From: Narbik Kocharians (narbikk@gmail.com)
Date: Tue Jan 09 2007 - 04:59:41 ART
If you enter the network command with the backdoor option in an attempt to
advertise a connected network, the local router will not advertise that
network to any of it's BGP peers. But the reason we use the network command
with the backdoor option is that we are receiving routing information
from two sources (Routing Protocols) for a given network, an IGP and EBGP,
and we like to prefer the IGP over the EBGP for the ADVERTISED network. In
this situation the only thing that the "backdoor" option does is it
increases the AD from 20 to 200. This can also be achieved by the distance
command and the result is the same.
On 1/8/07, Narbik Kocharians <narbikk@gmail.com> wrote:
>
> Josef,
> When the network command is used with a backdoor option, it is advertised
> to the ebgp peers. The reason for using the backdoor or the distance option
> is because the network was advertised through an IGP and BGP.
>
>
> On 1/8/07, Josef A <josefnet@gmail.com> wrote:
> >
> > One notable difference as stated in the quote from the documentation is
> > that
> > a network configured with the backdoor command is not advertised to ebgp
> >
> > peers, thus if you just manually alter the distance, the network will be
> > advertised. Since the bgp bestpath selection is not changed when a
> > network
> > is configured as backdoor you could actually have routing
> > inconsistencies
> > downstream moreso that the administrative distance is locally
> > significant.
> >
> > HTH
> > Josef
> >
> >
> >
> >
> >
> >
> > On 1/8/07, Yasmin Lara <ylara@sunsetlearning.com > wrote:
> > >
> > > What's the difference between assigning an administrative distance of
> > 200
> > > for
> > > a specific route and using the backdoor feature?
> > >
> > > For example:
> > >
> > > router bgp 300
> > >
> > > distance 200 192.168.100.1 0.0.0.0 99
> > >
> > > !
> > >
> > > access-list 99 permit 44.4.4.0 0.0.0.255
> > >
> > >
> > >
> > > vs.
> > >
> > >
> > >
> > > router bgp 300
> > >
> > > network 44.4.4.0 mask 255.255.255.0 backdoor?
> > >
> > >
> > >
> > >
> > >
> > > According to the documentation: "A backdoor network is assigned an
> > > administrative distance of 200. The objective is to make Interior
> > Gateway
> > > Protocol (IGP) learned routes preferred. A backdoor network is treated
> > as
> > > a
> > > local network, except that it is not advertised. A network that is
> > marked
> > > as a
> > > back door is not sourced by the local router, but should be learned
> > from
> > > external neighbors. The BGP best path selection algorithm does not
> > change
> > > when
> > > a network is configured as a back door." "Changing the default
> > > administrative distances is not recommended because changing the
> > > administrative distance may lead to routing loops."
> > >
> > >
> > >
> > > I don't understand why changing the distance may lead to routing loops
> > and
> > > I
> > > don't understand what difference is makes to set the distance for the
> > > route as
> > > 200 with the distance command or with the backdoor command. Am I
> > missing
> > > something?
> > >
> > >
> > >
> > > Thx!!!
> > >
> > >
> > >
> > > Yasmin.
> > >
> > >
> > _______________________________________________________________________
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> >
> > _______________________________________________________________________
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> >
>
>
>
> --
> Narbik Kocharians
> CCIE# 12410 (R&S, SP, Security)
> CCSI# 30832
> Network Learning, Inc. (CCIE class Instructor)
> www.ccbootcamp.com (CCIE Training)
>
-- Narbik Kocharians CCIE# 12410 (R&S, SP, Security) CCSI# 30832 Network Learning, Inc. (CCIE class Instructor) www.ccbootcamp.com (CCIE Training)
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