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This Question is Answered 1 Correct Answer available (4 pts) 2 Helpful Answers available (2 pts)
22389 Views 10 Replies Latest reply: Jun 24, 2011 7:38 PM by Hyeoncheol Cho RSS

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eBGP Multihop

Apr 19, 2009 2:25 PM

Chetan 539 posts since
Jul 26, 2008

Hi ,

 

Please refer to the topology on the following link :

 

 

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#ebgpmultihop

 

 

Explaination says that :

 

RTB indicates a neighbor that has direct connection, which is 129.213.1.2.

 

RTA indicates an external neighbor that does not have direct connection.

 

I didn't understand it. There are only two routers in the topology. Then one router has direct connection but not the other router..What's this ??

 

On RTA, what is the significance of using neighbor 180.225.11.1 instead of using neighbor 129.213.1.3 ?

 

Afterall both, 129.213.1.3 and 180.225.11.1 are referring to the same non-cisco router RTB..

 

Is it because of the fact that 180.225.11.1 means bgp router-id on RTB or what ??

 

Please help..

  • Paul Stewart  -  CCIE Security 7,575 posts since
    Jul 18, 2008
    Currently Being Moderated
    1. Apr 19, 2009 3:14 PM (in response to Chetan)
    Re: eBGP Multihop

    That is a difficult to interpret picture.  The second eBGP peer is actually behind RTB at IP 180.225.11.1.

     

    The deal with BGP and multi hop is this.  eBGP (a peering relationship with another AS) sets the IP TTL to 1 by default.  So when this traverses a router, it is decreased to 0 and dropped.  This is the same IP Header Field that TTL that is used with traceroute.  So as a eBGP packet traverses a router, it is dropped.  To remedy this, you use the ebgp-multihop paramater on a neighbor statement, after you define the neighbor.  If you don't specify a value, it will default to an initial TTL of 255.  I would say that this doesn't need to be that high, but that depends on how many hops the peers are apart.  iBGP allows multihop by default.  I have not looked at an iBGP packet to see what the initial TTL is, but I'd guess 255 or 127.

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  • Paul Stewart  -  CCIE Security 7,575 posts since
    Jul 18, 2008
    Currently Being Moderated
    Re: eBGP Multihop

    Looking at your example further, I see what you are talking about.  I wouldn't worry too much about that, just remember that multihop affects the IP TTL Field.  That is a confusing example.  The only thing I could be guessing is packets in one direction (RTA to RTB) are going through the router logic in RTB, where the return packets aren't.  In my last post, I said I thought the second peer was behind RTB, but I'm not sure what they are trying to show.  Again, just understand you need to use multihop when you send eBGP traffic through something that will decrement the TTL and drop the packet (i.e. a router).

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  • Paul Stewart  -  CCIE Security 7,575 posts since
    Jul 18, 2008
    Currently Being Moderated
    3. Apr 20, 2009 7:26 AM (in response to Chetan)
    Re: eBGP Multihop

    One more note on this, then I'll give pause for your response.  You mentioned that RTB is a "non-cisco" router.  I actually interpret third-party in this example to mean another party or network that is outside of your control, most likely the ISP.  That is why the policy could be such that there is no direct connectivity of BGP Peers.

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,429 posts since
    Oct 7, 2008
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    5. Apr 20, 2009 12:00 PM (in response to Chetan)
    Re: eBGP Multihop

    The concept of ebgp peers needing to be directly connected is per the RFC, not Cisco.  So that (or the workaround of multihop) is not reserved to Cisco devices.  Whether there's  a non-Cisco router in the mix wouldn't actually change anything here.

     

    Scott

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  • Paul Stewart  -  CCIE Security 7,575 posts since
    Jul 18, 2008
    Currently Being Moderated
    7. Apr 21, 2009 3:53 PM (in response to Chetan)
    Re: eBGP Multihop

    I have to be very careful talking about BGP (not my strong suit).  I just happen to understand eBGP multi-hop from my studies in CCIE Sec.  So  wouldn't necessarily consider myself an expert in that category, but thanks for the compliment.  Scott, on the other hand, is an expert on all things routing (including BGP).

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  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,429 posts since
    Oct 7, 2008
    Currently Being Moderated
    Re: eBGP Multihop

    hehehehe...  I dunno about that...  I just get to play around with lots of different things. 

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  • lius 1 posts since
    Feb 22, 2011
    Currently Being Moderated
    9. Feb 22, 2011 12:02 PM (in response to Chetan)
    Re: eBGP Multihop

    通过抓包之后看到的情况,确实和楼上的说法一致,改变了IP包头中的TTL值。

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  • Hyeoncheol Cho 28 posts since
    May 17, 2009
    Currently Being Moderated
    10. Jun 24, 2011 7:38 PM (in response to Chetan)
    Re: eBGP Multihop

    Chetan,

    I also have been studying BGP a bit these days and wanted to share a bit of my knowledge.

     

    Rule of thumb.

    1. For iBGP peering, there is no restirction to the number of hops in the peering condition.

     

    2. However, when it comes to the eBGP, one of the condition to estabilsh peering is that the peering router( you can consider the IP address of the neighbor) _MUST_ be on the same directly connected network. i.e, two routers in eBGP neighbor relation should have one interface connected on the same IP subnet. if not, the eBGP routers would not even generate an "open' message at all. neibhgor 180.225.11.1 ebgp-multihop is used to overcome this condition. once this command is used in the cnfiguration, the eBGP router understand that the peer does not directly connected.

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