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HSRP-EIGRP Configuration

Answered Question
Apr 10th, 2012
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I have two switches at different sites connected via fiber. The two sites have redundant SAN and VM hosts so I have their Vlans spanning both sites. My goal is to have redundancy for connectivity and routing to/from Vlan105 (SAN Vlan) at both the High School and Town Hall sites. But this config doesn’t look correct.


Shouldn't the HS_MDF switch have the following statement in the EIGRP configuration?



network 172.18.5.0 0.0.0.255

passive-interface vlan105

!--- above since we only let vlan 30 interface participate in eigrp


Am I right in assuming that if the TH-Core switch went down, the HS_MDF switch would assume the Vlan105 IP address but no other subnets would know about it via EIGRP?


Thanks,

-mike

Attachment: 
Correct Answer by Richard Burts about 5 years 4 months ago

Mike


Thanks for the additional information. In the larger context that you have explained I agree with your original point that you should add both the network statement and the passive-interface statement to EIGRP.


HTH


Rick

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Richard Burts Tue, 04/10/2012 - 14:03
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Mike


If you put the network statement for 172.18.5.0 under EIGRP then you would want to also put the passive interface command, assuming that you want EIGRP to advertise the subnet but not to form neighbor relationships on that VLAN or send any routing updates on that VLAN. If you have both network statements with one passive on one router it would seem logical that you should have it on both routers.


But I question whether it will do what you want. Redundancy for this environment is a bit tricky. If TH-Core goes down then HS_MDF will take over the virtual address. But the virtual address is not advertised by EIGRP. And who would you be advertising to? Your diagram shows only two switches. Is there more to the network than what you have showed us? Are there more networks in EIGRP than the two that you have shown us?


Given what you have shown us I do not see any real benefit in running EIGRP. If one switch goes down then there is not any other routing device to learn of an alternate path. And if the link for one VLAN/subnet goes down you would not fail over to reach the remote part of that subnet via the other path because the original subnet is still in the routing table as a locally connected subnet.


HTH


Rick

Michael Murray Tue, 04/10/2012 - 15:45
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Richard,


Thanks for you response.


I only included the portion of the config I had a question with. The scenario I am attempting to protect against is a switch failure at one of these two core sites. There are about a dozen branch locations that have fiber into each of these core sites. Picture the Police Station (on say subnet 172.18.3.0/24) that has fiber to TH and HS and accesses email on an Exchange server running in a VM on Vlan105. So if Town Hall goes down, all the VMs will move to the High School, and all of my branch sites should still have network connectivity to Vlan105. But if the High School is not configured to advertise subnet 172.18.5.0/24 then I am assuming that none of the routers at the branch sites (also participating in EIGRP on Vlan30 subnet) will have routes to the High School for that subnet.


-mike

kamran_Roostaee Tue, 04/10/2012 - 22:44
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Hi Mike

you are using HSRP on vlan 5 and Configuration shows that TH is active one and if TH fails HS will be active gateway. TH and HS are EIGRP neighbors and if you insert network 172.16.5.0 0.0.0.255, a route to this network through these 2 switches  will advertise to other neighbors, if TH goes down then route through TH will remove from other neighbors and they see just a route to network 172.16.5.0 through HS, so you need command "network 172.16.5.0 0.0.0.255" under both switch EIGRP configuration but if thee is no need to add int vlan 5 as passive interface and you can remove it.

ps. if you wants other switch does not use load balancing and use 2 routes to network 172.16.5.0 then you need to set metric in a manner that other switches see a better route through TH than HS

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Richard Burts Wed, 04/11/2012 - 10:59
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Mike


Thanks for the additional information. In the larger context that you have explained I agree with your original point that you should add both the network statement and the passive-interface statement to EIGRP.


HTH


Rick

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