[c-nsp] Sharing router uplinks?

JP Senior SeniorJ at bennettjones.com
Wed Aug 1 16:53:51 EDT 2012

Putting a web server (or any other) host device on the same subnet causes reachability issues to other subnets -- hacks/workarounds include ICMP redirects, static routing tables, and proxy arp on the subnet.  A server won't know which 'router' to take to get to which subnet.  This is an administrative disaster as you have to either permit ICMP redirects explicitly (Operating systems shouldn't/don't support this by default anymore), turn on evil proxy arp, have a full mesh IGP, or enable static routes on the hosts.

Hosts should only have a single exit point out of a subnet, through a router(or two, using FHRP).

As far as shared 'router' vlans or subnets, this is completely normal and common for distribution/core networks.

-JP Senior

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Erik Nelson
Sent: 01 August 2012 9:23 AM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] Sharing router uplinks?

I have always thought it is a best practice to not put servers or PCs on links/subnets that connect routers together. I also have always thought that router to router links should be 1:1. For example, the link from a top-of-rack or end-of-row router to the data center core should be a dedicated link. 

I have run into a situation where there is insistence that both of these practices not be observed. I am being asked to put many router uplinks on a single subnet connected to a single port on the core router. I am also being asked to put a web server on this same subnet. 

What do others think of this?  I have been unable to find anything on the web that says anything for or against. If anyone knows of authoritative guidelines on the web about this I would be very interested. 

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