in
IEOC CCIE Forums

IEOC - INE's Online Community

Welcome to INE's Online Community - IEOC - a place for CCIE and CCENT candidates to connect, share, and learn. Our Online Community features CCIE forums and discussions for all tracks including Routing & Switching, Voice, Security, Service Provider, Wireless,, and Storage. Through these online communities you can discuss your questions with thousands of your peers, hundreds of CCIE's and INE's own team of world renowned CCIE instructors and authors, Brian Dennis - Quintuple CCIE #2210, Brian McGahan – Triple CCIE #8593, Petr Lapukhov - Quad CCIE #16379, and Mark Snow - Dual CCIE #14073.
Latest post 04-15-2017 1:47 PM by Ellie. 13 replies.
Page 1 of 1 (14 items)
Sort Posts: Previous Next
  • 04-06-2017 6:12 PM

    BGP - Advertisements Between iBGP and eBGP peers

    Can anyone point me to a document that details the logic of how routes are advertised between peers.

    e.g. if an eBGP peer receives prefix from an eBGP neighbor it advertises those routes out these interfaces.

    I'm having a hard time finding something that goes over that just concerning route/prefix advertisement.

    • Post Points: 35
  • 04-07-2017 10:20 AM In reply to

    Re: BGP - Advertisements Between iBGP and eBGP peers

    the book Internet routing architectures has some explanations and diagram. in short, u get routes into a bgp table, path selection algorithm is run, then best path is marked and put into routing table.  tricky part is where or when  path/prefix attributes (in and out0 are applied.

    • Post Points: 5
  • 04-09-2017 11:03 AM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    I found this topic to be related to my question about BGP Advertisement that I have. I am some what confused about BGP Route Advertisment condition in general.

    The rule indicates that "BGP Best Route" From BGP Table will be advertised to "BGP Peer neighbor" if the route already exists within IGP Table (routing-Table). In that case, we can use "Network" Statement to advertise BGP Route to BGP Peer.

    Also, Another Rule indicates that "BGP BEST ROUTE" will get installed in Routing-Table after It has been selected as Best BGP Route. The question is that if The route should already exists inside of routing-table before we can advertise BGP Route via Network Statement, Why This also talks about that when BGP Best Route gets selected, BGP Best Route will get advertised to Routing-table. Per the first Criteria, my assumption is that BGP Best Route already existed inside of Routing-table other wise, That route would not have got selected to be advertised to BGP Peer or to get installed into Routing-table.

    Are going through Loop here as far as the conditions. I understand that BGP Best Route will get advertised to BGP Peer neighbor IF THE ROUTE Already exists inside of Routing-table as well as BGP Best Route will get installed in to Routing-table.

    My interpretation from above rules is that BGP Best Route already exists inside of routing-table. Otherwise, it would not have got selected to be advertised to BGP neighbor via Network statement. In that case, the second condition may be is not necessary by saying that BGP Best route will also get advertised to Routing-table because it was already there?

    Thank you for your time in advance.

    Best Regards

     

    • Post Points: 50
  • 04-09-2017 12:32 PM In reply to

    Re: BGP - Advertisements Between iBGP and eBGP peers

    For BGP to advertise a route to a network it owns, the network must be in the local RIB. If it is, and the network statement in "router bgp <as>" matches that of the route in the RIB, BGP will advertise that route.

    For BGP to advertise a route that it doesn't own, one received via an UPDATE, it has the option to install that route in the routing table, even if another route already exists via an IGP. It will install the route if the administrative distance of BGP is lower than that of the IGP.

    Hopefully I understood you correctly.

     

    • Post Points: 20
  • 04-09-2017 12:57 PM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Thank you for your explanations. Per what I understood from your explanations, would it mean that If BGP is advertising the route that it owns via network statement, The BEST route that is being advertised via Network statement to other BGP Peers will no longer gets installed in to the routing-table. The reason is because the route already existed in Routing-table. other wise, It could not get advertised to other BGP Peers. Is that correct understanding from this scenario?

    In that case, The only BGP BEST Route that will get advertised to Routing-table is when BGP Router has received the route from another BGP Peer (BGP Neighbor) and BGP Route is not the route that local router owns. Is that correct explanations of the situations.

    Thank you

     

    • Post Points: 20
  • 04-09-2017 12:59 PM In reply to

    Re: BGP - Advertisements Between iBGP and eBGP peers

    I believe your understanding is correct as you just explained it.

    • Post Points: 5
  • 04-09-2017 5:55 PM In reply to

    Re: BGP - Advertisements Between iBGP and eBGP peers

    yes, indeed, nice explanation!

    • Post Points: 20
  • 04-10-2017 5:05 PM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Great. Thank you both for your feedback. I appreciate your time. That was one of BGP questions that I had been carrying for a few years honestly. !

     

    It got resolved now.

     

    • Post Points: 5
  • 04-13-2017 7:13 AM In reply to

    • peety
    • Top 25 Contributor
    • Joined on 02-23-2011
    • Redmond, WA, USA
    • Elite
    • Points 19,825

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Ellie:

    The rule indicates that "BGP Best Route" From BGP Table will be advertised to "BGP Peer neighbor" if the route already exists within IGP Table (routing-Table). In that case, we can use "Network" Statement to advertise BGP Route to BGP Peer.

    That assumes synchronization is still on - almost everybody turns this off. Regardless, you can use network statement to advertise any other route to a BGP peer; you don't need or want a network statement to get a BGP route to a BGP peer.

