From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Tue Sep 30 2003 - 21:06:00 GMT-3
This behavior does not relate to split horizon. A BGP speaking
device will only advertise its best routes. BGP does this both to
prevent traffic black holes and to minimize the size of the global BGP
table.
The global routing table is around 130,000 prefixes now.
Suppose that you had 3 BGP peers from which you are taking a full table.
If the above behavior was not true, you would be advertising over
250,000 prefixes to each of these peers. If they in turn had two more
peers... You get the picture.
Ken, regarding your previous posting about how the BGP
attributes should be applied, the following table holds true:
_____________ ______________ ________________
| Attribute | BGP Policy | Traffic Flow |
|-------------|--------------|----------------|
| Weight | IN | OUT |
|-------------|--------------|----------------|
| Local Pref | IN | OUT |
|-------------|--------------|----------------|
| AS-Path | OUT | IN |
|-------------|--------------|----------------|
| MED | OUT | IN |
-------------|--------------|----------------
Weight and Local-Preference should be set inbound. They affect
how traffic leaves your AS. Weight is locally significant to the router
and is Cisco proprietary. Local-Preference is locally significant to
your entire AS.
AS-Path and MED should be set outbound. They affect how traffic
returns to your AS. AS-Path is a transitive attribute (will propagate
to all ASes). MED is a non-transitive attribute (will not propagate
beyond your directly connected AS). AS-Path can be used when you are
multi homed to a single provider, or multi homed to multiple providers.
MED should only be used when you are multi homed to a single provider.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Cristian Henry H
> Sent: Tuesday, September 30, 2003 10:45 AM
> To: Ken.Farrington@barclayscapital.com
> Cc: ccielab@groupstudy.com
> Subject: Re: BGP Experts Required here.
>
> In mi opinion that behavior is absolutly normal.
>
> First Router12 learn route 168.1.0.0 from Router76 with MED 33 and the
> same route from AS350 with MED 42, so comparing MEDs it choose the
route
> from Router76 as valid one. Then Router12 send its "valids" routes to
> Router76 so it should send back the route 168.1.0.0 learned from
> Router76 (MED33), but by slip-horizon it doesn't send it.
>
> If we start the same secuence but starting with Router76 learning the
> route from both neighbors, it will choose the MED33 from AS350, so it
> will send it to Router12 (not split horizon here). Then Router12 will
> have both entries for the route.
>
>
> Ken.Farrington@barclayscapital.com ha escrito:
> >
> > Hello all,
> > I have a question re the setup below.
> >
> > I have two routers peering eBGP with an external AS 350 and iBGP
between
> the
> > routers which (just for info) are separate locations.
> >
> > Now if I receive a route (say 168.1.0.0) that has a metric inbound
on
> both
> > routers (router76 route for this network has metric of 33 ,and
Router12
> has
> > metric for this route of 42) it looks as if the router that has the
> lowest
> > metric from the route, does not accept the iBGP route from the other
> router
> > with the higher metric, but the router with the higher metric
accepts
> the
> > iBGP route from the router with the lower metric.
> >
> > If I receive a route where the metric is not set and all attributes
are
> the
> > same, it send each other the iBGP route (like 168.41.0.0) and uses
it's
> > local exit point via eBGP to that network.
> >
> > Please can someone confirm that this is normal operation for iBGP.
> >
> > Many thx indeed,
> > Ken
> >
> > \ 168.1.0.0 168.224.0.0 168.41.0.0 /
> > \ External /
> > \ AS 350 /
> > \__________________________________/
> > |192.168.42.253 |192.168.41.254
> > | |
> > |eBGP |eBGP
> > | |
> > | |
> > | AS 300 |
> > +--------+ +--------+
> > |Router76|---------------------|Router12|
> > +--------+ iBGP +--------+
> > 192.168.42.253 192.168.41.253
> >
> > on Router76
> >
> > router bgp 300
> > neighbor 150.5.36.129 remote-as 300
> > neighbor 192.168.42.254 remote-as 350
> >
> > Router76>sh ip int brief
> > Interface IP-Address OK? Method Status
> > Protocol
> > Ethernet0 unassigned YES unset administratively
down
> down
> > Ethernet1 192.168.42.253 YES NVRAM up
> up
> > Loopback0 150.5.32.65 YES NVRAM up
> up
> >
> > Network Next Hop Metric LocPrf Weight Path
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> > *> 168.1.0.0 192.168.42.254 33 0 350 i
> This
> > entry only appears once
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> > *>i168.224.0.0 192.168.41.254 31 100 0 350 i
> > * 192.168.42.254 42 0 350 i
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> > * i168.41.0.0 192.168.41.254 100 0 350 i
> This
> > entry appears on both
> > *> 192.168.42.254 0 350 i
> This
> > entry appears on both
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> >
> > On Router12
> >
> > !
> > router bgp 300
> > neighbor 150.5.32.65 remote-as 300
> > neighbor 192.254.41.254 remote-as 350
> > !
> >
> > Router12>sh ip int brief
> > Interface IP-Address OK? Method Status
> > Protocol
> > Ethernet0 unassigned YES unset administratively
down
> down
> > Ethernet1 192.168.41.253 YES NVRAM up
> up
> > Loopback0 150.5.36.129 YES NVRAM up
> up
> >
> > Network Next Hop Metric LocPrf Weight Path
> > * 168.1.0.0 192.168.41.254 42 0 350 i
> > *>i 192.168.42.254 33 100 0 350 i
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> > *> 168.224.0.0 192.168.41.254 31 0 350 i
> This
> > entry only appears once
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> > *> 168.41.0.0 192.168.41.254 0 350 i
> This
> > entry appears on both
> > * i 192.168.42.254 100 0 350 i
> This
> > entry appears on both
> >
------------------------------------------------------------------------
> ----
> > ---------------------------
> >
> >
This archive was generated by hypermail 2.1.4 : Wed Oct 01 2003 - 07:24:40 GMT-3