        <---obviously, a locally-configured sp is refercned on this line...
  bridge-domain 421

This configuration was taken off a 7600 router that is playing the role of backhaul aggregator in a mobile broadband environment.

I tried searching for an explanation of service instances, but it seems that the only thing I can find is how to configure one - not what it is or when you would deploy one.

I have this problem too.
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lamav Thu, 03/18/2010 - 15:02

That definition says absolutely nothing....and the link takes me to a command lookup tool, so Im not sure what thats about....

Thanks, anyway

Jon Marshall Thu, 03/18/2010 - 15:30


Have a look at this link and when you have read it and fully understand it come back and explain it to me

7600 EVC

Seriously, i am still digesting it myself but a service can be many things and the EVC framework allows you to apply mutiple services to the same physical port. An example of a service would be a dot1q trunk, a Q-in-Q tunnel etc.


mustafiz Thu, 03/18/2010 - 15:48

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From the Command lookup tool from the previous post if you select IOS -> then select all IOS Commands and Paste "service instance" in the next field, it will give you a few links. I copied a bit to give you some idea. There are more if you click on the link. Probably you already did this, thought I should mention it anyway.



Usage Guidelines

A service instance is a configuration object (container) that holds all  management and control-plane attributes and parameters that apply to  that service instance on a per-port basis. Different service instances  that correspond to the same EVC must share the same name. Service  instances are associated with a global EVC object through their shared  name.

After you enter the service  instance ethernet command, the device enters Ethernet  service configuration mode, and these configuration commands are  available:

defaultSets the service instance to its default  state.

ethernet lmi ce-vlan  map—Configures Ethernet Local Management Interface (Ethernet LMI)  parameters. See the ethernet lmi ce-vlan map command at: http://www.cisco.com/en/US/docs/ios/cether/command/reference/ce_01.html.

exit— Exits  Ethernet service configuration mode and returns to global configuration  mode.

no—Negates a  command or returns a command to its default setting.

group (service  group)—Allows a member to be added to a service group. The service group  must already exist when the group command is  issued.


The following example shows how to define an Ethernet service instance  and enter Ethernet service configuration mode for an EVC:

Router(config-if)# service instance 333 ethernet test
lamav Thu, 03/18/2010 - 15:54

Jon, I think I may have seen this link before...I'll review it.

Mustafiz, thanks for the command lookup stuff, but once again, it says nothing. There's no explanation of the function it really serves, when you would configure one, etc....


lamav Fri, 03/19/2010 - 08:33

Jon, I began reading that document, and other than the drawing, I didnt get much out of it....

I bet Giuseppe has configured service instances before and knows what they are for....


Jon Marshall Sat, 03/20/2010 - 17:32


Agreed, i suspect Giuseppe might have more insight into this. While we are waiting , this is what i have got from the links so far

EVC = Ethernet virtual circuit. Basically a way for a carrier to provide an ethernet type service to a customer. This ethernet service could be any of the follwing -

802.1q trunk
802.1q tunnel
Local connect
Scalable EoMPLS (EoMPLS xconnect)
Multipoint Bridging (L2 bridging)
Multipoint Bridging (VPLS, SVI-based EoMPLS)
L3 termination

A service instance is simply a way to apply one of the above services to a specific port on the 7600.

Your config example -

service instance 1110 ethernet   <--- simply the identifier and type of service ie. it's an ethernet service with id of 1110

  encapsulation dot1q 421          <--- this means match any frame that has a vlan tag of 421

  rewrite ingress tag pop 1 symmetric   <--- this means when a packet is received on the port strip the outer vlan tag of 421 which is the outermost tag. Also, because of the symmteric keyword it means add the vlan tag of 421 for any frame leaving the port.

  service-policy output samsung-downlink-10M        <---obviously, a locally-configured sp is refercned on this line...

  bridge-domain 421  <-- this bit is still a bit hazy. From the config do you have any other ports that have been configured with this bridge-domain id of 421 ? If so could you post their config as well.

Basically a bridge-domain is used to emulate what we think of a classic L2 broadcast domain where mac-addresses are used for forwarding decisions.

Where your example is a bit confusing to me is that the bridge-domain is using a vlan ID of 421 which is the vlan ID you are matching and removing with the other commands in the config. From all the examples in the link it seems the bridge-domain vlan ID is different from the vlan ID you are matching and removing/adding.

Hopefully Giuseppe can shed some light on this.



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