1 2 Previous Next 19 Replies Latest reply: Dec 12, 2019 11:33 PM by Martin RSS

    BGP as IGP or base for eBGP ?


      can we use BGP as IGP and how often is used ?

        • 1. Re: BGP as IGP or base for eBGP ?

          not good option; BGP convergence is very slow ; slow as RIP (if not slower);  BGP is more application rather then routing protocol (or defined as something in between).  this application approach give us flexibility in managing/administering routes and traffic engineering. 

          it was designed to handle lot of number of routes/networks known as prefixes.  BGP carries over 500,000 Internet routes

          • 2. Re: BGP as IGP or base for eBGP ?

            What about BGP between ISPs; and do we need to run IGP if we want to have iBGP peerings? 

            • 3. Re: BGP as IGP or base for eBGP ?

              technically you can run BGP without any IGPs as long as TCP session can be established - see example of this in TCP/IP book.  but I would not recommended; I would run some sort of dynamic protocol and BGP over it; I would not use static routes. 

              BGP peering can be established as long as you have IP reachability and TCP permission (allow to be established)


              ISPs usually run OSPF or IS-IS and MPLS over that; then BGP VPNs will use MPLS labels to carry multiple independent routes

              MP-BGP over MPLS with multiple VRFs handles L3 VPNs  among companies that single ISP provider serves

              • 4. Re: BGP as IGP or base for eBGP ?

                It will be difficult to manage, depend on dynamic routing protocol like EIGRP or OSPF and run BGP over it if needed.

                • 5. Re: BGP as IGP or base for eBGP ?

                  why is ISIS preferred over OSPF in ISP environment

                  • 6. Re: BGP as IGP or base for eBGP ?

                    its internal architecture that includes separated 3 protocols or blocks that form ISIS lets this protocol to be easily adopted for various applications. ISIS started as connectionless network protocol and only later was adopted to support IP.   it still does support both.  ISIS is itself network layer protocol and can run with or w/o IP - unlike BGP or OSPF that rely on IP network layer.

                    In short, ISIS structure can be easily adopted to support various application that you can build (more easier than OSPF)

                    also, when apply for standardization, IS IS has added enhancement for MPLS TE -traffic engineering. 

                    • 7. Re: BGP as IGP or base for eBGP ?

                      this is a very frequently asked question here - for some reason people seem to like to try to fit square shapes into round holes - human nature i suppose


                      for most task in any job you need to select the right tool. if you learn about the tools themselves then you learn why they are good for some things and not so great for others. the more you learn about bgp the more you will discover the answer to your question for yourself.

                      • 8. Re: BGP as IGP or base for eBGP ?
                        Steven Davidson

                        BGP "as an IGP" is not really technically accurate.  It's not an IGP by definition.  BGP "instead of or in lieu of an IGP" is what you're talking about and it isn't practical for several reasons.  Mainly from a feature parity point-of-view but also because it really wasn't designed for that purpose and so it doesn't have the same convergence characteristics and the complexity characteristics are different.  Devices that you find at the various building block levels of a network support different features and you might not find BGP features (built-in) at the access and aggregation level.  In large enterprise environments it is very common to see BGP used to connect different blocks of the enterprise network together by way of a core but in this case BGP isn't taking the role of an IGP.  So in other words, image you have a very large network (not an ISP, an enterprise).  Rather than to run IGP, end to end, you segment your enterprise network into multiple BGP autonomous systems.  You would have a core AS.  You would then have different ASes for different functional areas.  You might have a WAN access AS.  You will have a server farm AS.  You will have an Internet AS (where you draw -> null0 and into a default free zone).  You might have an Extranet AS (for business-to-business connectivity).  All of these ASes are stitched together via eBGP peer relationships to the enterprise core AS.  Then, within each non-core AS, you would run independent IGPs.  They could all be using the same type of IGP (such as OSPF) or different non-core ASes could use different IGPs.  For example maybe you're going to run EIGRP in you WAN aggregation AS but everywhere else you want to run OSPF.  It doesn't really matter because they all exist in different BGP autonomous systems and those details are hidden from the core AS and all other ASes.  The core AS could run an IGP or it could run only BGP.  Where I work our core AS doesn't run an IGP internally because all routers in our core are layer 2 adjacent so all peers are established over point-to-point links for a full mesh of BGP peers across the dozen or so routers that make up the core.

                        • 9. Re: BGP as IGP or base for eBGP ?

                          A couple of advantages I can think of ISIS over OSPF are:


                          - it is easily extensible. Relying on TLV fields, allows ISIS to be easily extended to suppot other protocols.
                          Support for IP is in fact an extension itself since ISIS was initially conceived for CNLP, not IP.

                          - It was designed by the ISO organization, not by IETF. Making it slower to release but cleaner than OSPF with all of its additional RFCs and adds-on such as STUB, NSSA, etc, which are not needed in a SP FLAT backbone anyway.


                          These are just my personal considerations.

                          • 10. Re: BGP as IGP or base for eBGP ?



                            Read this topic:

                            BGP running without IGP question.

                            • 11. Re: BGP as IGP or base for eBGP ?

                              can u explain "not rely on IP network layer"

                              • 12. Re: BGP as IGP or base for eBGP ?

                                to proof that ISIS does not need IP network protocol is the fact tat ISIS uses MAC addressing in hellos filed.  There are several reserved MACs for ISIS: ending 000005 is for all ISIS routers; 0000004 is for all ISIS devices.  adjacencies for L1 level are 000014; L2 are 000015

                                OSPF uses M-cast IPs of and 0.6 whereas ISIS uses MACs

                                • 13. Re: BGP as IGP or base for eBGP ?

                                  so it is true,  you must set NSAP to id IS-IS instance for routing protocol running on an IS as well as IP address even thou you only need IP to run.

                                  • 14. Re: BGP as IGP or base for eBGP ?

                                    yes, it is weird but you must have ISIS network ID along with IP addressing; this is not much but u do need to have ISIS addressing scheme;




                                    router isis
                                    net 49.0001.0000.0000.000a.00
                                    interface ethernet0/0
                                    ip address
                                    ip router isis
                                    interface serial 2/0
                                    ip router isis
                                    ip address


                                    outer isis
                                    net 49.0001.0000.0000.000b.00
                                    interface ethernet0/0
                                    ip router isis
                                    ip address
                                    interface serial2/0
                                    ip router isis
                                    ip address
                                    interface serial5/0
                                    ip router isis
                                    ip address

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