BGP main loop prevention mechanism

[zillah2004]

I have read this :

((Since BGP does not use a discernible metric value as IGPs do, the main loop prevention mechanism built into iBGP is that fact that routes learned from an iBGP neighbor cannot be advertised onto another iBGP neighbor. Therefore,

this stipulation implies that all iBGP speaking devices must establish direct peering relationships with all other iBGP devices within your autonomous system)).

I am aware that we have to have full mesh peering relations within IBGP, that means, any IBGP speaker will not pass route update to other IBGP speaker unless it has peer relationship with it . In this way, we are preventing loops from happening .

But I could not get what he mean by his comparison when he says :

"Since BGP does not use a discernible metric value as IGPs do" ?

[simonstoll]

Hi

Actualy the only usable metric do prevent loops in bgp is the as-path attribute. No EBGP peering neighbors do accepct an update which includes it's own AS. So this is used in EBGP to prevent loops. But in IBGP there is no change is this attribute, so there comes the rule you mention with the IBGP full mesh. Of course you can overcome this limitation by either using Route-Reflectors (which add two additional attributes to prevent loops, CLUSTER_LIST and ORIGINATOR_ID) or by using Confederations which splits your AS into multiple SUB-AS's which use EBGP to connect each other.

What is meant by "discernible metric"? As I'm german speaking, i had to ask GOOGLE first :-) Well I think they mean that BGP has no metric like a linkstate or distance vector protocol like hopecount or bandwidth which changes whith each new router hop, but has only the AS-Path Attribute (plus many other attributes, but those are not usable to prevent loops) which stays the same aslong as the updates do not pass over an AS border.

Hope that helps

Simon

[zillah2004]

Thanks Simon for this explaination.

Could you kindly explain what did you mean by below :

"But in IBGP there is no change is this attribute".

Thanks.

[simonstoll]

The AS-Path is not changed as-long as the update does not cross a AS Border. So as IBGP is only within an AS, there is no change of this attribute.