Clarification on the path column in the get router info bgp network command

[Joe2]

Hi,

In the output of the "get router info bgp network command", specifically in the path column, there is at the end a <-/-> or <-/number> or <number/-> or <number/number>

Could you please clarify what does this represent?

Here is an example:

FG# get router info bgp network
VRF 0 BGP table version is 1, local router ID is x.x.x.x
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight RouteTag Path
*>i10.2.3.0/24 172.21.4.18 0 100 0 111 i <2/2>
*>i 172.21.1.18 0 100 0 111 i <1/1>
* i 172.21.4.18 0 100 0 111 i <2/->
* i 172.22.1.18 0 100 0 222 i <1/->

Thanks,

[Joe2]

It seems that:

<x/y>

'x' means the received path ID (set by peer).

'y' means the sending path ID (set by this FortiGate).

But can someone clarify their purpose?

Thanks,

[Demir21]

Hi,

<x/y> x means the received path id(set by peer); y means the sending path id(set by this router)

The assignment of the Path Identifier for a path by a BGP speaker is purely a local matter.

The Path Identifier must be assigned in such a way that the BGP speaker is able to use the (Prefix, Path Identifier) to uniquely identify a path advertised to a neighbor. This value is generated from the Fortigate when the route is installed in order to always recognize the route, in case of multipath.

[Joe2]

Hello,

Thank you for your sharing that info.

Since this is automatically generated by the FortiGate, I assume that we can't change those values or optimize it, is that correct?

Also, is there any rules related to how this functions? Meaning how those IDs are generated?

Sometimes there are no value <1/-> or <-/2>  which mean no ID, why is that the case?

Thanks,

[Demir21]

Hi, 

I would suggest to check on this community link: https://community.fortinet.com/t5/FortiGate/Technical-Tip-Usage-of-BGP-multipath-and-description-of-... which better explains your questions. Let me know if you have any further questions.

[Joe2]

Hello,

Thank you for sharing that link - I already did check that link prior to creating this post, but it doesn't mention how those path IDs are created? and if there is any meaning for that assignment? or if those number have any impact related to routing or path preference.

Thanks,

Joe

[pavankr5]

Dear Customer,

In your example output:

• <2/2> indicates that the network 10.2.3.0/24 is received from a neighboring BGP router with AS number 2 and is being advertised to another neighboring BGP router with AS number 2.

• <1/2> indicates that the network 172.21.1.18 is received from a neighboring BGP router with AS number 1 and is being advertised to another neighboring BGP router with AS number 1.

• <2/-/> indicates that the network 172.21.4.18 is received from a neighboring BGP router with AS number 2, but it is not being advertised further to any other neighboring BGP router.

• <1/-> indicates that the network 172.22.1.18 is received from a neighboring BGP router with AS number 1, but it is not being advertised further to any other neighboring BGP router.

These path attributes provide information about the BGP routing path for the specific network entry.

Please let us know if you have any queries.

Thanks, 

Pavan

[Joe2]

Hello,

Thanks Pavan for your reply.

It doesn't seem that those value represent BGP AS number, as we are not using AS number 1 or 2.

Thanks,
Joe

[Fern-X]

They're Path Identifiers, as per 'Advertisement of Multiple Paths in BGP' (RFC7911); and, 'Assignment of the Path Identifier for a path by a BGP speaker is purely a local matter'.