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Re: BGP - weight

  • From: E.B. Dreger
  • Date: Sun Feb 15 14:35:22 2004

SH> Date: Sun, 15 Feb 2004 16:50:02 +0000
SH> From: Sven Huster

[ editted and reformatted for clarity ]

SH> The core sends to R1, which believes the best path is via R2
SH> and sends it back to the core as that's the only way to reach
SH> R2.  Then the core again sends it to R1 and all the same
SH> again.


SH> As this is a small network internally everything is routed
SH> via static routes.

Except for the smallest of networks, I try to avoid static
routes.  It's additional work and opportunity for error.  Using
BGP + TCP MD5 auth, OSPF auth, hardcoded ARP entries, per-port
MAC address restrictions, prefix lists, route maps, etc., one can
run a dynamic network and still keep security under control.

SH> R1 and R2 have full BGP views from the transit providers as
SH> well as partial view from the peers.

Why not arrange the routers and switch in a single VLAN?  (Or did
I misunderstand your earlier ASCII-art diagram?)  I usually use
something like:  local sinkhole  virtual router (HSRP/VRRP; maybe XRRP now)  physical router #1  physical router #2
	:	:	:	:	:	:	: [routing] switch #2 [routing] switch #1

Let R1, R2, and R3 speak directly over ethernet without routing
through core.  If they already do, verify that you're setting
nexthop correctly.

Multihop routing sessions often can be made to work, but they're
a tricky "house of cards".  Remember, classic IP routing forwards
to a { MAC addr | PVC | endpoint } based on destination IP addr.
You can't do fancy rewriting at each hop; that's part of why PBR
and label switching were invented. ;-)

Note: I am _not_ suggesting PBR for this situation.

SH> They [R1 and R2] run iBGP with R3 and the core.

You have a partial mesh in which R1 and R2 do not exchange routes
with each other?

EBD> router bgp xxxx
EBD>  [no] bgp bestpath compare-routerid

SH> All devices use the default settings in this respect.
SH> R1-3 are Cisco routers, the core Extreme Alpine.

Somewhere along the line Cisco changed the default from "bgp
bestpath compare-routerid" to the converse.  I forget when,
although a quick Google search leads me to believe it was around
12.0/12.0S/12.0ST.  I can't comment on Extreme.

Again, though, I'm going out on a limb with this one.  I'd bet on
static routes, topology, and [lack of] IGP before BGP path
selection algorithm.

SH> It seems to be a temp problem, which we just figured out once


SH> it went away based on netflow data and traffic dumps. So there
SH> is no data available for this right now.

If you catch any non-traceroute packets with expiring TTL, see if
you can grab routing info from all the boxes involved.  I'm
confused how these devices are building their RIBs...

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