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EBGP loopback config

Hi,

I just started learning BGP protocol today, i have some confusion, when we using loopback interface to connect other ebgp using different AS, we have to mention three commands to get neighbor up using non-directly connected  interface, the two which i  highlighted below is mandatory, i try remvoe this cmd no neighbor 2.2.2.2 update-source lo 0, but the neighbor won't down, its still show BGP state = Established

. (R1 and R2 already advertise loop interface via RIP v2), please explain why and how this neighbor up and even i remove no update source cmd.

R1<---->R2

S111#sh run | sec bgp

router bgp 111

no synchronization

bgp log-neighbor-changes

neighbor 2.2.2.2 remote-as 222

neighbor 2.2.2.2 ebgp-multihop 2

no auto-summary

AS222#sh run | sec bgp

router bgp 222

no synchronization

bgp log-neighbor-changes

neighbor 1.1.1.1 remote-as 111

neighbor 1.1.1.1 ebgp-multihop 2

no auto-summary

AS111#sh ip bgp summ

BGP router identifier 192.168.2.1, local AS number 111

BGP table version is 1, main routing table version 1

Neighbor        V          AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

2.2.2.2         4        222      32      33        1    0    0 00:30:15        0

AS111#sh ip bgp neighbors

BGP neighbor is 2.2.2.2,  remote AS 222, external link

  BGP version 4, remote router ID 192.168.1.2

  BGP state = Established, up for 00:30:24

  Last read 00:00:12, last write 00:00:11, hold time is 180, keepalive interval is 60 seconds

  Neighbor capabilities:

    Route refresh: advertised and received(new)

    New ASN Capability: advertised and received

    Address family IPv4 Unicast: advertised and received

  Message statistics:

    InQ depth is 0

    OutQ depth is 0

1 ACCEPTED SOLUTION

Accepted Solutions

EBGP loopback config

Hi Feroz,

Just as Jon mentioned, when BGP was established, you have update-source command in you config, hence the TCP session  between both the host was established ( BGP relies on underlying TCP for session establishment). Once the session is built, even though you remove update-source command, it will not affect the existing TCP session, as no new TCP is getting built between the two, the TCP three-way handshake process is already completed and now it is only keepalive which  would be sent periodically to keep session up. One can argue that if keepalive are sent to and from loopback ip's why session is not torn, reason is that you already have route to loopback ip's in routing table and since for BGP TCP session is already established, it will continue to send keepalives.

Also, as you noticed that when you clear the BGP session, it went to active state as it was then when TCP was trying to establish session with its peer with mentioned IP in the configs, though those IP's were reachable, BGP by default when forming eBGP session sets the TTL to 1.

One way to circumvent is as you know ebgp multihop.

Another way to do the same thing is to disable connect check for  that neighbour.

Regards,

Smitesh


PS: Please rate helpful  posts.

5 REPLIES

EBGP loopback config

So you didn't have update source set on either side? How long did you wait to see if it went down? Default timers for BGP is 60 seconds keepalive and 180 seconds hold time.

Daniel Dib
CCIE #37149

Daniel Dib CCIE #37149 Please rate helpful posts.
New Member

Re: EBGP loopback config

till now it's up,

AS111#sh ip bgp neighbors

BGP neighbor is 2.2.2.2,  remote AS 222, external link

  BGP version 4, remote router ID 192.168.1.2

BGP state = Established, up for 02:05:08

  Last read 00:00:17, last write 00:00:16, hold time is 180, keepalive interval is 60 seconds

  Neighbor capabilities:

    Route refresh: advertised and received(new)

    New ASN Capability: advertised and received

    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:                0          0

    Keepalives:           126        125

    Route Refresh:          0          0

    Total:                127        126

  Default minimum time between advertisement runs is 30 seconds

For address family: IPv4 Unicast

  BGP table version 1, neighbor version 1/0

  Output queue size : 0

  Index 1, Offset 0, Mask 0x2

  1 update-group member

                                 Sent       Rcvd

  Prefix activity:               ----       ----

    Prefixes Current:               0          0

    Prefixes Total:                 0          0

    Implicit Withdraw:              0          0

    Explicit Withdraw:              0          0

    Used as bestpath:             n/a          0

    Used as multipath:            n/a          0

                                   Outbound    Inbound

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

    Total:                                0          0

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

  Address tracking is enabled, the RIB does have a route to 2.2.2.2

AS111#sh run | sec bgp

router bgp 111

no synchronization

bgp log-neighbor-changes

neighbor 2.2.2.2 remote-as 222

neighbor 2.2.2.2 ebgp-multihop 2

no auto-summary

AS111#

AS222#sh run | sec bgp

router bgp 222

no synchronization

bgp log-neighbor-changes

neighbor 1.1.1.1 remote-as 111

neighbor 1.1.1.1 ebgp-multihop 2

no auto-summary

AS222#

New Member

Re: EBGP loopback config

i try clear ip bgp * now it's went to Active state

AS222#sh ip bgp summary

BGP router identifier 2.2.2.2, local AS number 222

BGP table version is 1, main routing table version 1

Neighbor        V          AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

1.1.1.1         4        111     151     150        0    0    0 00:02:50 Active

AS222#sh ip bgp neighbors

BGP neighbor is 1.1.1.1,  remote AS 111, external link

  BGP version 4, remote router ID 0.0.0.0

  BGP state = Active

  Last read 00:03:01, last write 00:03:01, hold time is 180, keepalive interval is 60 seconds

  Message statistics:

Hall of Fame Super Blue

EBGP loopback config

It is Active now because you have removed the command and now a new session cannot be formed.

I can't test it but i assume that when you removed the command initially the BGP session was already up and running so it just continued to use the loopback address in it's updates.

This is a common case with BGP, where you need to reset the session before your new configuration is used.

Jon

EBGP loopback config

Hi Feroz,

Just as Jon mentioned, when BGP was established, you have update-source command in you config, hence the TCP session  between both the host was established ( BGP relies on underlying TCP for session establishment). Once the session is built, even though you remove update-source command, it will not affect the existing TCP session, as no new TCP is getting built between the two, the TCP three-way handshake process is already completed and now it is only keepalive which  would be sent periodically to keep session up. One can argue that if keepalive are sent to and from loopback ip's why session is not torn, reason is that you already have route to loopback ip's in routing table and since for BGP TCP session is already established, it will continue to send keepalives.

Also, as you noticed that when you clear the BGP session, it went to active state as it was then when TCP was trying to establish session with its peer with mentioned IP in the configs, though those IP's were reachable, BGP by default when forming eBGP session sets the TTL to 1.

One way to circumvent is as you know ebgp multihop.

Another way to do the same thing is to disable connect check for  that neighbour.

Regards,

Smitesh


PS: Please rate helpful  posts.

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