Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

isl encapsulation on C2811 Router w/ IOS12.3+ possible?

We have a C2621 as our LNS connecting to our ISP's LAC. I need to upgrade my underpowered C2621 router and have purchased a C2811 to replace it. I was hoping to simply copy over the config from the 2621 to the 2811 but it seems ISL doesn't work in the same manner as it did in 12.2. Our ISP is setup at the moment using ISL encapsulation on their VLAN interface facing my router, and I prefer to keep it that way for the moment.

I need to know if it's possible to setup ISL encapsulation on Cisco 2811-IOS12.4.x subinterfaces because the below configuration does not work as is on my new C2811. If I try to enter the "encapsulation isl" command, i only have dot1q as an encapsulation option. Is there some global config command I need to use. Or am I S.O.L. with IOS12.4 and this command. It's in the 12.4 Command what am I doing wrong? Current Image is c2800nm-advsecurityk9-mz.124-5 and I am trying to configure the fastethernet ports.

interface FastEthernet0/1.213

description someplace 213

encapsulation isl 213

ip address 10.x.x.x

no ip redirects

no ip mroute-cache

Let me know if there are enough details for someone to help.



Re: isl encapsulation on C2811 Router w/ IOS12.3+ possible?

I have not seen an isl option in the ISR series routers. I ran a quick check through Software advisor, and it shows ISL Vlan and 802.1q ISL Vlan mapping feature as supported and shows 12.4 and 12.3T trains. I dont think that the software advisor is showing the right info in this case. I am sure you are pretty much left with 802.1q which asks for changing the ISP configuration as well.

I checked on a 2821 router with 12.3 code and it only supports 802.1q.


PS: Please remember to rate helpful posts!


Re: isl encapsulation on C2811 Router w/ IOS12.3+ possible?


in addition to Sankar´s post, I found this in the 2800 ISR FAQ´s:

Q. What trunking protocols are supported?

A. Only 802.1Q is supported. Inter-Switch Link (ISL) is not supported; it is proprietary to Cisco and is not commonly deployed.

There is a previous post inquiring about ISL, I quote the comprehensive and very useful explanantion from Rick Burts:

´Many vendors, including Cisco, will recognize a need and introduce proprietary features in advance of a standard being developed and will then adopt the standard when it is developed.

ISL is an example of this. Cisco introduced ISL as it recognized the need to have a trunking protocol in Ethernet switching in advance of the adoption of dot1Q. After the standard was adopted Cisco began to move to it. Some products supported both products but increasingly Ethernet switches support only dot1Q and not ISL. So I believe that the reasons and opportunities to use ISL are getting fewer and fewer.´



New Member

Re: isl encapsulation on C2811 Router w/ IOS12.3+ possible?

Thanks for the help. This would make sense now. Where did you find that FAQ though? I can't seem to find it anywhere.