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New Member

BGP Keeps on Dropping...

Error:  

Jan  23 02:00:21.003: %BGP-3-BGP_NO_REMOTE_READ: x.x.x.x connection timed out - has not accepted a message from us for 20000ms (hold time), 0 messages pending transmition.

Jan  23 02:00:21.003: %BGP-3-NOTIFICATION: sent to neighbor x.x.x.x passive 4/0 (hold time expired) 0 bytes

Jan  23 02:00:21.003: %BGP_SESSION-5-ADJCHANGE: neighbor x.x.x.x IPv4 Unicast topology base removed from session  BGP Notification sent

i already change the hold down timer and keepalive timer to lower value but it won't change. bgp still dropping frequently

added policy map config but still having the same result.

policy-map parent

class class-default

    shape average 3300000 103000 0

can anyone help me figuring out waht is the exat issue?

Thank you

1 ACCEPTED SOLUTION

Accepted Solutions
Silver

Re: BGP Keeps on Dropping...

Firuz,

as the ping with 1500 bytes works, and the mss is 536 bytes, this looks like that one of the peers has ip tcp mss 536 configured. I would removed that,  and then clear the BGP session. Then, the BGP session should have a mss of 1460 bytes.

Best Regards,

Jose.

19 REPLIES
Cisco Employee

BGP Keeps on Dropping...

Hi Arnold,

It seems as if the BGP communication with the flapping peer was somehow unreliable, lossy or blocked by some kind of firewall with short timeout periods. The message basically tells you that you have sent your BGP peer a message but it did not respond in proper time (in this case, 20 seconds). That can be caused by the fact that it either

  • did not receive your message, or
  • did not have the CPU power to process your message and send you a response, or
  • sent you a response but the response did not make it back to you

Each of these possibilities has to be evaluated individually. However, without knowing more about the network between you and the flapping BGP peer, it is hard to give any guidance.

The policy-map you have created - what is it good for? Where did you apply it? If it is not required by your other QoS settings, I strongly suggest removing any QoS related additions you have added during the examination of this issue.

Best regards,

Peter

New Member

BGP Keeps on Dropping...

Thank you Peter,

Yeah, i already removed the QoS settings but still i have the same issue. I also rebooted the router but still having the same result.

Is there any factor that i need to check?

Cisco Employee

BGP Keeps on Dropping...

Arnold,

Please try to focus on the three areas I have pointed out. They simply have to be carefully inspected. Hard to tell anything more at this point - we don't know anything about your network at all.

What messages does the opposite BGP router print out? Perhaps they contain some interesting debugging info.

Best regards,

Peter

BGP Keeps on Dropping...

I agree with Peter, we will need to know more about your network. Is this router the only router with a BGP connect? Is it connected directly to your ISP? Also, are you running an IGP throughout your network or relying on static routing?

New Member

BGP Keeps on Dropping...

Hi John,

This is an IBGP peering and it is directly connected, the peer/opposite router has EBGP peering but it works fine.

no IGP are running in the network.

They have the same print out for the opposite BGP router.

Thank you

BGP Keeps on Dropping...

So what I take from this, is that, you have two routers directly connected A and B via an iBGP peer, and one of the routers A and B has an eBGP connection to C, is this correct? And the problem is with the iBGP peer?

New Member

BGP Keeps on Dropping...

Hi John,

Yes, that is correct! My problem is my  iBGP peer...

Thank you.

BGP Keeps on Dropping...

Peter gave some pretty good ideas. It sounds like there are problems with keepalives getting through. Can you check the log history of those two routers? It might tell you what's going on. That's a good place to start.

New Member

BGP Keeps on Dropping...

Hi John,

It's working now! First, it didn't work by changing the hold down and keepalive timer, rebooted the router and the same by adding policy map.

What i've done to make it work is that I physically transferred the connection between router A and B.

Is there any relationship between the BGP peering (BGP Flapping) and the port itself?

How much are the data traffic need to affect or to drop the BGP hello packet?

If traffic is huge, is it all right to lower down the value of hold down and keepalive time?

Or what is the effect if the value of hold down and keepalive time is lower than the default?

Thank you so much!

BGP Keeps on Dropping...

