Troubleshooting BGP Neighbor Adjacency

Lessons Discussion
1

This topic is to discuss the following lesson:

2

Rene,

Great lesson.

Thanks

Hamood

3

Thank you Rene!

How easy is to find if you have a duplicate AS number. I am referring to the AS private ranges?

Best,
MS/

4

Hi Marty,

You can debug it on the router, it will tell you that an update is refused since it sees its own AS number in the path. Here’s an example:

https://networklessons.com/bgp/mpls-layer-3-vpn-bgp-allow-as-in/

Rene

5

Hi Rene,

Did you mean iBGP instead of eBGP?

“Problem solved! The only difference with EBGP is that we don’t have to change the TTL with the ebgp-multihop command.”

Rgds,

Shannon

6

Hi Rene,

How we can increase the BGP convergence time. can you please explain in brief.

Thanks,
Ajay

7

@Shannon
Problem solved! The only difference with EBGP is that we don’t have to change the TTL with the ebgp-multihop command

This is correct as written, because in the context of the paragraph, Rene is talking about iBGP. So written another way, it would read, “iBGP’s difference with EBGP is that we don’t have to change the TTL with the ebgp-multihop command” which is a true statement.

8

@Shannon
Good catch on the repeating lines. I will ask Rene to fix.

9

Hi Andrew,

Many thanks for the response!

I’ll have to re-read the notes on the lesson again to better understand your response. Bit murky still. Not to worry, this is on me to figure out first.

Rgds,

Shannon

10

@Ajay

There are quite some different things you can do to increase the BGP convergence time. Is there anything in particular you want to know or just a general overview?

Rene

11

@Shannon thanks for reporting the typo, it has been fixed.

12

Hi Rene,

Noted with thanks!

Rgds,

Shannon

13

Hi Rene,

I need general overview of increasing the BGP convergence time.

Regards,
Ajay

14

Hello Ajay!

At the moment we don’t have a section covering BGP convergence timers and tuning, but INE has a great blog post about it that you can find here:

http://blog.ine.com/2010/11/22/understanding-bgp-convergence/

I hope this is helpful!

Laz

15

Hello Rene,

Could you please tell me why a router cannot have two BGP autonomous systems configured ?

Thanks,
Harsha.

16

Hello Harsha

By design, BGP is made so that each router belongs to a single AS. But this doesn’t limit its functionality. You can still peer with routers from multiple AS as well as to routers in the same AS. There is however a way to allow a router to appear to neighbors to be a member of a second AS using what is called the Local-AS feature. This is usually done for migrations when moving from one ISP to another, or from one MPLS provider to another, and is usually provided as a temporary solution during the migration process.

BGP is designed to function with only a single AS assigned to each router.

I hope this has been helpful!

Laz

17

Hello Laz,

           Thank you so much for the reply. I appreciate the help.

Thanks,
Harsha.

1 Like
18

whenever the neighbor in active state , that means open message is synchronizing between both R1 & R2 routers.
so in that circumstances R1 & R2 both should ping each other but the telnet will not works , that means the L2 & L3 is working fine but issue with L4 i e. issue with TCP port 179

whenever the neighbor in idle state , that means there is surely an L2 or L3 issue.

so my question is that , in what circumstances BGP neighborship will stuck in connect state.

Although we do not generally seeing that problem i.e. neighborship stuck in connect state.

Thanks in advance…

19

Hi Laz ,

Thanks for your great post , could you please share the link / blog for Local-AS feature , how it works.
Also please check my following comments & help to share your acknowledgement / feedback.
whenever the neighbor in active state , that means open message is synchronizing between both R1 & R2 routers.
so in that circumstances R1 & R2 both should ping each other but the telnet will not works , that means the L2 & L3 is working fine but issue with L4 i e. issue with TCP port 179

whenever the neighbor in idle state , that means there is surely an L2 or L3 issue.

so my question is that , in what circumstances BGP neighborship will stuck in connect state.

Although we do not generally seeing that problem i.e. neighborship stuck in connect state.

20

Hello Tanmoy

The Active state can be reached via two situations. The first is when a BGP peering goes from Connect to Active. This will take place if the Connect three-way handshake fails. The other case is going from OpenSent to Active. This will take place again, if the TCP session fails. So theoretically, yes, two routers in Active state will be able to ping each other, but are in a situation where they have not yet successfully completed the TCP three way handshake.

If two routers are configured correctly to be BGP peers, and they remain in the idle state, then yes, it is most likely an L2 or L3 problem. However, it could also be a misconfiguration on one of the routers such as an incorrect neighbor IP address.

Like you said, we don’t generally see a “stuck in connected” state. This is clearly described in the BGP Neighbor Adjacency States lesson:

Connect: BGP is waiting for the TCP three-way handshake to complete. When it is successful, it will continue to the OpenSent state. In case it fails, we continue to the Active state. If the ConnectRetry timer expires then we will remain in this state. The ConnectRetry timer will be reset and BGP will try a new TCP three-way handshake. If anything else happens (for example resetting BGP) then we move back to the Idle state.

I hope this has been helpful! Stay healthy and safe!

Laz