Reduced Router Peering ?

polm

Diamond Member
May 24, 2001
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I am studying for my BCMSN and I came across this :

http://www.cisco.com/warp/public/cc/so/neso/lnso/cpso/gcnd_wp/gcnd_wp8.gif

"Using the routed gigabit ethernet connections between the backbone switches offers the following advantages :

* Reduced router peering for additional stability and scalability"

Can someone explain to me exactly what "router peering" is , and how "Using the routed gigabit ethernet connections between the backbone switches" offers a reduction in router peering ?
 

yoda291

Diamond Member
Aug 11, 2001
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If memory serves, router peering is simply the existence of a peering route. Normally reserved for BGP router, a peering route is the route between 2 seperate BGP devices. nowadays however, the term is so ambiguous, it could mean any route agreement. Essentially tho, figure that by making a routed gigabit connection between your core switches means you do not necessarily require routes between your distribution layer switches and all your core switches.

Example, I have 40 or so distribution layer switches in 20 buildings. I have 5 core switches, a 2 switch server module and a 2 switch edge module. Now, if I did not have routed gigabit connections between my core/backbone switches, I would need a routed connection between every distribution switch, server switch, edge switch to every core switch. (40 +2 +2) x 5 = 220 connections. Every distribution layer switch you added would require another 5 connections(10 if you have redundancy). if you had a routed connection between the backbone switches, you could get away with 1 connection per distribution layer switch(provided they were in redundant pairs) and 1 connection per edge and server switch(also assuming redundant pairs). Every switch you add in any module(core,server, or edge) only needs 1 connection to the nearest core switch. 220 connections vs 44 connections. you lose the fast convergence though. You gain orders of magnitude in reliability though.
 

spidey07

No Lifer
Aug 4, 2000
65,469
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router peering simply means a neighbor relationship between two routers. that is they are running a routing protocol, monitoring each others reachability and exchanging routes.

A lot of times people will do all sorts of complicated designs where a router is peering with 10 other routers through their redundant links. this is really bad design and leads to routing instability, increased convergence times, routing complexity, etc.

-edit- and its referrign to a layer3 design where core/distribution switches are connected with layer3 links. If they were layer2 links all the core/distribution switches would want to peer with everyone of the others resulting in n(n-1) peers. Bad.