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26855 Views 6 Replies Latest reply: Jun 12, 2013 12:56 AM by satwant RSS

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BGP Router ID

Feb 28, 2010 8:46 AM

Deepak 311 posts since
Jun 25, 2008

There are following two steps comes in BGP path selection process...

 

Prefer the route with the lowest neighbor BGP router ID value.

 

If the BGP router IDs are the same, prefer the route with the lowest neighbor IP address.

 

Just wanted to know is there a possiblity of having BGP router IDs to be same, if yes then in please explain the scenario.

 

Thanks in advance,

Deepak

  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    1. Feb 28, 2010 9:02 AM (in response to Deepak)
    Re: BGP Router ID

    Hi Deepak

     

    I did not think you could have the same router-id

     

    Regards Conwyn

     

    router bgp 2
    no synchronization
    bgp router-id 1.1.1.1
    bgp log-neighbor-changes
    neighbor 10.0.0.1 remote-as 1
    no auto-summary

     

    *Mar  1 00:02:27.587: %BGP-3-NOTIFICATION: sent to neighbor 10.0.0.1 2/3 (BGP identifier wrong) 4 bytes 01010101

    FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 002D 0104 0001 00B4 0101 0101 1002 0601 0400 0100 0102 0280 0002 0202 00

    r1(config-router)#bgp router-id 2.2.2.2
    r1(config-router)#
    *Mar  1 00:05:25.219: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Up

  • Petr Lapukhov 4xCCIE/CCDE 71 posts since
    Jul 9, 2008
    Currently Being Moderated
    3. Feb 28, 2010 12:07 PM (in response to Deepak)
    Re: BGP Router ID

    BGP Router-IDs are supposed to be unique values, and this is normally achieved by using global IPv4 addresses as the IDs. The purpose of BGP Router-ID is to recover from TCP session collision, i.e. when both speakers open TCP connections simultaneosuly. A local system will reject the OPEN message with the same Router-ID as the local one. However, it does not compare Router-IDs of two different peers as this is not needed for the purpose of collision recovery.   That is, consider the topology below:

     

    X----A----Y

     

    If the BGP speakers X and Y peer with A, they could use the same Router-ID value as long as it does not match A's Router-ID. Hence the best-path selection step - if the two speakers with the same IDs advertise the same path, break the tie based on the peering transport IP address.

     

    HTH,

     

    Petr

  • Conwyn 7,907 posts since
    Sep 10, 2008
    Currently Being Moderated
    Re: BGP Router ID

    Hi Deepak

     

    An example below

     

    Regards Conwyn

     

    r0#show run | section router

    router bgp 1
    no synchronization
    bgp router-id 1.1.1.1
    bgp log-neighbor-changes
    neighbor 10.0.0.2 remote-as 2
    no auto-summary
    r0#show ip bgp summary
    BGP router identifier 1.1.1.1, local AS number 1
    BGP table version is 1, main routing table version 1

    Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
    10.0.0.2        4     2       7       7        1    0    0 00:04:32        0

     

    r1#show run | section router
    router bgp 2
    no synchronization
    bgp router-id 2.2.2.2
    bgp log-neighbor-changes
    neighbor 10.0.0.1 remote-as 1
    neighbor 10.0.0.6 remote-as 3
    no auto-summary
    r1#show ip bgp summ
    BGP router identifier 2.2.2.2, local AS number 2
    BGP table version is 1, main routing table version 1

    Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
    10.0.0.1        4     1       8       8        1    0    0 00:05:40        0
    10.0.0.6        4     3       6       6        1    0    0 00:02:14        0

     

    r2#show runn | section router
    router bgp 3
    no synchronization
    bgp router-id 1.1.1.1
    bgp log-neighbor-changes
    neighbor 10.0.0.5 remote-as 2
    no auto-summary
    r2#show ip bgp summary
    BGP router identifier 1.1.1.1, local AS number 3
    BGP table version is 1, main routing table version 1

    Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
    10.0.0.5        4     2       7       7        1    0    0 00:03:39        0

  • satwant 1 posts since
    Dec 31, 2010
    Currently Being Moderated
    6. Jun 12, 2013 12:56 AM (in response to Deepak)
    Re: BGP Router ID

    great explanation....

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