CCNA - Configuration ISL

Author

Configuration ISL

Ybentrepreneur

2002-11-29, 3:47 pm

I am kinder Confused about configuration ISL.

For Example.... How would I configure such a setup?

I have Five Routers and six Switches...

The Names of my Routers are: NY, NJ, Orlando, Virginia, And Atlanta.

The NY Router contains Two Switches: NYA & NYB.

I configure Switch NYA with Trunk ON!

So now my question:
Should I Make the Router in the NY LAN a 2600 Series Router in order to Configure ISL?

How would I Configure ISL using a 2500 series router??

Is it a Must to use a 2600 router to Configure ISL?

Thanks all

dbowen

2002-11-30, 8:59 pm

The purpose of ISL is to run multiple vlans over a single port (or bundle of ports) for linking vlans across different switches or routing between vlans.

Say I have two 13-port switches, each contiguous 4 ports is set into a vlan (so I have 3 vlans + 1 trunk port, which operates in all vlans). If I wanted vlan 1 on switch 1 to merge with vlan 1 on switch 2 WITHOUT trunking, I'd have to crossover ports from vlan 1 on both switches. Same thing applies to merging vlans 2 and 3 across switches (just like chaining hubs except you're physically chaining vlans--not efficient). Now enter the trunk port. Instead of crossing-over 3 sets of ports, I just crossover the trunk ports. So vlans 1, 2, 3, etc. all run over this one wire; the switches handle getting vlan traffic from one switch to the appropriate vlan/port on the other switch.

I can't remember all the router series specs, but you definitely can't run ISL encapsulation over an Ethernet port; you must use a Fast Ethernet port. So the upshot is...if your router doesn't support Fast Ethernet, then you definitely can't implement ISL on that router (as far as I know, same rule applies to switches).

If your router supports Fast Ethernet, try exploring 'encap isl' (or something like that--you might need an extra argument, including vlan number; check the argument help feature, you know, question mark after everything) on your physical Fast Ethernet interface (or one of its subinterfaces). If that flies, you're good to go.

Basically, patch the trunk port on your switch to the trunk port on your router. Assuming you want to route between vlans (router-on-a-stick model), assign IP addresses to subinterfaces on the physical Fast Ethernet (trunk) port, and bind each vlan to a subinterface. If you're just trunking vlans across switches, just enable trunking on the desired Fast Ethernet ports and crossover the ports.

If you're implementing router-on-a-stick, start with a simple model, for example:

vlan 1 --> 10.1.1.0/24 --> subinterface F0/0.1
vlan 2 --> 10.2.2.0/24 --> subinterface F0/0.2
vlan 3 --> 10.3.3.0/24 --> subinterface F0/0.3

Matching up all the 1's, 2's, and 3's makes learning this a little more clear.

Good luck.

dbowen

2002-11-30, 10:21 pm

Also, the default gateway for a computer on any vlan should point to the IP address of the corresponding subinterface of the Fast Ethernet port on the router.

For example, if VLAN 1 and IP address 10.1.1.254/24 are bound to F0/0.1, then the default gateway for a PC on VLAN 1 is 10.1.1.254.

Remember, this varies per vlan (and corresponding subinterface).

Have fun. You'll learn quite a bit from setting up router-on-a-stick and trunking across vlans.

Also, try removing vlans from a trunk port. By default, all vlans are transported across a trunk port unless you specify which vlans not to carry. I frequently, and apparently mistakenly, call this "pruning vlans from the trunk" (sounds good to me--all these tree words: pruning, trunk; however, I understand you only truly "prune" vlans from the vtp database).

Ybentrepreneur

2002-12-01, 4:44 pm

so in a Vlan, Its not really wise to set IP Addresses on switchs, Correct? Or should O set the switches Default-Gateway address to be that of the f0/1 interface...

dbowen

2002-12-02, 7:39 pm

A switch is typically considered a Layer 2 device; however, you can still assign it an IP address for management purposes. And, depending on the breed of your switch, you may be able to assign host IP addresses to your individual VLANS for management and gateway purposes.

So I wouldn't say it's not wise to assign an IP address to a switch--it's just not always necessary. For example you can route between VLANS just fine without assigning an IP address to the switch itself or its VLANS. But if you want to ping that switch, telnet to it, configure it via a web-browser, or do SNMP monitoring you must assign an IP address.

And, typically, a switch doesn't route so you don't have to set the switch's default gateway either.

Just make sure your PC's are pointing to the correct VLAN gateway (remember, that's the IP address of the corresponding FastEthernet subinterface). PC to PC traffic within a VLAN doesn't hit the gateway, but PC to PC traffic outside of a VLAN will hit the gateway (then the router takes care of forwarding the packets to the proper VLAN gateway).

darronb

2003-06-17, 9:07 am

I hope this post is not to old.

Like the above I have a 2621 router with two fast Ethernet ports. I have 8 2950 SX switches, all connected over a fibre link.

At one end of the 'Switch chain' I have the fa 0/0 port of the router connected to a switch, and the other end of the switch chain, I have the fa 0/1 port of the router connected.

I understand (i Think about Router-on-a-stick and sub interfaces), but how would you set up the ports on the router for this config.

I need this config so I will have a backup if one line fails.

NOTE. I have two subnets for this network, and need to implent two VLANS

darronb

2003-06-19, 12:11 am

Hi Guys

Can anyone help me out here !!!!!!!!!

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