Daisy Chaining Question

Unanswered Question
Mar 19th, 2008

I have a network where (6) Layer 2 switches are daisychained together with a router on each end:

rtr1--sw1--sw2--sw3--sw4--sw5--sw6--rtr2----xxxxxx--back2rtr1

Can you think of anyway to provide redundancy if a link failure occurs between one of the switches without making them Layer 3(or spending extra money)? The routers will never know if a link fails between the switches because its directly connected switch will keep the router interface up. Any suggestions? The 2 routers are running HSRP, but when a link fails between the sw's, both routers become active and continue to advertise the route causing a black hole in some instances. All Vlans are being trunked in both directions. Your suggestions are appreciated.

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Jon Marshall Wed, 03/19/2008 - 15:20

Hi

Does depend on what interfaces ie fibre/UTP you want to interconnect the switches with but my suggestion

Pick any two of the switches, preferably your most powerful, and make them the top level switches. So

rtr1 -> sw1 - sw2 <- rtr2

The 2 switches will be interconnected with a layer 2 switch. Then for each of the remaining switches run 2 connections, one to sw1 and one to sw2.

You can load balance your vlans by making sw1 STP root primary for odd vlans and sw2 secondary for odd vlans.

sw1 is secondary for even vlans and sw2 is primary for even vlans.

You can then match the vlan HSRP active gateway to the STP root switch.

HTH

Jon

cuyler.mccoy Wed, 03/19/2008 - 19:14

Thanks for your quick reply Jon. Each of the devices mentioned are in separate buildings connected via fiber as shown in the basic depiction I showed. All of the switches are 2950s so they don't support Layer 3. The routers are running HSRP.

I guess my question is if there is a fiber cut, let's say between switch 3 & 4, how can the blackhole situation be mitigated when both routers will become active in HSRP for whichever VLANs were between the SWs. I really don't see a resolution without adding additional equipment to provide true redundancy.

I really appreciate you taking the time to respond.

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