BGP route validity

Answered Question
May 27th, 2008

I was wondering about BGP's decision making process when it comes to whether a route is considered valid or not.

I know that a route can be considered valid sometimes when it is not even accessible in the case of NBMA networks or external routes learned from iBGP neighbors. I know that a route is considererd invalid if the receiving router AS is in the AS path. I also know that route filters can make a route invalid, but does anyone have a comprehensive list of what makes a route valid.

What is the step by step decision making process a router goes through when attempting to answer the question of route validity?

Thanks in advance.

Correct Answer by mohammedmahmoud about 8 years 8 months ago

Hi,

Kindly check this Cisco document for a similar list:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#background

BGP relies on the IGP for the actual traffic forwarding, it reports prefixes and their next-hop, route recursion and the actual forwarding depends on the IGP, and thus its a matter of IGP design and convergence, as long as the next-hop is reachable (according to the IGP), then the route is a candidate of BGP best path selection.

BR,

Mohammed Mahmoud.

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asanchez_77 Wed, 05/28/2008 - 01:14

as far as I know, BGP first check its prefixes received(BGP table) for a proper path to a network and after the BGP algorithim procedure, the resulting route its therefore installed in the routing table. A BGP router commonly receive FRT so teorically its resposability is to properly choose one or another path.

any suggestions?

alex

izackvail Wed, 05/28/2008 - 05:49

I found this list:

Is the route VALID? To be valid:

* The route must be synchronized with the Interior Gateway Protocol (unless synchronization is turned off).

* The route must appear in the IP routing table (see previous bullet point).

* The NEXT_HOP must be reachable.

* The AS_PATHs received from an external AS must not contain the local AS, or they will be discarded.

* The local routing policy must permit the route. If the neighbor is filtering the route, they won't use it.

I should note that this didn't come from a Cisco document.

I still wonder why BGP would consider a route valid even if it is inaccessible?

I guess this is just an imperfection in the algorithm that determines NEXT_HOP reachability. According to this list "the NEXT_HOP must be reachable".

Correct Answer
mohammedmahmoud Wed, 05/28/2008 - 08:16

Hi,

Kindly check this Cisco document for a similar list:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml#background

BGP relies on the IGP for the actual traffic forwarding, it reports prefixes and their next-hop, route recursion and the actual forwarding depends on the IGP, and thus its a matter of IGP design and convergence, as long as the next-hop is reachable (according to the IGP), then the route is a candidate of BGP best path selection.

BR,

Mohammed Mahmoud.

izackvail Wed, 05/28/2008 - 08:30

Thank you Mohammed that is exactly what I was looking for.

I am familiar with the Best Path Selection process. I have seen it referenced in many books but I haven't seen that route validity section. Thanks again!

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