CCIE - ccie written

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ccie written

bkhanh

2003-10-23, 3:09 pm

hi:
I am preparing my ccie written exam.
There was 1 question on the practice exam I got really confused about:

Here is the question:

What is the effect of bgp backdoor command :

A. Changes the distance of an iBGP route to 20.
B. Changes the distance of an eBGP route to 200.
C. Changes the distance of an IGRP route to 200.
D. Changes the distance of an IGP route to 20.
E. Does not change the distance of the route.

the practice exam said B. is the correct answer. While reading about BGP backdoor command on cisco doc. site, it merely said that the BGP backdoor command makes the IGP route becomes a prefered route. No mention of changing distance, infact changing distance is one of the method making a route is prefered over the others but it is discouraged in this case of BGP, backdoor command should be used to have the IGP route chosen instead.

So why B is the correct answer?

I appreciate any help here.

thanks

ZacDogg

2003-10-23, 9:31 pm

B is correct. The normal AD for a route learned from an EBGP peer is 20, but when it is specified as a backdoor network the AD will be set to 200, making every route learned from an IGP more preferred.

Zac

darthfeces

2003-10-26, 12:09 am

BGP Backdoor

Consider the above diagram, RTA and RTC are running EBGP and RTB and RTC are running EBGP. RTA and RTB are running some kind of IGP (RIP, IGRP, etc.). By definition, EBGP updates have a distance of 20 which is lower than the IGP distances. Default distance is 120 for RIP, 100 for IGRP, 90 for EIGRP and 110 for OSPF.

RTA will receive updates about 160.10.0.0 via two routing protocols:
EBGP with a distance of 20 and IGP with a distance higher than 20.

By default, BGP has the following distances, but that could be changed by the distance command:

distance bgp external-distance internal-distance local-distance

external-distance:20
internal-distance:200
local-distance:200

RTA will pick EBGP via RTC because of the lower distance.

If we want RTA to learn about 160.10.0.0 via RTB (IGP), then we have two options:

1- Change EBGP's external distance or IGP's distance which is NOT recommended.

2- Use BGP backdoor

BGP backdoor will make the IGP route, the preferred route.

Use the following command: network address backdoor.

The configured network is the network that we would like to reach via IGP. For BGP this network will be treated as a locally assigned network except it will not be advertised in BGP updates.

Example:

RTA#
router eigrp 10
network 160.10.0.0

router bgp 100
neighbor 2.2.2.1 remote-as 300
network 160.10.0.0 backdoor

Network 160.10.0.0 will be treated as a local entry but will not be advertised as a normal network entry would.

RTA will learn 160.10.0.0 from RTB via EIGRP with distance 90, and will also learn it from RTC via EBGP with distance 20. Normally EBGP will be preferred, but because of the backdoor command EIGRP will be preferred.

bkhanh

2003-10-26, 12:42 pm

hi:
thanks for your reply. I do have the exact same bgp backdoor doc you just sent me. My question is that, it is not explicitly mentioned in the doc. that behind the scene, the backdoor command indeed changes the AD EBGP's AD to 200 to be equal to that of the IGP, and hence the internal route IGP is prefered. I guess that it does the AD changed behind the scene. My only issue is that why can the tech. writer be more specific about it?. The doc only said that "by using the BGP backdoor command, it makes the IGP route the prefered one", but it never actually mentioned the real action behind.

thanks for communicating, you cheer me up any how for I don't feel that I am a lonely soldier out there...

CCIE blue...

ckb

bkhanh

2003-10-26, 1:54 pm

hi darthfeces:
another point I want to point out here for
your opinion. I quote from the doc that we
both have below:

RTA will pick EBGP via RTC because of the lower distance.

" If we want RTA to learn about 160.10.0.0 via RTB (IGP), then we have two options:

1- Change EBGP's external distance or IGP's distance which is NOT recommended.

2- Use BGP backdoor "

If 1- is not recommended for us to do then
why BGP backdoor command does it as option
B- dictated (option B is the the one almost
everyone chose) for that practice question. While I am not so sure about it because
changing AD is not recomended based on the
doc. above. So why should BGP backdoor does what it is not recommended to begin with?

I read from somewhere that the Backdoor command triggers the IGP route to be chosen blindly without manipulating AD.

confuse as ever, but will go on to the next question on my practice exam....

dumfart

2003-10-26, 2:02 pm

See pages 326-327 in Internet Routing Architectures. If you don't have this book then you absolutely must get it.

bkhanh

2003-10-26, 3:39 pm

hi dumfart:
No, I don't have the book yet but
will get it very soon. Thank you.

ckb

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