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Cisco HSRP question?

Posted on 2007-04-05
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Last Modified: 2008-01-09
Have a question about cisco’s HSRP.   My environment currently has 2 Cisco 3845’s.  One of them is a cold stanby for the other…to use it if the primary failed we’d have to change the IP and physically move over a Serial DS3 cable and go from there.  The network…she is flat.  No vlans no internal subnets.   All one big ugly layer 2 storm at this point.  

I’m thinking about vlanning the network.  Basically, we have 3 floors.  I was going to put each floor in a different vlan on a different subnet and then one other vlan for the servers and use the routers to route between them.   At that point redundancy becomes a must.  So I am thinking of implementing HSRP and using it.  However, there is no routing protocol in use on the network.  Any routes put in are static.  Simply because there really are no other upstream or downstream routers (smaller company) routers.  So question:
A. Does HSRP mirror config from primary to secondary from each router involved or do I need to make sure that the configs are the same statically?  It appears as tho it does not as you simply add the standby address to the interfaces on both routers?  Anyway just curuious not the end of the world.
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/cs009.htm

B. What about the DS3?  I mean right now if it goes down we loose connectivity to a remote site (it’s a serial connection to the primary router).  How do I connect that serial side to the other router as well so in the case that the primary goes down the secondary picks up?  I’m thinking at this point that this isn’t possible.  Being the DS-3 is serial I cant just split the cable and connect it to both routers and expect anything to work.  Will this continue to be a manual switch if it goes down?  

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Question by:stamperb
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Sorenson earned 350 total points
ID: 18858540
HSRP is fairly straight forward.  For your design I would look at the track command to watch the serial interface so that the router with the interface up has the default routes.  This will require you to set a allow preempt as well.

HSRP does not mirror the configs (and neither does glbp) you will need to set the configs manually.

As you begin to build out the vlans and are looking for the redundancy, I would check out the GLBP instead of HSRP.  The commands are similar to HSRP, however it will load balance answer mac addresses for gateway queries, load balancing between your two routers.  The glbp is aware of the its peer members and takes over their mac addrs, in case of a failure, so it gives you both redundancy and load balancing (at least for the inter vlan routing).  I believe glbp is supported on the 3845s.

Depending on the amount of traffic you are pushing, remember that traffic between each vlan will be limited to a fraction of what the interface on the 3845 supports (ie 100mb, with 3 or 4 vlans may only give 20-40mb throughput between vlans.  I would suggest looking at a L3 switch to do the inter vlan routing (or a pair using HSRP or GLBP).

Hope that helps.


 
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by:stamperb
ID: 18858842
OK that is awesome.  Just a few more questions arise with this.  The GLBP seems to fit my needs perfectly.  I'm still a little hazy on the DS-3 config tho.  Right now both 3845's have a NM-1 T3/E3.  I'm in a taller building so the cable from the router connects to a Fiber Driver that takes this back down to the entry point to the building.  How do I "Split" this coacable so I can have both routers hooked up.  Using the track command to make sure only one interface is up at a time makes since.  But do they make a splitter for the ds-3 coax cable thing? <-sounds technical i know :-)
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by:stamperb
ID: 18858871
Also it would appear that the standby track and standby preempt are featuers of HRSP so are you suggesting I use both GLBP and HSRP together?  And if so are you meaning GLBP on the Ethernet interfaces and HRSP on the DS-3 interfaces
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by:stamperb
ID: 18858905
Ignore that last question I see the track command available with GLBP as well.
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by:stamperb
ID: 18860208
Same goes for a T1...i mean it comes in serial...how would you hook it up to both routers so that it could pick up on the other?  Do they make "splitter" cables for such a thing?
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by:mikebernhardt
mikebernhardt earned 150 total points
ID: 18860493
Regarding the serial, you can't "split" it. It is a point-to-point link as you know. You need to set up 2 DS-3s, or at least a T1 for the backup (maybe you don't need full bandwidth for your backup path?).
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by:stamperb
ID: 18860915
Thats great.  Last part then.  Can I use this for just the Ethernet interfaces (routing the vlans) to provide the availability there and not on the serial interfaces?  Is it all or nothing or interface specific?  I presume looking at the commands that its interface specific?
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by:mikebernhardt
ID: 18861003
It's interface specific. So even if you can't currently provide failover for your exit (the DS-3), at least you can provide high availability for your LANs.
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