BGP neighbour dropping?

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Dec 20th, 2007
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We've experiencing bgp dropping that also causes our phones on remote site to drop but it will be up again in sec. Can someone help me what could be causing it.

below is the sho bgp results:


3750Switch#sho ip bgp neighbors x.x.x.x

BGP neighbor is x.x.x.x, remote AS 7474, external link

BGP version 4, remote router ID y.y.y.y

BGP state = Established, up for 21:02:37

Last read 00:00:00, hold time is 10, keepalive interval is 3 seconds

Configured hold time is 10, keepalive interval is 3 seconds

Minimum holdtime from neighbor is 0 seconds

Neighbor capabilities:

Route refresh: advertised and received(new)

Address family IPv4 Unicast: advertised and received

Message statistics:

InQ depth is 0

OutQ depth is 0

Sent Rcvd

Opens: 4 3

Notifications: 1 2

Updates: 39 33

Keepalives: 2721668 2449710

Route Refresh: 0 0

Total: 2721712 2449748

Default minimum time between advertisement runs is 30 seconds


For address family: IPv4 Unicast

BGP table version 178, neighbor version 178/0

Output queue sizes : 0 self, 0 replicated

Index 1, Offset 0, Mask 0x2

1 update-group member

Sent Rcvd

Prefix activity: ---- ----

Prefixes Current: 27 18 (Consumes 936 bytes)

Prefixes Total: 27 18

Implicit Withdraw: 9 0

Explicit Withdraw: 0 0

Used as bestpath: n/a 18

Used as multipath: n/a 0


Outbound Inbound

Local Policy Denied Prefixes: -------- -------

Total: 0 0

Number of NLRIs in the update sent: max 16, min 0


Connections established 3; dropped 2

Last reset 21:04:21, due to BGP Notification sent, hold time expired

Connection state is ESTAB, I/O status: 1, unread input bytes: 0

Local host: x.x.x.x, Local port: 64932

Foreign host: x.x.x.x, Foreign port: 179


Enqueued packets for retransmit: 1, input: 0 mis-ordered: 0 (0 bytes)


Event Timers (current time is 0x1E6B03675):

Timer Starts Wakeups Next

Retrans 26239 977 0x0

TimeWait 0 0 0x0

AckHold 22737 19460 0x0

SendWnd 0 0 0x0

KeepAlive 0 0 0x0

GiveUp 0 0 0x0

PmtuAger 0 0 0x0

DeadWait 0 0 0x0


iss: 32448291 snduna: 32928771 sndnxt: 32928771 sndwnd: 15776

irs: 514194396 rcvnxt: 514626744 rcvwnd: 15624 delrcvwnd: 760


SRTT: 300 ms, RTTO: 303 ms, RTV: 3 ms, KRTT: 0 ms

minRTT: 0 ms, maxRTT: 604 ms, ACK hold: 200 ms

Flags: higher precedence, retransmission timeout, nagle


Datagrams (max data segment is 1460 bytes):

Rcvd: 46518 (out of order: 0), with data: 22737, total data bytes: 432347

Sent: 45056 (retransmit: 977), with data: 25263, total data bytes: 480479

3750Switch#

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royalblues Fri, 12/21/2007 - 00:00
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You have very aggressive timers set on your BGP session. this could be a problem if the latency between the endpoints is higher


Narayan


marikakis Fri, 12/21/2007 - 07:46
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I agree with you that the timers are aggressive.

However, the latency between the 2 endpoints of an external BGP session is normally counted in milliseconds.

From Athens/Greece to New York you can certainly find links with RTT just 136 msec. Half a second round-trip delay is noticable and unacceptable for an end-user, so no question about a session via a single direct link (or even a multi-hop session a few hops away).

The problem with aggressive timers demonstrates itself when another more fundamental one appears, namely a constant flap of the physical layer.

A malfunctioning physical layer that goes up and down, will cause your BGP to also flap, and the BGP flaps will be often if your timers are aggressive. BGP flaps can potentially affect other parts of your network and cause general instability.


In summary, timers need to be fixed to protect rest of you network and your router's CPU, rather than the session itself.

Also check your physical layer (cabling, contact your circuit provider), since this is the usual suspect for constant BGP flaps.

Do not forget to also check that the BGP peer router is ok.


Kind Regards,

M.

redrobish Thu, 01/03/2008 - 18:52
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Thanks marikakis


Could you tell what could be the timer we could set? The link is just w/in the country.


thnx again

dodgerfan78 Thu, 01/03/2008 - 18:58
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I work for an ISP and we use 15 second keep alive and 45 second hold time and it seems to work fine.


router bgp 65000

timers 15 45


Just thought I'd give you some other real world config, I am sure others have different values.


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