EIGRP Multiple neighbor adjacencies

Unanswered Question
Feb 13th, 2008

One of our clients have two 6509E core switches. These switches are connected to a 3620 WAN router via a Layer 2 switch.The vlan thar makes this connection is Vlan2. EIGRP is the routing protocol. In the rouitng protocol configuration, i have passive defaulted all vlan interfaces and allowed EIGRP to run only on Vlan 2. The core switches peer with each other as well as the WAN router. Now there is another router that has to get connectivity to the LAN.This is via a wireless link. The wireless device would get connected to the branch router. At the HQ side wireless device gets connected to the topmost floor edge switch and that Vlan would be carried to the core switches. Now since this is a separate Vlan lets say Vlan 3, for EIGRP to run over this i have to give "no passive-interface vlan 3", since everything is passive defaulted. Now the concern is that in addition to core switch forming adjacency to the new branch router, the core switches would also form adjacency with eaach other via Vlan 3. So each core switch would form two adjacencies with each other, one via Vlan 2 (already in production) and the other via Vlan 3 (the new Vlan).

Now the concern is that, if this is indeed the case, will there be any effect on the network, with respect to stability of performance issues. As of the route table consists of almost 50 entries. This customer is a bank with a lot of online services and we would not like to have a situation that would cause any connectivity issues.

I have this problem too.
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Richard Burts Wed, 02/13/2008 - 08:47

Prasant

I would not think that having neighbor relationships over 2 VLANs would cause any kind of connectivity problem or have any negative impact on stability or performance.

If you want to make sure that the switches will use VLAN 2 as the primary interface for forwarding traffic you could put some additional delay configured on the VLAN 3 interface. A higher delay on the VLAN 3 interface will produce a higher metric in EIGRP and make the VLAN 2 interface more favorable for forwarding traffic.

HTH

Rick

Jon Marshall Wed, 02/13/2008 - 10:34

Prasant

Just to add to Rick's post.

We always use 2 vlans to peer across a L2 trunk link between our 2 6500's in our main buildings so there will not be an issue.

Indeed a couple of our 6500's were once accidentally misconfigured and non of the vlan interfaces were made passive. The 6500's ended up peering with each other over about 30 separate interfaces !!.

I wouldn't bother with trying to modify the metrics on one of the interfaces as these peerings are only used to exchange routes and so whether it is just vlan 2 peering or vlan 2 & 3 peering the paths will still be the same.

Edit - okay just to clarify what i mean. When i say the path will be the same i mean the L2 trunk between the 6500 switches not which vlan interface it routes through.

HTH

Jon

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