    Ellie:

    Also, Another Rule indicates that "BGP BEST ROUTE" will get installed in Routing-Table after It has been selected as Best BGP Route. The question is that if The route should already exists inside of routing-table before we can advertise BGP Route via Network Statement, Why This also talks about that when BGP Best Route gets selected, BGP Best Route will get advertised to Routing-table. Per the first Criteria, my assumption is that BGP Best Route already existed inside of Routing-table other wise, That route would not have got selected to be advertised to BGP Peer or to get installed into Routing-table.

    Are going through Loop here as far as the conditions. I understand that BGP Best Route will get advertised to BGP Peer neighbor IF THE ROUTE Already exists inside of Routing-table as well as BGP Best Route will get installed in to Routing-table.

    There's a BGP RIB ("routing table") and a local RIB ("routing table"). As BGP learns prefixes ("routes" with "mask") and "paths", these go into the BGP RIB. The best path to a particular prefix is _offered_ to the local RIB, but does not need to be accepted by the local RIB (see "RIB-failure"). Regardless of whether the local RIB accepted the route, the best path is still available to propagation to other BGP neighbors. The best path learned from an EBGP peers is advertised to all neighbors, whereas a best path learned from an IBGP peer is advertised to all EBGP neighbors only. Route reflection does alter this process slightly.

    Ellie:

    My interpretation from above rules is that BGP Best Route already exists inside of routing-table. Otherwise, it would not have got selected to be advertised to BGP neighbor via Network statement. In that case, the second condition may be is not necessary by saying that BGP Best route will also get advertised to Routing-table because it was already there?

    I think you've got it all backwards here. BGP best route definitely exists in the BGP RIB; otherwise it can't be a best route within the BGP process. If it's the best route, it's eligible for propagation to other BGP neighbors regardless of Network statement. You do not need a network statement, and I would argue that you do not want a network statement, to get the route to propagate onward.

    CCIE R&S #34583

    Now based in Redmond, WA

    • Post Points: 20
  • 04-15-2017 11:52 AM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Thank you Peety for your feedback. I just got a chance to review your information. That is a good point that you brought up with "Synchronization" which I had not thought about it. Would you mean that the rule of Network statement BGP Route advertisement with the condition that route should already exist inside of Routing-Table comes to play JUST When "Synchronization" is enabled?

    IF "Synchronization" is disabled, We can say that Network statement can advertise BGP routes regardless if route already exists inside of Local RIB (Routing-Table)? Is that correct BGP Conditon in regards to BGP Advertisment via Network statement to BGP Peers?

     

    Thank you all for your time again. I appreciate it.

    • Post Points: 20
  • 04-15-2017 12:01 PM In reply to

    • peety
    • Top 25 Contributor
    • Joined on 02-23-2011
    • Redmond, WA, USA
    • Elite
    • Points 19,825

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Ellie:

    Thank you Peety for your feedback. I just got a chance to review your information. That is a good point that you brought up with "Synchronization" which I had not thought about it. Would you mean that the rule of Network statement BGP Route advertisement with the condition that route should already exist inside of Routing-Table comes to play JUST When "Synchronization" is enabled?

    No, the network statement and synchronization are essentially exclusive. Network pertains to putting routes INTO the BGP protocol from another protocol (which could include connected and static as "protocols"). Synchronization pertains to propagating routes that are already in BGP onward to other BGP neighbors only if they exist in the IGP.

    Ellie:

    IF "Synchronization" is disabled, We can say that Network statement can advertise BGP routes regardless if route already exists inside of Local RIB (Routing-Table)? Is that correct BGP Conditon in regards to BGP Advertisment via Network statement to BGP Peers?

    Yes, that is correct.

    CCIE R&S #34583

    Now based in Redmond, WA

    • Post Points: 20
  • 04-15-2017 12:46 PM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Thank you Peety for your quick reply. I agree with the rule of Synchronization and also being off in most recent IOS.

    Per my understanding, If the route is locally originated, we use Network statement to Advertise to other BGP Peers. Based on what we discussed, since Synchronization is disabled on most recent IOS, regardless if that route exists inside of Routing-table (Local RIB), The route via Network Statement will get advertised to BGP Peers via Network statement.

    Another types of Routes are routes that came to us from Neighbors through different methods. for those routes, when we select the best-route if multiple Paths are advertised to us from neighbors, we will do two things with that Best Route. We will advertise Best route to remote BGP neighbor. Secondly, we will install Best-Route to Local RIB (Routing-table) as well.

    Would all of these mentioned above consider correct conditions.

    Thank you again for your time.

     

    • Post Points: 20
  • 04-15-2017 1:06 PM In reply to

    • peety
    • Top 25 Contributor
    • Joined on 02-23-2011
    • Redmond, WA, USA
    • Elite
    • Points 19,825

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Those appear to be correct. There's also the option of redistribution from other protocols instead of the network statement.

    CCIE R&S #34583

    Now based in Redmond, WA

    • Post Points: 20
  • 04-15-2017 1:47 PM In reply to

    • Ellie
    • Top 75 Contributor
    • Joined on 08-14-2011
    • Dallas, TX
    • Elite
    • Points 5,145

    Re: BGP - Advertisements Between iBGP and eBGP peers

    Great. Thank you for your response. I appreciate your time.

    I agree with you on the option of Redistribution as another method. When the routes are being redistributed to BGP, Origin Code will be changed to "?". When we use Network Statement or Summary address within BGP Table, Origin Code will be appeared as "i".

    Thanks

    • Post Points: 5
Page 1 of 1 (14 items)
IEOC CCIE Forums Internetwork Expert CCIE Training
About IEOC | Terms of Use | RSS | Privacy Policy
© 2010 Internetwork Expert, Inc. All Rights Reserved