OSPF SHAM LINK bouncing normal ?

[sarahr202]

Hi everybody

Braving sham link first time.

Please consider the following :

CE1---R1(PE)------R2(p)------R3(PE)--------CE3

R1 has a sham link to R3

R1:

R1#show running-config | section ospf
router ospf 4 vrf ZEE
 log-adjacency-changes
 area 1 sham-link 10.10.10.1 30.30.30.3 cost 2
 network 0.0.0.0 255.255.255.255 area 1

interface Loopback10
 ip vrf forwarding ZEE
 ip address 10.10.10.1 255.255.255.255

router bgp 1
 bgp log-neighbor-changes
 neighbor 3.3.3.3 remote-as 1
 neighbor 3.3.3.3 update-source Loopback0
 !
 address-family ipv4
  neighbor 3.3.3.3 activate
  no auto-summary
  no synchronization
 exit-address-family
 !
 address-family vpnv4
  neighbor 3.3.3.3 activate
  neighbor 3.3.3.3 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf ZEE
  redistribute ospf 4 vrf ZEE
  no synchronization
  network 10.10.10.1 mask 255.255.255.255
 exit-address-family

R3;

R3#show running-config | section ospf
router ospf 2 vrf ZEE
 log-adjacency-changes
 area 1 sham-link 30.30.30.3 10.10.10.1 cost 2

interface Loopback30
 ip vrf forwarding ZEE
 ip address 30.30.30.3 255.255.255.255
end

router bgp 1
 bgp log-neighbor-changes
 neighbor 1.1.1.1 remote-as 1
 neighbor 1.1.1.1 update-source Loopback0
 !
 address-family ipv4
  neighbor 1.1.1.1 activate
  no auto-summary
  no synchronization
 exit-address-family
 !
 address-family vpnv4
  neighbor 1.1.1.1 activate
  neighbor 1.1.1.1 send-community extended
 exit-address-family
 !
 address-family ipv4 vrf ZEE
  no synchronization
  network 30.30.30.3 mask 255.255.255.255
 exit-address-family

++++++++++++++++++++++++++++++++++++++++++++++++

I see the sham link bouncing :

*Mar  1 10:11:49.453: %OSPF-5-ADJCHG: Process 2, Nbr 16.16.16.1 on OSPF_SL0 from LOADING to FULL, Loading Done
R3#
*Mar  1 10:12:04.489: %OSPF-5-ADJCHG: Process 2, Nbr 16.16.16.1 on OSPF_SL0 from FULL to DOWN, Neighbor Down: Interface down or detached
R3#
*Mar  1 10:12:29.369: %OSPF-5-ADJCHG: Process 2, Nbr 16.16.16.1 on OSPF_SL0 from LOADING to FULL, Loading Done
R3#

On R1:

*Mar  1 10:26:35.533: %OSPF-5-ADJCHG: Process 4, Nbr 35.35.35.3 on OSPF_SL0 from LOADING to FULL, Loading Done
R1#
*Mar  1 10:27:01.245: %OSPF-5-ADJCHG: Process 4, Nbr 35.35.35.3 on OSPF_SL0 from LOADING to FULL, Loading Done
R1#
*Mar  1 10:27:35.557: %OSPF-5-ADJCHG: Process 4, Nbr 35.35.35.3 on OSPF_SL0 from LOADING to FULL, Loading Done
R1#

Is it normal ? Does sham link bounce like that?

Thanks

[Renan Abreu · Accepted solution]

Can you check if you're learning the loopbacks used to the sham-link via OSPF? this is not allowed.

[sarahr202]

Thanks Renan

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

the trick with sham links is that the sham link endpoints must be published only with MP iBGP and not in OSPF in VRF.

In your case the network area command is too general and includes the loopback used for the sham link. So you need to modify the configuration of Ospf avoiding that on the PE nodes the sham addresses are advertised by OSPF in VRF.

This is the reason for what you see.

Hope to help

Giuseppe

[sarahr202]

Thanks Giuseppe