IBGP Routes

[janm0a1]

Hello,

I have some

IBgp

routes on my route table with no ASN number. 

Will these routes be permanent?

Will they ever timeout?

My thinking is that these routes will not remain after a period of time, is that correct?

[David Ruess]

No. If they are in your BGP table and unless you have a policy configured to remove them then they will remain there. When

iBGP

neighbors exchange updates they don't utilize the

AS path

because the loop prevention mechanism would not allow it. YOu have 2 BGP neighbor types:

eBGP

- Their loop prevention mechanism is

AS path

(dont accept routes with its own AS in the PATH). eBGP neighbors advertise their

AS path.

iBGP ->iBGP 

- Their loop prevention mechanism is dont advertise

 iBGP

learned routes to

iBGP

neighbors. You can modify this behavior with rout-reflectors. If they also relied on

AS path

then that would be another obstacle. For that reason you wont see

iBGP

learned routes with an AS the same as their own because its internal and no need.

-David

[Giuseppe Larosa]

Hello @janm0a1 ,

>> I have some

IBgp

routes on my route table with no ASN number.

This is normal , they are routes that are originated within your AS.

Before advertising to a true eBGP peer the local AS is appended to the routes and they can be advertised, in

iBGP

the local AS is not added to the

AS path

as it is the same for all

iBGP

peers and it would not provide any protection from routing loops.

This is also the reason why the

iBGP

  requires a full mesh of

iBGP

sessions unless BGP route reflectors or BGP confederations ( or both in a great ISP ) are used.

The BGP RR servers are allowed to reflect = propagate routes between their clients (

iBGP

peers defined as clients on RRS BGP config)

the safe reflection is performed by adding two BGP attributes to the

prefix:

BGP Originator ID =  BGP Router ID of the iBGP peer that originated the prefix

cluster list =  list of reflections it contains either the RRS BGP Router-id of the cluster -id if configured.

A shorter cluster list is preferred when choicing the best path.

Both are used to avoid issues , if a route cluster list contains the local

cluster-id

it is not propagated to the clients as it has been originated by one of them.

if a router would receive a route with

Originator ID

equal to its BGP router id it would not install it.

BGP confederations work by using mini ASes taken from the private AS space 64512 - 65535, but the configuration needs to be changed on all routers. BGP RR are more used for this reason as their introduction requires changes only on RRS.

RRS need to be fully meshed between them as.

>> My thinking is that these routes will not remain after a period of time, is that correct?

No, this is not correct these routes are normal

prefixes originated within your AS this is why the AS path

attribute is still empty as I have written above before advertising to an external eBGP peers the local AS is appended on the left in the

AS path

attribute so from the point of view of a direct eBGP peer those routes have

AS path =  your ASN

Hope to help

Giuseppe

[janm0a1]

Good Information thanks!

[M02@rt37 · Accepted solution]

Hello @janm0a1,

iBGP (internal BGP), routes

exchanged between routers within the same autonomous system (AS) don't typically carry AS numbers in their route entries. This is because

iBGP routers 

share routes within the same AS, and the AS number remains constant within the AS.

iBGP routes

, once learned, are not subject to a default timeout. Unlike some other routing protocols,

iBGP routes

don't have a built-in mechanism to expire after a certain period of time. These routes will remain in the routing table until there's a change in the network topology or configuration that causes the route to be withdrawn.

iBGP routes

will persist as long as the routes themselves are being advertised by the advertising router and the receiving router continues to consider them valid. If there's a change in the network, such as a link failure or a configuration change that affects the BGP advertisements, the routes may be withdrawn and updated in the routing table accordingly.

[janm0a1 · Accepted solution]

Great thanks for the info!