BGP two questions

[acbennyma]

With the following BGP lab, In the lab, I want to verify two things.

I know RTA learn two network 192.168.169.0/24 and 192.168.167.0/24

via AS200 and AS300. If the no. of AS path is the same, then how RTA determine 192.168.169.0/24 use 172.0.0.2

as next hop and why 192.168.167.0/24 use 172.0.0.22 as next hop ?

why RTB only can learn 10.0.0.0/24 via 172.0.0.21. How about via RTD ? I tried to

show ip bgp neighbor 172.10.10.2 advertise-route in RTD but it haven't 10.0.0.0/24, why this happen?

CONNECTION----RTA(AS100)Parellel link to-->RTB(AS200), RTC(AS300) Parellel link to-->RTD(AS400)

[Harold Ritter]

For eBGP, If all other attributes are equals and it comes to to using the RID a the tigh breaker, the first prefix learnt will be kept even though the second one might have been learnt from a peer with a lower RID. This provides more stability.

For more information on the BGP Best Path Selection Algorithm, refer to the following URL:

http://www.cisco.com/warp/customer/459/25.shtml

As for your second question, RTD will not advertise 10.0.0.0/24 to RTB since it sees its best path for this prefix coming from RTB.

Hope this helps,

[acbennyma]

For the second question, As u said RTD will not advertise 10.0.0.0/24 to RTB since it sees its best path for this prefix coming from RTB.

Then why RTD still advertise 10.0.0.0/24 to RTC

[Harold Ritter]

Because the best path on RTD is from RTB, hence it is advertised to RTC.

Hope this helps,

[acbennyma]

sorry, I still don't understand how this work.

after analysis your description, So Why RTC can learn

192.168.167.0/24 and 192.168.169.0/24 via RTA.

If RTA know the best route for RTC to those network is via RTD, then RTA will not advertise it to RTC.

But now.............

any document talking about this case ?

Thanks for your help !!!

[Harold Ritter]

RTA advertises 192.168.167.0/24 and 192.168.169.0/24 to RTC because RTA's best path is not via RTC.

The rule can be summarized as follow: A router takes all the paths it receives for a specific prefix from all of its peers, runs the BGP best path selection process and advertises the best path to all of its peers except to the peer from which the best path was received.

Hope this helps,

[acbennyma]

By refering to this theorem, (BGP best path selection process and advertises the best path to all of its peers except to the peer from which the best path was received )

can u search any link taking about this since I have some confuse about it.

For the next problem is if I want to perform load balance to 192.168.167.0/24 and 192.168.169.0/24 from RTA.How can I do ?

Thank for your help !!!

[acbennyma]

Any Expert can teach me

Thank you very much !

[Harold Ritter]

This is not a Cisco IOS specific behavior but rather an RFC based one. For this reason the best document to read to understand it thoroughly is RFC1771.

http://www.ietf.org/rfc/rfc1771.txt?number=1771

As for your second question, you can only loadshare between different paths if they are via the same peer AS, which is not the case in your scenario.

For more information on the Multipath feature, refer to the BGP Multipath section in the following URL:

http://www.cisco.com/warp/customer/459/25.shtml

Hope this helps,

[acbennyma]

sorry, I don't know what you mean. is it can't to perform load balance?

I just know I can use load sharing by using route map and access-list with weight.

Also I have no CCO account to access your link

http://www.cisco.com/warp/customer/459/25.shtml

Pls help !

[Harold Ritter]

If by loadsharing you mean select one peer as the best path for one prefix and the other peer for the other prefix, then yes I agree that you should be able to do that by changing the BGP attributes.

I will forward you the document offline.

Hope this helps,

[acbennyma]

Hi Hritter,

Thanks you for your attachment.

After reading the document, I try to use maximum-path 2

to see whether load balance occur, it also fail. As I know the weight of both path is the same, also AS_Path is the same.

But it is also fail. What configuration I need to add ? Actaully, Can this connection to perform load balance, I still

have no idea up to now. Any configuration sample for reference ?

Thanks you for your great help !

RTA#sh ru

router bgp 100

no synchronization

bgp log-neighbor-changes

network 10.0.0.0 mask 255.255.255.0

neighbor 172.0.0.2 remote-as 300

neighbor 172.0.0.2 version 4

neighbor 172.0.0.22 remote-as 200

neighbor 172.0.0.22 version 4

maximum-path 2

[Harold Ritter]

As I was explaining in a previous post, in your scenario you will not be able toi use the multipath feature (maximum-path) since the prefixes received from AS 400 are learnt via two different ASes (200 and 300) and that the multipath feature only works when paths are received via the same neighboring AS. You could change your scenario to replace AS 300 with AS 200 and verify thet the multipath works.

The other way to do it is to set the local preference to 110 for 192.168.167.0/24 coming from RTRB and the same for 192.168.169.0/24 coming from RTRC. This will provide you with some sort of loadsharing.

router bgp 100

neighbor 172.0.0.22 route-map fromB in

neighbor 172.0.0.2 route-map fromC in

route-map fromB permit 10

match ip address 1

set local-preference 110

route-map fromB permit 20

route-map fromC permit 10

match ip address 2

set local-preference 110

route-map fromC permit 20

access-list 1 permit 192.168.167.0

access-list 2 permit 192.168.169.0

Hope this helps,

[acbennyma]

Thanks ! hritter

In other words, is it means there is only one situation that BGP load balance can be performed is

that routes are received from the same AS via different links?

Also in your suggest configuration, it I make Router bgp 100 to perform load sharing. Can I use weight instead of local-preference ? As I know, local-preference will be used if the As also have iBGP.

Please advise.

Thanks you very much !

[Harold Ritter]

The answer to your first question is yes.

For the second question, you can use local preference or weight. Local preference will be evaluate as part of the BGP best path selection even if you only have one router.

Hope this helps,