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Sorry if this is an XY problem -- struggling to understand a few concepts.

To put it simply, I just don't understand when iBGP is needed. Been reading the CCNP Enterprise - Core and the example provided is confusing me.

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For start, it's mentioned that "R2 could form an iBGP session directly with R4, but R3 would not know where to route traffic from AS65100 or AS65300 when traffic from either AS reaches R3, because R3 would not have the appropriate route forwarding information for the destination traffic" and I just don't get this.

  • Is OSPF required between R2 and R3? (Why is it not iBGP? -- Is it just for the sake of example?)
  • Is EIGRP required between R3 and R4? (Again, why is it not iBGP? -- Is it just for the sake of example?)
  • How does the relation between R2 and R4 matter in this case? I imagine it's related to the BGP path attributes, but want more assurance.
  • Why can't the information coming from AS65100 or AS65300 be useful to R3 in this current design?
  • When they mention "redistributing the BGP table [...]", does that imply eBGP?
  • Ancillary to point listed immediately above, what is the protocol used between {R1 and R2} and {R4 and R5}?
  • What is the quote unquote solve for a situation like this? Sorry if that's too broad. The book mentions that "establishing iBGP sessions between all the same routers in a full mesh allows for proper forwarding between autonomous systems", but is there any way to make this situation work without using strictly iBGP in AS65200?

Hopefully the scope of this is clear enough!

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Is OSPF required between R2 and R3? (Why is it not iBGP? -- Is it just for the sake of example?)

No, it's just an example. Any routing protocol (or static routes) will work

Is EIGRP required between R3 and R4? (Again, why is it not iBGP? -- Is it just for the sake of example?)

No, it's just an example. Any routing protocol (or static routes) will work

How does the relation between R2 and R4 matter in this case? I imagine it's related to the BGP path attributes, but want more assurance.

Not sure what you're asking here. If AS 65200 is a transit AS, one assumes you want BGP attributes to pass through it. That won't happen if you redistribute into the IGP.

Why can't the information coming from AS65100 or AS65300 be useful to R3 in this current design?

Possibly, but in this example, it's not a good practice for the reasons (scalability, etc) they mention

When they mention "redistributing the BGP table [...]", does that imply eBGP? Ancillary to point listed immediately above, what is the protocol used between {R1 and R2} and {R4 and R5}?

No. eBGP and iBGP are in the same table (it's the same protocol). BGP is the protocol.

What is the quote unquote solve for a situation like this? Sorry if that's too broad. The book mentions that "establishing iBGP sessions between all the same routers in a full mesh allows for proper forwarding between autonomous systems", but is there any way to make this situation work without using strictly iBGP in AS65200?

R3 needs to know how to reach all the destinations that R2 and R4 do in order to provide transit. Anything that makes that happen is a solution. A good exercise is to try to think up some yourself.

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  • "Not sure what you're asking here. If AS 65200 is a transit AS, one assumes you want BGP attributes to pass through it. That won't happen if you redistribute into the IGP." > W this current design, how could you achieve that? (Could you provide a concrete example?) Also, /why/ won't that happen? "Possibly, but in this example, it's not a good practice for the reasons (scalability, etc) they mention" > Would you please elaborate? Are the path attributes the primary concern? Part of my fundamental issue w this is that I don't understand why R3 doesn't have the information it needs.
    – qux
    Jul 7 at 16:57
  • The second diagram uses static routes to solve the problem. Otherwise, R2 wouldn't know how to reach R4 and vice versa. Imagine that your BGP router is receiving the full Internet routing table (~750,000 routes). OSPF or EIGRP will have problems with that.
    – Ron Trunk
    Jul 7 at 17:09
  • And yes, BGP attributes are an important factor.
    – Ron Trunk
    Jul 7 at 17:09
  • For my own notes, are the routing tables shown under diagram #2 a working solve for this? I'm trying to make sure I understand correctly.
    – qux
    Jul 7 at 17:25
  • It's not R3 that needs the routes, it's R2 and R4. R2 doesn't know how to get to 10.45.1.0 unless it knows to forward traffic to R3.
    – Ron Trunk
    Jul 7 at 17:26

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