Routing on a Stick

Unanswered Question
Dec 22nd, 2007

Hi

For routing on a stick with native vlan configured, can the interface on the sw connecting to the external router be a Fast Ethernet and the interface on the router is a ethernet.

Please help.

I have this problem too.
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pciaccio Sat, 12/22/2007 - 11:29

Yes you can have two dissimilar port types. Just make sure that you code the speed and duplex the same on both sides. If not then you will see errors on the port and interface... Pls rate...

royalblues Sat, 12/22/2007 - 11:45

Trunking was supported only on fastethernet ports. Something rings my mind that this was relaxed but not sure (somebody correct me here)

For routing on a stick, you need to have a corresponding subinterface for each vlan. so unless your ethernet interface supports dot1q/ISL trunking and subinterfaces, it will not work

HTH

Narayan

Peter Paluch Sun, 03/06/2011 - 01:36

Hello Nik,

What are you exactly interesting in answering? I believe Jon has provided a complete answer when referencing the article.

Just to reiterate: the 802.1Q is principially a software functionality, i.e. it does not require any special hardware support to work. The 802.1Q tagging does not modify the basic structure of an Ethernet frame (Destination MAC, Source MAC, Type, Payload, CRC), thus no modifications to the Ethernet hardware are necessary (except allowing slightly larger Ethernet frames - up to 1522 bytes). The 802.1Q functionality is provided simply by adding a 4B tag (new EtherType, Priority, CFI bit, VLAN ID) immediately between the Source MAC and the original Type field, thereby using the Payload field of a basic Ethernet frame to carry all its data. As such, it does not matter on which Ethernet interface type it runs, be it 10Mbps, FastEthernet up to the latest 40/100 Gbps Ethernet variants.

This is what the theory says. However, Cisco may - on its sole discretion - decide that it will not provide this software functionality on selected router platforms and interface combinations. That is what the document referenced by Jon says: originally, the 2600 routers with Ethernet interfaces did not support 802.1Q encapsulation simply because it was not made available by the IOS. Starting with IOS 12.2(2)T, the 802.1Q support is available also on Ethernet interfaces. There may be other router platforms on which similar limitations exist or existed in the past.

However, the basic idea remains: the 802.1Q is a software issue and it does not depend on any specific Ethernet interface variant. However, there may be additional limitations imposed by router and switch vendors that will prevent you from actually using the 802.1Q on selected interfaces. That is, however, purely their technical and marketing decision. The 802.1Q happily runs on any Ethernet interface without limitations - the only issue to consider is that the maximum frame length grows by 4 bytes, to 1522 bytes. Older Ethernet controllers may find this problematic to process.

Best regards,

Peter

glen.grant Sun, 03/06/2011 - 04:07

  Anyway you look at it  I would not want to route multiple vlans across a 10 meg half duplex link anyway   unless you are just playing in a lab to prove concept ..

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