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iBGP/eBGP default route propagation

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.

Everyone's tags (1)
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

iBGP/eBGP default route propagation

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

3 REPLIES
Cisco Employee

iBGP/eBGP default route propagation

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

New Member

iBGP/eBGP default route propagation

All interlinks are one hop BGP peerings.

Cisco Employee

iBGP/eBGP default route propagation

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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