General OSPF Configuration

Posted on 2009-02-23
Last Modified: 2012-08-13

I am trying to resolve some OSPF related issues for 2x Nortel 8310 aggregation switches. The current network is setup in the following manner:

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For this lab environment, we only have 1 link from the Nortel to the BGP router. Eventually we'll have redundant links for each switch. Both BGP routers A,B has eBGP with their upstream providers and iBGP between them. The BGP routers have a /30 to the Nortel switches and OSPF is running on those interfaces.

The 2x Nortel switches have MLT/IST trunks between them, and they act as a single logical switch to the edge switches. However, they are technically 2 separate switches with shared VLANS (if it is included across the IST trunk). Port 1/1 on Nortel A and Port 1/1 on Nortel B are on the same VLAN with RSMLT (similar toVRRP, HRSP in Cisco terminology). The problem I am having is:

Lets say we have a /23 IP block for these switches to sub allocate to customer/nodes. The problem is how do  we set it up the OSPF advertisement without conflicts? Since each Nortel runs OSPF to the upstream BGP router, it would advertise a certain IP block that it manages. This would work fine if each switch were to allocate a /24 block that doesnt overlap however in doing so, the redundancy scenario (Port 1 on both Nortels = same VLAN) would not work.

What would be the best way to setup OSPF so that each switch actually manages the /23 blocks without any advertisement conflicts? From a non Nortel perspective, this setup would be similar to having 2 independant switches trunked with VLAN sharing capabilities, and each switch manages the same /23 block.

Any suggestions are appreciated.

Question by:iesolutions
    1 Comment
    LVL 11

    Accepted Solution

    You could try increasing the cost of one of the switches so that only one is prefered. Or use route-filtering to block the other switches advertisements

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