bgpd: update AS value of a hidden bgp instance

[askorichenko]

'import vrf CUSTOM' could define a hidden bgp instance with the default AS_UNSPECIFIED (i.e. = 1) value.
When a
router bgp AS vrf CUSTOM
gets configured later on, replace this AS_UNSPECIFIED setting with a requested value.

[askorichenko]

Let config have

• a custom "vrf D" defined and
• a bgp router in that vrf that imports default vrf

frr# show run
Current configuration:
...
!
vrf D
exit-vrf
!
router bgp 65001 vrf D
 !
 address-family ipv4 unicast
  import vrf default
 exit-address-family
exit
!

This operation defines a 'hidden bgp' in the default vrf

• it has bgp->as = AS_UNSPECIFIED (i.e = 1)
• a flag (bgp->flags & BGP_FLAG_INSTANCE_HIDDEN) == BGP_FLAG_INSTANCE_HIDDEN

To define a real bgp router in the default vrf now, this hidden structure would be selected for update.
But bgp_create() function would miss to update "bgp->as, bgp->as_pretty" values.
The router would remain in AS = 1.

frr# conf
frr(config)# router bgp 65001 
frr(config-router)# exit
frr(config)# exit
frr#
frr#
frr# show run 
Current configuration:
...
!
vrf D
exit-vrf
!
router bgp 65001 vrf D
 !
 address-family ipv4 unicast
  import vrf default
 exit-address-family
exit
!
router bgp 1
exit

This patch of bgp_create() adds an overwrite of as, as_pretty values for a hidden bgp instance.

[ton31337]

Here we don't have bgp->peer allocated, and bgp->peer->cmp = (int (*)(void *, void *))peer_cmp; will segfault after goto peer_init;.

Also, we should respect AS notation...

[donaldsharp]

I'd like to see a topotest for this situation... Unless @ton31337 doesn't think we need it.

[ton31337]

Agree, a topotest would be great 👍