cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
779
Views
0
Helpful
3
Replies

IBGP with three routers need IGP?

prmaha0009
Beginner
Beginner

Hi All

 

I have below issues. 

 

Topology

 

R2-->R1-->R3 in IBGP

R9 -->R3 EBGP

R4-->R2 EBGP

 

I am not using IGP/Static routes. I have created loopbacks on each router and use route reflector on R1 for R3 & R2 as client.

I see loopback IP is getting advertise/Learn on R2 & R3 (Showing best path) but unable to ping it. 

 

So Question is, Does IBGP need IGP when we are using more than 2 routers? 

If no, then where is the issue? I did use next hop self on R1 but it didnt help. 

2 Accepted Solutions

Accepted Solutions

Peter Paluch
Cisco Employee
Cisco Employee

Hi,

So Question is, Does IBGP need IGP when we are using more than 2 routers? 

This is not a matter of how many routers you have but rather whether they are all on the same directly connected network. Remember that when BGP advertises a network, the IP address in the next hop value is a standalone attribute that can be set to the IP address of:

  • The router that injects this network into BGP (classic intra-AS iBGP)
  • The advertising AS border router in the next autonomous system (classic eBGP)
  • The AS border router that received this network via eBGP (intra-AS iBGP with next-hop-self)

There are also other options of manually influencing the next hop attribute via route-maps.

What I am saying is that the next hop attribute can be set to various values. Now you have to ask yourself: Can each of my routers directly reach every possible next hop IP address that can show up in a received BGP advertisement? If yes, you do not need an IGP - but that also means that your entire AS consists of a single broadcast domain and a single subnet - effectively a single LAN with all routers connected to it. Surely, that's not how most AS are designed.

So in realistic cases, the next hop address of a received BGP route is an IP address that is not on a directly connected network which means that you first need to have a route to that next hop address, and that is why you need IGP.

Feel welcome to ask further!

Best regards,
Peter

View solution in original post

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

One more thing to consider.

When pinging R3's loopback from R2, your packets are sourced from R2's interface toward R1. Is this network between R1 and R2 advertised so that R3 knows about it? We always have to keep thinking about the route back to the sender. It may be possible that the packets from R2 hit R3 but R3 is unable to respond back.

Best regards,
Peter
 

View solution in original post

3 Replies 3

Peter Paluch
Cisco Employee
Cisco Employee

Hi,

So Question is, Does IBGP need IGP when we are using more than 2 routers? 

This is not a matter of how many routers you have but rather whether they are all on the same directly connected network. Remember that when BGP advertises a network, the IP address in the next hop value is a standalone attribute that can be set to the IP address of:

  • The router that injects this network into BGP (classic intra-AS iBGP)
  • The advertising AS border router in the next autonomous system (classic eBGP)
  • The AS border router that received this network via eBGP (intra-AS iBGP with next-hop-self)

There are also other options of manually influencing the next hop attribute via route-maps.

What I am saying is that the next hop attribute can be set to various values. Now you have to ask yourself: Can each of my routers directly reach every possible next hop IP address that can show up in a received BGP advertisement? If yes, you do not need an IGP - but that also means that your entire AS consists of a single broadcast domain and a single subnet - effectively a single LAN with all routers connected to it. Surely, that's not how most AS are designed.

So in realistic cases, the next hop address of a received BGP route is an IP address that is not on a directly connected network which means that you first need to have a route to that next hop address, and that is why you need IGP.

Feel welcome to ask further!

Best regards,
Peter

Peter Paluch
Cisco Employee
Cisco Employee

Hello,

One more thing to consider.

When pinging R3's loopback from R2, your packets are sourced from R2's interface toward R1. Is this network between R1 and R2 advertised so that R3 knows about it? We always have to keep thinking about the route back to the sender. It may be possible that the packets from R2 hit R3 but R3 is unable to respond back.

Best regards,
Peter
 

Thanks peter.. Issue resolved. i was having subnet issue coz of that it was having problem. Thanks for ur Update. Nice in detail update. Kudos :D

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: