Route Updates on Layer 3 Switch

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Jun 11th, 2007
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Assume, a router is connected to L3 switch. Switch has uplink to router and downlink to other switches plus other host ports.

Let us say, switch has a few vlans and EIGRP routing was on. The uplink on the switch to router is not a L3 port. (other switches are not running L3). What port swith use to update routing table and how switch know what port should be used?


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Overall Rating: 4.5 (2 ratings)
Richard Burts Mon, 06/11/2007 - 07:21
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I am not sure that I really understand your question completely. Lets start with a clarification about the link between the switch and the router. You say that it is not to be a layer 3 port on the switch. You do not say what it would be. This situation could work if the link were a trunk and the router interface were set up for trunking. In this case routing updates would be generated for each VLAN interface and carried over the trunk. And routing updates from the router would be carried over the trunk and put into its routing table by the switch. If the switch port were an access port then it would be a member of a single VLAN (or subnet). The switch and router would become neighbors over the access link and exchange routing updates over the single VLAN/subnet.



smallrain_2 Mon, 06/11/2007 - 09:40
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Thanks Rick! I knew my explaination was bad but you answered exactly what I needed. Btw, do you know any book/link/sample copnfigs about routing(EIGRP) on L3 switch.

Thanks again.

Amit Singh Mon, 06/11/2007 - 07:25
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The vlan interface on the L3 switch, in which the which the port resides which connectes to the router will form the EIGRP neighbor relationship with the router. Once the nighbor relation is successful, the router and switch will exchange the routes. You have to make sure the EIGRP is enabled on the vlan interface i;e the Layer3 SVI of the port which is connected to the router. If all the downstream switch have the SVI interfaces configured on them and are running EIGRP they will form the neighbor relationship with the upstream switch and will exchange the routes with each other.

HTH,Please rate if it does.

-amit singh


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