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Question on possibly using route switch as standby backup to router

  • From: william(at)
  • Date: Wed Mar 17 18:22:42 2004

(On topic to nanog for a change...)

  I'll be soon going through resetup of one of our primary hosting POPs 
(moving to different DC and upstream provider) and as a result have
opportunity to  make some changes to the configuration, etc and want to 
set it up so there is standby backup available to the main router. Note 
that I'm not  going to setup complete cisco HSRP, I don't have interest in 
going this far. The only purpose is to provide service for customers in 
the POP for short period  of time in case of router failure or when router 
has to be taken down for upgrades. And while the router itself has direct 
connections to other routers on our network, I'm fine with network being 
split into segments while the router is down, I do however want customers 
in that POP to be able to use the primary upstream in that POP (and I'll 
as a result need to announce only the ip blocks related to that segment 
in case router goes down)

  The concept I have in mind is to use our main cisco switch there that also
happened to have a router card and could I think do bgp. The idea is to connect
one or two upstream GB connections directly into the switch (currently we 
connect to upstream through port on the router) and then setup vlans from 
there to go to the router through its gigabit interface (most likely 
etherchannel logically on the router to be able to expand to multiple 
interfaces if it is ever needed).
  Futher instead of doing typical /30 interconnect to upstream, I would like
to use /29 there and assign one ip to their router, one to our router and 
one ip would be on route switch card (which can see each vlan as separate
interface). For BGP instead of establishing session directly between
interfaces of one and another router, I want to use separate /30 that would
be announced to upstream through EBGP (but not go beyond just between these
routers) and this /30 would contain ip address to be used for primary
BGP session for announcing our routes (for those familiar this is how 
cogent does it). The idea is then that when everything is working and 
primary router is ok, it will announce this /30 and bgp ip to upstream 
and thereafter be able to establish bgp session and send all the routes 
there, but if router is down, same /30 begins to be announced by route 
switch which could take over routing.

  Now my main config problem here is that I need a way to have main router 
announce something to route switch that would suppress its announcement of 
this same /30 though ebgp. 
  Additionally I need to find a way to let the individual customer servers
(these on separate vlans connected from the router and through the switch,
each vlan has one ip on the main router and one on the route-switch vlan)
know which is the correct default gateway. In theory I can of course have 
route-switch always be the default router for those customer machines, 
but I'd like to avoid this and use router instead. And you have to remember 
here also that while some customer machines are linux and solaris and can 
talk ospf and receive default router through that, many servers are windows
and I really really would like to avoid using IGP routing protocol on 
microsoft software. So if possible I'd like to completely avoid using IGP 
protocol for sending default and try doing it some other way.

And suggestions on above two items? Did any of you  do anything similar and 
perhaps documented it somewhere?

P.S. My router is 7500. The route switch is currenly 5500 with RSM card, 
but I maybe upgrading to 6500 switch soon, but would like the setup to 
work on either one.

William Leibzon
Elan Networks

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