An excellent book on this subject is High Availability Networking with Cisco
by Vincent Jones ISBN 0201704552.
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Priscilla Oppenheimer
Sent: Thursday, May 31, 2001 3:11 PM
To: [EMAIL PROTECTED]
Subject: Re: Redundancy design question [7:6646]
Well, you have pinpointed the problem with many redundant campus network
designs. They may not be redundant into the WAN.
To meet your goals, you may need a backup WAN connection of some sort.
Depending on the level of performance you want for the backup and the
amount of traffic that you have, you could use a low-speed and low-cost
backup such as ISDN or even an analog modem.
You'll need to think about the cost, benefits, risks of not doing anything,
etc.
How often do failures occur with your current WAN? (Mean Time Between
Failure)?
When problems occur, how quickly do they get fixed? (Mean Time To Repair)
What's the cost of downtime?
Any layer 8 (politics) issues you need to deal with? Like will you lose
your job and/or credibility if the WAN connection is down for a long time?
When provisioning backup WAN links, you should learn as much as possible
about the actual physical circuit routing also. Different carriers
sometimes use the same facilities, meaning that your backup path is
susceptible to the same failures as your primary path.
Be sure to analyze your local cabling in addition to your carrier's
services. Perhaps you have designed an ISDN link to back up a Frame Relay
link. Do both of these links use the same cabling to get to the demarcation
point in your building network? What cabling do the links use to get to
your carrier? The cabling that goes from your building to the carrier is
often the weakest link in a network. It can be affected by construction,
flooding, ice storms, trucks hitting telephone poles, Bob the back-hoe
operator, etc.
Priscilla
At 03:09 PM 5/31/01, Jon wrote:
>I've been reading about designing physical redundancy into networks, by
>having hot standby devices and using HSRP between them. As an example, if
>a site has a single router and a single core switch, these are points of
>risk. By adding a second core switch and a second router, any hardware
>failure should be overcome by the standby device taking over. If all the
>servers and wiring closet switches are multi-homed to both core switches,
>users shouldn't notice that a fault has occured. (I assume that the loss
>of a wiring closet switch is acceptable -- perhaps local spares are
>sufficient).
>
>However, if I only have one WAN circuit coming into the facility, it can
>only be connected to one router at a time, right? So, if the active
>router fails, how does the WAN connectivity fail over, short of an
>operator moving the cable to the second router? I'm not trying to address
>WAN circuit redundancy or multi-homing, that's a different worm-can to
>open.
>
>Is there some way to have both routers connected to the same WAN circuit?
>Something along the lines of a WYE-cable that connects both routers to the
>demarc connection? Or is this something that the circuit provider would
>address with their equipement (for a fee, I'm sure)?
>
>If this has been hashed over in the past, I couldn't find it in the
>archives. So, if we've covered this before, could someone share the key
>search words to locate the discussion?
>
>-jon-
>
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Priscilla Oppenheimer
http://www.priscilla.com
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