Well the whole purpose of keepalive are for reachability concerns. Think of them as a heartbeat. If Router A stops hearing heartbeats from Router B, it considered Router B to be dead. Just make sure, when you tune your timers, not to make them so short, that if there is some lag or a little delay, that the BGP peering doesn't go down.

New Member

BGP Keeps on Dropping...

Thank you John/Peter... Appreciate your help.

Arnold

Cisco Employee

BGP Keeps on Dropping...

hi,

check for mtu related issue on the link between the 2 router.

ping the neighbor at 1500 (it's ethernet right?) with df-bit set.

if your ping does not go through you might have identified the issue.

what happens in this cases is that as long as only little updates or keepalives are sent between peers no issue is exposed as the message being sent are smaller than max mtu.

when big bgp updates are sent the router try to push multiple updates in the same packet until the negotiated tcp mss window is full. a bgp update is also counted as a keepalive, which is no small keepalive is being sent while big updates are going through.

If due to mtu issues the big updates are dropped along the way the peering will go down for hold time expiration as one of the 2 routers (or both) are not receving keepalives (or updates which are considered keepalives).

this is what happens when icmp packets are filtered along the way.

you can also quickly checked the negotiated mss in the bgp peering of both routers and check if the value is coherent with actual link mtu.

example:

iBGP is running between R1 and R4

R1#sh ip bgp ne 10.100.1.4 | inc max data segment

Datagrams (max data segment is 1460 bytes):

R4#sh ip bgp ne 10.100.1.1 | inc max data segment   

Datagrams (max data segment is 1460 bytes):

New Member

BGP Keeps on Dropping...

Hi Riccardo.

From your explanation, I have the same problem

I have eBGP but when I advertise all my routing table to remote peer. My neighborship is broken

logs :

Jun 11 22:48:04: %BGP-3-BGP_NO_REMOTE_READ: 192.168.4.58 connection timed out - has not accepted a message from us for 180000ms (hold time), 1 messages pending transmition.

Jun 11 22:48:04: %BGP-5-ADJCHANGE: neighbor 192.168.4.58 Down BGP Notification sent

But when I filter some routes its working fine

I think you are right this this MTU issue

Router#show ip bgp neighbors 192.168.4.58 | i  max data segment

Datagrams (max data segment is 536 bytes):

The same output from remote peer

But Ping goes successfull

Router#ping 192.168.4.58 source 192.168.4.57 df-bit size 1500    

Type escape sequence to abort.

Sending 5, 1500-byte ICMP Echos to 192.168.4.58, timeout is 2 seconds:

Packet sent with a source address of 192.168.4.57

Packet sent with the DF bit set

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms

How I can resolve this problem?

Thanks for help

Regards

Silver

Re: BGP Keeps on Dropping...

Firuz,

as the ping with 1500 bytes works, and the mss is 536 bytes, this looks like that one of the peers has ip tcp mss 536 configured. I would removed that,  and then clear the BGP session. Then, the BGP session should have a mss of 1460 bytes.

Best Regards,

Jose.

New Member

BGP Keeps on Dropping...

Hello Jose

Where it configured ? I never see in configuration mss 536. Could you tell where exactly see ?

Silver

BGP Keeps on Dropping...

One of the peers should have:

R1(config)#ip tcp mss 536

Best Regards,

Jose.

Cisco Employee

BGP Keeps on Dropping...

Hi Firuz,

I guess Jose put you on the right track. TCP traffic is somehow limited to 536 bytes by some devices in between.

If you don't see anything on the intermediate routers remember to also check other devices such as firewalls/load balancers etc.

Riccardo

New Member

Re: BGP Keeps on Dropping...

One of the peers should have:

R1(config)#ip tcp mss 536

Best Regards,

Jose.

No I dont have this configuration on both peers. But in middle there are many L2 switches (no routers between peers).

Should I configure on peers ip tcp mss 1460 ?

Or I need to find why

Datagrams (max data segment is 536 bytes) ?

New Member

BGP Keeps on Dropping...

I think I find a problem.

One peer is path-mtu capable, but other end no. I wanna enable globally ip tcp path-mtu-discovery
I hope this helps.

Do u know any restriction for this command?

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