From: Darren Ward (dward@xxxxxxxxxx)
Date: Sun Sep 02 2001 - 12:51:17 GMT-3
Hi Peng,
As far as I see the backdoor command you are telling the router that if it hear
s that route from an EBGP Neighbor pop it in the table with an admin distance o
f 200 that way IF there is an IGP known route it will be preferred over the EBG
P route that know has the AD 200. (Also as you said it will not pass on the rou
te as Internal)
Otherwise you learn it as an AD 20 which over rules any IGP's normal AD.
By not originating the route it means that it will not advertise the local IGP
route via BGP because the 'backdoor' parameter was attached, without the backdo
or parameter it's a plain old BGP network statement and since the route is know
n via an IGP it would announce it of course.
I think it will still advertise the neighbor announced route with it's AS in th
e path as per normal it just will not originate one of its own.
Anyone correct the last statement I haven't had time to test it yet?
Darren
Michael Wong wrote:
> OK, lets start with the "network" only command. On page 238, Doyle's book sta
tes .....
>
> ".... the network command causes the EBGP discovered routes to be treated as
local BGP routes. Network 172.17.0.0 is advertised to Lillehammer via EBGP, for
instance, and is entered into the routing table. The command network 172.17.0.
0 is added to Lillehammer's configuration, even though 172.17.0.0 is not really
a local route. Because the address is in the routing table, the network comman
d matches it and makes it a local route."
>
> ".... By first being an EBGP route, 172.17.0.0 is changed into a local BGP ro
ute with the network command. Because 172.17.0.0 is now considered a local rout
e at Lillehammer, it is assigned an AD of 200. The RIP route to 172.17.0.0 now
has a lower AD and becomes the preferred route ...."
>
> The above makes perfect sense and I can get this part to work .... yes it doe
s take a little time for it to appear, but other than that, no problems. Howeve
r my issue is with the "network backdoor" command. On page 241, Doyle's book st
ates ....
>
> "The network backdoor command has the same effect as the network command. The
EBGP route is treated as a local BGP route, and the AD is changed to 200. The
difference is that the address specified by the network backdoor command is not
advertised to EBGP peers."
>
> OK, so basically I understand this to be the RIP route will take over as it s
till has the lower AD, however the only difference is that the address specifie
d in the network command will not be advertised ..... cool !!!! No problems ...
... I get the picture and the logic about why you don't want the network to be
advertised etc., but it seems that when the "backdoor" command is used, the rou
tes no longer become local ???? By the way they also don't get advertised.
>
> Am I understanding this correctly and I'm having these issues due to dodgy IO
S ????
>
> Thanks .... MW :)
>
> -----Original Message-----
> From: afiddler [mailto:afiddler@wi.rr.com]
> Sent: Saturday, 1 September 2001 9:02 am
> To: Peng Li; Michael Wong
> Subject: Re: BGP Backdoor (Doyle Vol II page 240)
>
> My study buddy and I went through this lab just a few weeks ago. It seems
> to work as stated. Lillehammer does not really have the RIP route, but
> advertises it so that it looks like an IBGP route. With a much higher AD,
> this route is not preferred as long as the RIP route exists. As soon as the
> RIP route disappears, the next best route is the IBGP route from
> Lillehammer, which advertises it with an origin of IGP.
>
> Perhaps I just do not understand the issue you are having. I would be happy
> to set this up again in my lab and provide some results to you if that would
> help.
> ----- Original Message -----
> From: "Peng Li" <lipeng@canada.com>
> To: "Michael Wong" <Michael.Wong@nec.com.au>; <ccielab@groupstudy.com>
> Sent: Friday, August 31, 2001 6:03 AM
> Subject: Re: BGP Backdoor (Doyle Vol II page 240)
>
> > Hi,
> > I think there's several examples in the book either Jeff overlooked or
> doesn't elaborate in much detail or maybe some misunderstanding If I dare to
> challenge.
> >
> > One of the AM example is what you discovered. According to my
> understanding and lab results, the "network xxx backdoor" does't change the
> Ebgp into IBGP with changing AD from 20-200. This is not the way it works.
> When you finish you config of AM command, you should shut down the EBGP
> neibor and see the difference. It works now. The reason is that it takes
> time for EBGP tcp connection to setup and get routes with AD20,
> approximately 40-50 seconds. By this time, the rip already got the route and
> by using "network" command . The rip learned route is already entered in
> Local BGP table with Weitht 32768 much hiher than the later learned EBGP
> with Weight of 100, this cause the Router deny EBGP routes and prefer IBGP
> one. at the same time, he keeps the RIP one in RT.
> >
> > Hope it helps and correct me if I'm wrong.
> >
> > Take care.
> > My lab is Oct.10 in Beijing.
> >
> > ----- Original Message -----
> > From: "Michael Wong" <Michael.Wong@nec.com.au>
> > To: "Groupstudy - CCIELAB (E-mail)" <>
> > Sent: Friday, August 31, 2001 1:40 AM
> > Subject: BGP Backdoor (Doyle Vol II page 240)
> >
> >
> > > BGP gurus .....
> > >
> > > Has anyone managed to get BGP backdoor to work properly ??? I'm going
> through Doyle's example on page 240 and I can't seem to get the BGP backdoor
> command to work properly.
> > >
> > > The funny thing is that I am able to get the correct results and change
> the EBGP route to a local BGP route and make RIP take precedence over the
> local BGP route with the "network 172.18.0.0" command, however when I use
> the same network command and just add "backdoor" to it, the RIP routes do
> not appear .... strange I thought ????
> > >
> > > The RIP routes are definitely getting through as when I close the BGP
> sessions, the RIP routes appear in the routing table. It seems that when the
> "backdoor" command is added to the network command, BGP does not modify the
> EBGP to a local BGP route and the route table still has an AD of 20.
> > >
> > > Any thoughts ????
> > >
> > > Thanks peoples ..... MW
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > **Please read:http://www.groupstudy.com/list/posting.html
> **Please read:http://www.groupstudy.com/list/posting.html
**Please read:http://www.groupstudy.com/list/posting.html
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