Actually what does this mean on Nexus9K BGP Status?

[mhiyoshi]

Dear all,

When I confirm BGP configuration on Nexus9K, then if I check with "show ip bgp summary" on the condition below, it looks to establish however what is the (No Cap) mean? I appreciate if there is any configuration guide about this.

Best Regards,

Masanobu Hiyoshi

[Sergiu.Daniluk]

Hello,

I believe "No Cap" means "no capabilities". Basically when you have different AF configured on your 2 BGP routers: for example one with AF ipv4 unicast, while the other with AF ipv6 unicast.

Regards,

Sergiu

[Sergiu.Daniluk]

So I tested it in the lab, and indeed. If the BGP session is up and running, then one of the peers remains without a common AF, the (No Cap) appears:

n9k1(config-router-neighbor)# sh run bgp 

router bgp 100
  neighbor 192.168.1.2
    remote-as 100
    address-family ipv4 unicast
    address-family ipv6 unicast

n9k2(config-router-neighbor-af)# sh run bgp 

router bgp 100
  neighbor 192.168.1.1
    remote-as 100
    address-family ipv4 unicast



n9k1(config-router-neighbor-af)# sh ip bgp summary | last 3

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.1.2     4   100      50      55        4    0    0 00:00:10 0  

n9k2(config-router-neighbor-af)# show ip bgp summary | last 3

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.1.1     4   100      57      59        4    0    0 00:02:13 0    



n9k1(config-router-neighbor)# no address-family ipv4 unicast
n9k1(config-router-neighbor)# show ip bgp summary 
BGP summary information for VRF default, address family IPv4 Unicast

n9k2(config-router-neighbor-af)# show ip bgp summary | last 3

Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
192.168.1.1     4   100      26      22        0    0    0 00:04:44 0 (No Cap)

Regards,

Sergiu

[mhiyoshi]

Dear msdaniluk,

Thank you for your precious information and verification.

I see that "NO CAP" means "No Capability" it makes sense to me.

However please let me check one thing, so now the BGP peer can be established but

one of the Router's BGP status shows "NO CAP", no impact without such cosmetic output right? 

Best Regards,

Masanobu Hiyoshi

[Sergiu.Daniluk]

Not quite. As you can see in my outputs snip, after removing the ipv4 AF, the neighborhood goes down on the local Nexus.

So the "No CAP" on the remote Nexus, indicates that there is a problem on it's peer.

Regards,

Sergiu

[paul driver]

Hello @Sergiu.Daniluk 

Cheers for the validation, Without you testing that I would have definitely said the peering would have been torn down.
Did you test this on simulation software if is so what was it, virl, gns,eve?

[Sergiu.Daniluk]

Hello,

I tested it in VIRL, but I expect the same behavior on real hardware.

Cheers,

Sergiu

[paul driver]

Hello
Interesting just tested this in gns3 and it torn down the peering (Cisco NX-OSv 9000) however tbh I would trust virl over gns3

[Sergiu.Daniluk]

Yep, I trust VIRL over GNS3 as well.

[paul driver]

Hello
That dosnt seem right- what youve shown there is your disabling ipv4 address family for that neighour so i dont see how that peering is staying up?
You shouldnt need to but did you clear the bgp session?

clear ip bgp * soft
show ip bgp nei 1.1.1.x | in cap

[mhiyoshi]

Hi Paul,

Thank you for your info. I've checked this like below however it still shows "No Cap" but the BGP state is Established.

Based on this situation I think The "No Cap" means simply no capability which the counter part BGP nexus switch does not configure "address-family ipv4 unicast".

'Nexus9K-2# clear ip bgp * soft

Nexus9K-2# show ip bgp neighbors 1.1.1.1 | in cap
  Neighbor capabilities:
  Dynamic capability: advertised (mp, refresh, gr) received (mp, refresh, gr)
  Dynamic capability (old): advertised received
  Route refresh capability (new): advertised received
  Route refresh capability (old): advertised received
  4-Byte AS capability: advertised received
  Graceful Restart capability: advertised received

Nexus9K-2# show ip bgp neighbors 1.1.1.1 | in "BGP state"
  BGP state = Established, up for 00:10:26'

[f00z]

Yes that is exactly what it means. For example if you configure an ipv4 bgp session and enable ipv4 and ipv6 address families over it on one side, but only ipv4 on the other side it will show no capability under the show bgp all summary for the ipv6 section.

Older devices like IOS sometimes won't even bring up the BGP session if there's a capability mismatch.

Check the show ip bgp neighbor to verify, you will see something like :

Address families advertised to peer:
IPv4 Unicast
Address families received from peer:
IPv4 Unicast

Extended Next Hop Encoding Capability: advertised received
Receive IPv6 next hop encoding Capability for AF:
IPv4 Unicast

[mhiyoshi]

Hi f00z

Thank you for your info. I think the real device also shows like this, so it might be tips to know this situation.

Nexus9K-2## sh ip bgp neighbors | be Address
  Address family IPv4 Unicast: advertised received

*When the counter part BGP nexus configres as "no address-family ipv4 unicast"
After that the output does not show received status.

Nexus9K-2## sh ip bgp neighbors | be Address
  Address family IPv4 Unicast: advertised

Regards,

Masanobu Hiyoshi