Cisco Router interfaces definition

Unanswered Question
Feb 8th, 2010

Hi,

I'm trying to define interfaces of a 3925 CISCO router. This router will be interconnected with multiples WAN interfaces (WAN across macrowave network, WAN across optical fiber network,...). Theses WAN interfaces will be 100Mbps or 1Gbps interfaces, so I expect to use the three natives CISCO ports (GigabitEthernet 0/0,GigabitEthernet 0/1,GigabitEthernet 0/2) to interconnect with theses interfaces. For all additional WAN acces, I will add routed port HWIC card.

1) I expect to integrate 1000BASE-T SFP transceiver module into HWIC card. The CISCO technical notes notice this SFP is used for Category 5 copper wire. So is it a minimal requirement? I suppose this SFP could be used with upper copper wire category without impact the performance of the global link, couldn't it?

This router will be also interconnected with LAN through cipher unit and a backup 3925 CISCO router configuring with HSRP link.

2) For theses two acces I wonder if routed port are necessary or if layer 2 port can't be enough?

3) More generally, what is the difference between layer 2 and layer 3 port ?

(I know that contrary to layer 2 interfaces,  you can assign IP address on routed interface, but you can also set up link with layer 2 interfaces by means of Vlan-Interfaces. In the same way, routed interface can act as layer 2 interface by means of  Sub-interfaces.)


Thank you for your responses

Best Regards

Julien

I have this problem too.
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Paolo Bevilacqua Mon, 02/08/2010 - 04:09

Hi, from the questions you're asking comes to mind you're the end user, and have not worked much with routers before  ?

Take no offense, but wouldn't be better that you engange a professional to design your network ?

Much frustration and costly mistakes are avoided this way.

julien_eads Mon, 02/08/2010 - 05:32

Thank for your response but it's not very constructive and it's doesn't really help me. Contrary to what you can think, I have a background in network design and if I ask this question here, That's because nobody could answer me among my circle.

Jon Marshall Mon, 02/08/2010 - 08:47

julien_eads wrote:

Hi,

I'm trying to define interfaces of a 3925 CISCO router. This router will be interconnected with multiples WAN interfaces (WAN across macrowave network, WAN across optical fiber network,...). Theses WAN interfaces will be 100Mbps or 1Gbps interfaces, so I expect to use the three natives CISCO ports (GigabitEthernet 0/0,GigabitEthernet 0/1,GigabitEthernet 0/2) to interconnect with theses interfaces. For all additional WAN acces, I will add routed port HWIC card.

1) I expect to integrate 1000BASE-T SFP transceiver module into HWIC card. The CISCO technical notes notice this SFP is used for Category 5 copper wire. So is it a minimal requirement? I suppose this SFP could be used with upper copper wire category without impact the performance of the global link, couldn't it?

This router will be also interconnected with LAN through cipher unit and a backup 3925 CISCO router configuring with HSRP link.

2) For theses two acces I wonder if routed port are necessary or if layer 2 port can't be enough?

3) More generally, what is the difference between layer 2 and layer 3 port ?

(I know that contrary to layer 2 interfaces,  you can assign IP address on routed interface, but you can also set up link with layer 2 interfaces by means of Vlan-Interfaces. In the same way, routed interface can act as layer 2 interface by means of  Sub-interfaces.)


Thank you for your responses

Best Regards

Julien

Julien

The difference between a L2 and L3 port -

Pretty much th difference between L2 and L3 ie. at a fundamental level L2 deals with mac-addresses and L3 deals with IP addresses.

A L3 routed port receiving a packet will do a L3 lookup ie. the destination IP address to determine where to send the packet.

A L2 port receiving a packet will do a L2 lookup ie. the destination mac-address to determine where to send the packet.

Now obviously a L3 port after doing the L3 lookup then has to do a L2 lookup but the key point is a L3 port is really concerned with the IP address as destination.

From a practical point of view one of the benefits of running routed ports is that they do not extend the L2 broadcast domain and they do not run STP which can limit the amount of damage a broadcast storm/STP loop could cause.

Jon

julien_eads Wed, 02/10/2010 - 05:15

Thank you to have taken time to answer me Jon.

Actually I had understood the fundamental difference between layer 2 and layer 3 in the sens that L2 deals with mac-addresses and L3 deals with IP addresses. But I had not necessarly in mind the practical point of view you explain. So, thank you very much for this explanation.

But my request would be the following :

In a pratical use, You can need to interface your router with other equipment (not necessarly LAN equipments) in using Vlan-Interface. So you have the possibilities to use Sub-Interface of a routed port but in my case, the routed port being restrained it's better for me to use layer 2  interfaces. But I wonder if you will have the same fonctionnalities in using layer 2 interface instead of layer 3 interface.

For exemple, for having already used hierarchical shaping, I saw you couldn't use this fonctionnality on Vlan-Interface whereas it is available on layer-3 interface.

In a same way, Will you have the same performances in using layer 2 interface instead of layer 3 interface? For exemple if I set up GRE tunnel from Vlan-Interface instead of L3 interface? It's maybe not comparable, but for having tested GRE performance on Catalyst 3750 L2 interfaces, performances were disastrous.

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