about bgp and ORF

[ariela]

Hi folks,

what is Outbound Route Filtering?

I see that:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00800b5d61.html

but I don't understand when and how to use it ...

Any advice will be appreciated

Regards

Andrea

[mheusinger]

Hi,

assume you have rather restrictive filters (prefix-list) in place and accept only 1000 routes from the full internet BGP table. This means your neighbor is sending 200000 prefixes and you discard 199000 of them.

In this situation ORF comes in to optimize the BGP update procedure. Basically your BGP router would send its incoming prefix-list to the neighboring BGP peer. This peer would then apply your inbound prefix-list to its updates before sending the updates and only deliver the 1000 routes you are willing to accept.

So the gain is 199000 prefixes less to send, i.e. faster convergence, and your router has less CPU load with update processing.

Hope his helps! Please rate all posts.

Regards, Martin

[ariela]

Hi Martin,

thanks for your answer.

that's quite similar to 'transmit side loop detection' mechanism, isn't it?

Both only for eBGP peering, correct?

examples:

1. transmit side loop detection

router bgp 100

nei 10.0.0.1 remote-as 1

nei 10.0.0.1 filter-list 1 out

!

ip as-path 1 deny _1_

ip as-path 1 permit any

2. ORF

router bgp 100

nei 10.0.0.1 remote-as 1

nei 10.0.0.1 capability orf prefix-list send

nei 10.0.0.1 distribute-list 10 in

!

access-list 10 permit ....

or

router bgp 100

nei 10.0.0.1 remote-as 1

nei 10.0.0.1 capability orf prefix-list send

nei 10.0.0.1 prefix-list routes_in in

!

ip prefix-list routes_in ...

Please let me know

Thanks for your support

Regards

Andrea

[mheusinger]

Hi Andrea,

I see a difference inbetween the two things.

Loop detection is for loop detection, whereas prefix-lists can achieve various results. Mainly in conjunction with ORF they block unwanted prefixes. As an example: one would probably not want to accept RFC1918 IP networks or BOGONs or the like. They would not create any loop, but could be harmful for your internal IP routing (AD of eBGP is 20, OSPF f.e. 110).

So ORF does not really address BGP loops, but does allow for efficient update processing.

Hope this helps! Please rate all posts.

Regards, Martin

[ariela]

ok, that's clear.

but my syntax is correct?

thanks

Andrea