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iBGP/eBGP default route propagation

Todd Bren
Beginner
Beginner

Hi guys, see the attached diagram for these question:

What routers will receive udpates originated on the routers below supposing each node has default BGP configurations (and no sync enabled)?

  • Route originated on A
  • Route originated on C
  • Route originated on E
  • Route originated on H

Is the below correct?

Route A:

  • received on C only
  • missing on the rest

Route C:

  • received on A, D, E, F, G, H, I, J, K
  • missing on B only

Route E:

  • received on C, H, I, F, J, K, G, D
  • missing on A, B

Route H:

  • received on E, C, I, F, D, J, K, G
  • missing on A, B

Thank you.

1 Accepted Solution

Accepted Solutions

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hi Todd,

In that case:

Route originated on A: Received by C only, not forwarded further

Route originated on C: Received by A, D, E, forwarded from E to H, from H to I, from I to J and F but F will drop it (AS_PATH), from J to K, from K to G but G will drop it (AS_PATH). Resulting set: A, D, E, H, I, J, K

Route originated on E: Received by C and H, forwarded from H to I, from I to J and F but F will drop it (AS_PATH), from J to K, from K to G but G will drop it (AS_PATH). Resulting set: C, H, I, J, K

Route originated on H: Received by E and I, forwarded from E to C and from I to F, forwarded from F to D, forwarded from I to J, from J to K, from K to G, from G to D. Resulting set: E, I, C, F, D, J, K, G

Best regards,

Peter

View solution in original post

3 Replies 3

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hi Todd,

In your exhibit, the links interconnecting the routers are physical links or BGP peerings? In other words, how are the BGP adjacencies configured - which routers are BGP peers and which are not? This is crucial to answer your question correctly.

Best regards,

Peter

All interlinks are one hop BGP peerings.

Peter Paluch
Hall of Fame Cisco Employee Hall of Fame Cisco Employee
Hall of Fame Cisco Employee

Hi Todd,

In that case:

Route originated on A: Received by C only, not forwarded further

Route originated on C: Received by A, D, E, forwarded from E to H, from H to I, from I to J and F but F will drop it (AS_PATH), from J to K, from K to G but G will drop it (AS_PATH). Resulting set: A, D, E, H, I, J, K

Route originated on E: Received by C and H, forwarded from H to I, from I to J and F but F will drop it (AS_PATH), from J to K, from K to G but G will drop it (AS_PATH). Resulting set: C, H, I, J, K

Route originated on H: Received by E and I, forwarded from E to C and from I to F, forwarded from F to D, forwarded from I to J, from J to K, from K to G, from G to D. Resulting set: E, I, C, F, D, J, K, G

Best regards,

Peter

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