layer 3 engine interface,

Answered Question
Dec 13th, 2008

Hi everybody!

What types of interfaces can be regarded as layer 3 layer 3 engine interface?

svi,loopback, are layer 3 engine interfaces? If they are layer 3 engine interfaces, How can they mac addresseses as they are virtual interfaces?

For example , my cisco book shows an output:

switch# show adjacency vlan 99 detail.

protocol interface address

ip vlan99 192.168.1.2(5)

00A5E44ffffeeefef0800

the first six octets is the mac address of next hop, the remainder 6 octes is the mac address of int vlan99, the 0800 shows the "Ip protocol"

thanks a lot!

I have this problem too.
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Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Sarah,

loopbacks as pure logical interfaces are in the route-processor and only there.

SVIs are hosts of associate L2 Vlans as I described in my first post here.

Packets destinated to loopbacks and to SVIs (and also I guess to routed physical ports, that are those ports with the command no switchport and an ip address on it they become L3 interfaces as well) are process switched they can include : routing protocol messages, packets of telnet/SSH sessions, SNMP gets and so on.

Note: I realized that probably I've gone out of topic in my first post

Hope to help

Giuseppe

Correct Answer by Jon Marshall about 8 years 1 month ago

In addition to Giuseppe's post, Catalyst switches support either a unique mac-address per L3 vlan interface or they use the same mac-address for all vlan interfaces.

Because Catalyst switches can support per-vlan mac-address tables and can map multiple IP addresses to the same mac-address using the same mac address for multiple L3 vlan interfaces is not a problem.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml#topic1

Jon

Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello Sarah,

I don't see any trouble in this.

An SVI needs to have a MAC address when it transmits and when traffic destinated to the SVI is sent.

From the L2 point of view an SVI is an host on the L2 broadcast domain (that is the group of ports in the same vlan).

Traffic transmitted by SVI or destined to SVI has to go through a L2 port that is in the associated vlan and in STP forwarding state to access/(comes from) the outer world.

The SVI is up/up until at least one L2 port (either access or trunk) is in STP forwarding state (autostate feature).

So for me an SVI is an interface on the Route Processor, via an internal link in the backplane it connects to the L2 vlan.

This is clear in C6500 sup2/MSFC2 in Hybride mode:

there is a special trunk 15/1 that is the trunk to the MSFC (routing processor)

you are allowed to remove a vlan from the list of the allowed vlans on 15/1, after doing so the corresponding SVI is placed in down/down.

We could define SVIs as semi-logical interfaces while loopback are pure logical interfaces and then we can list LAN physical interfaces and WAN physical interfaces.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sat, 12/13/2008 - 12:58

Hello Sarah,

I don't see any trouble in this.

An SVI needs to have a MAC address when it transmits and when traffic destinated to the SVI is sent.

From the L2 point of view an SVI is an host on the L2 broadcast domain (that is the group of ports in the same vlan).

Traffic transmitted by SVI or destined to SVI has to go through a L2 port that is in the associated vlan and in STP forwarding state to access/(comes from) the outer world.

The SVI is up/up until at least one L2 port (either access or trunk) is in STP forwarding state (autostate feature).

So for me an SVI is an interface on the Route Processor, via an internal link in the backplane it connects to the L2 vlan.

This is clear in C6500 sup2/MSFC2 in Hybride mode:

there is a special trunk 15/1 that is the trunk to the MSFC (routing processor)

you are allowed to remove a vlan from the list of the allowed vlans on 15/1, after doing so the corresponding SVI is placed in down/down.

We could define SVIs as semi-logical interfaces while loopback are pure logical interfaces and then we can list LAN physical interfaces and WAN physical interfaces.

Hope to help

Giuseppe

Correct Answer
Jon Marshall Sat, 12/13/2008 - 13:34

In addition to Giuseppe's post, Catalyst switches support either a unique mac-address per L3 vlan interface or they use the same mac-address for all vlan interfaces.

Because Catalyst switches can support per-vlan mac-address tables and can map multiple IP addresses to the same mac-address using the same mac address for multiple L3 vlan interfaces is not a problem.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml#topic1

Jon

sarahr202 Sat, 12/13/2008 - 14:41

Thanks Jon and Giuseppe!

Actually I was reading about punt adjacency.

I learned that packets destined to ip addresses ,assigned to Interfaces on layer 3 engine, will be punted to layer- 3 engine.

So I am wondering what interfaces could be on layer-3 engine besides svi? Can I say loop-back interfaces are also on layer-3 engine or just svi?

thanks a lot!

Correct Answer
Giuseppe Larosa Sat, 12/13/2008 - 15:17

Hello Sarah,

loopbacks as pure logical interfaces are in the route-processor and only there.

SVIs are hosts of associate L2 Vlans as I described in my first post here.

Packets destinated to loopbacks and to SVIs (and also I guess to routed physical ports, that are those ports with the command no switchport and an ip address on it they become L3 interfaces as well) are process switched they can include : routing protocol messages, packets of telnet/SSH sessions, SNMP gets and so on.

Note: I realized that probably I've gone out of topic in my first post

Hope to help

Giuseppe

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