Help on BGP timers

[kenyusuf]

Hi all,

I need help on the below issue :

Assuming default BGP timers being used in a MPLS IPVPN network. How long does it take to withdraw affected routes from routing table of remote CEs if WAN BGP peer of one CE in the same VRF goes down? 

Suggest some show commands to verify the result.

[rais]

If BGP keepalive time is set to x, usually holddown time is 3x. Once your neighbor goes down, it could take up to 3x units [usu. seconds] for the router to declare the neighbor down and flush its routes.

Thanks.

[kenyusuf]

@ rais Thanks for the reply.
Can anyone give some commands where i can verify this?

Thanks so much.

[e.ciollaro]

That's right but widthdrawn propagation form PE to remote CE has also to be take in account. Considering that BGP process has a minimal interval between update (30sec by default) I think in the worst case (BGP update sent immediatly before CE neighborship goes down) it could take an extra time equal to 30 * numero_of_hop between PE and remote CE.

Bye,

enrico

.

[kenyusuf]

Thx so much guys.

May i know which commands to check these, i am not able to locate them.

Please share some commands to how to verify this.

Thanks so much.

[e.ciollaro]

Hi,

you can check the timers negotiated with a neighbour using show ip bgp neighbour <ip>, Es:

R1#sh ip bgp neigh 192.168.12.2
BGP neighbor is 192.168.12.2,  remote AS 65002, external link
  BGP version 4, remote router ID 10.2.2.2
  BGP state = Established, up for 00:01:43
  Last read 00:00:43, last write 00:00:42, hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
    Route refresh: advertised and received(old & new)
    Address family IPv4 Unicast: advertised and received
  Message statistics:
    InQ depth is 0
    OutQ depth is 0
                         Sent       Rcvd
    Opens:                  1          1
    Notifications:          0          0
    Updates:                1          1
    Keepalives:             3          3
    Route Refresh:          0          0
    Total:                  5          5
  Default minimum time between advertisement runs is 30 seconds

 For address family: IPv4 Unicast
  BGP table version 6, neighbor version 6/0
 Output queue size : 0
  Index 1, Offset 0, Mask 0x2
  1 update-group member
                                 Sent       Rcvd
  Prefix activity:               ----       ----
    Prefixes Current:               4          2 (Consumes 104 bytes)
    Prefixes Total:                 4          2
    Implicit Withdraw:              0          0
    Explicit Withdraw:              0          0
    Used as bestpath:             n/a          1
    Used as multipath:            n/a          0

                                   Outbound    Inbound
  Local Policy Denied Prefixes:    --------    -------
    Bestpath from this peer:              1        n/a
    Total:                                1          0
  Number of NLRIs in the update sent: max 4, min 4
  Minimum time between advertisement runs is 12 seconds
  Connections established 1; dropped 0
  Last reset never
Connection state is ESTAB, I/O status: 1, unread input bytes: 0
Connection is ECN Disabled, Mininum incoming TTL 0, Outgoing TTL 1
Local host: 192.168.12.1, Local port: 179
Foreign host: 192.168.12.2, Foreign port: 18431

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

Event Timers (current time is 0x2E1DC):
Timer          Starts    Wakeups            Next
Retrans             6          0             0x0
TimeWait            0          0             0x0
AckHold             6          4             0x0
SendWnd             0          0             0x0
KeepAlive           0          0             0x0
GiveUp              0          0             0x0
PmtuAger            0          0             0x0
DeadWait            0          0             0x0

iss: 2368926251  snduna: 2368926437  sndnxt: 2368926437     sndwnd:  16199
irs:  936357534  rcvnxt:  936357712  rcvwnd:      16207  delrcvwnd:    177

SRTT: 169 ms, RTTO: 1204 ms, RTV: 1035 ms, KRTT: 0 ms
minRTT: 72 ms, maxRTT: 328 ms, ACK hold: 200 ms

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/command/irg-cr-book/bgp-s1.html#wp3502759178

Bye,

enrico

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