Before asking question let me put here what I understood.
[ iBGP peers should not advertise routes learned from iBGP peers to other iBGP peers( as per BGP rules, which is there because AS-Path attribute functionality will be defeated..so this rule is good). The solution to this problem is Full mesh among iBGP peers OR Route Reflectors OR Confederations.]
We know lot of ISP's run IGP inside their core even though they have iBGP running. Why do they need this IGP(ospf or isis) if they have satified above required for iBGP.
My question is under what conditions IGP must be used while iBGP is serving well enough ?
You are correct when you say that if you have a full mesh IBGP you do not need an IGP within the AS. However this assumes you are using the physical interface addresses to peer with IBGP and best practice is to use loopbacks for the peering. If you use loopbacks you do now need an IGP so that each IBGP router knows how to reach the other routers loopbacks.
1] Yes to advertise Loopback interface you need an IGP to form IBGP neighbor using loopback which give you more Flexibilité.
2] In ISP network the other reason is to have IGP becasue of presence P router, P give you more flexibilite to update all PE Routers ( If P is configured with RR) that connect to CE router becasue the BGP update use loopback & That is reachable via IGP.
3] If service provider run without RR then you should have Full mesh as you told. BUt for full mesh you need that much of link which increace you capex, In that senario you need an central router or location know an P router or HUB where all link are terminated & every router configured with Loopback IP to form an full mesh BGP & P router won't need to configure BGP it run only IGP i.e. OSPF or ISIS.
The IGP should carry your infrastructure routes and loopbacks, be kept as small as possible and thus be quick to converge. BGP builds upon and relies upon your IGP for next-hop reachablility - this becomes self evident in a multipath IGP scenarios where more that one route exists between two iBGP routers.
BGP is slower to converge and carries customer routes.
The old Cisco ISP Essentials book is a great read on this subject and is available as a PDF with a few good google searches.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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