BGP Path Attributes iBGP vs eBGP Explained

Here’s a breakdown of BGP attributes that are either considered by iBGP neighbors only or eBGP neighbors only, along with the attributes that apply to both, but may have different behaviors or implications depending on whether the neighbor is iBGP or eBGP.

Attributes Considered by iBGP Neighbors Only:

These attributes are shared within an AS but may not be propagated or considered by eBGP neighbors:

1. Local Preference:

   • Used by: iBGP
   • Ignored by: eBGP
   • Description: The Local Preference (Local Pref) attribute is used to influence outbound traffic within an AS. It is not sent to eBGP neighbors. An eBGP neighbor won’t see this attribute because it’s meant for internal path selection.
   • Example: An iBGP router receiving an update with a higher Local Preference will prefer that path, but an eBGP neighbor will not receive or consider the Local Preference attribute.

2. Next-Hop Behavior:

   • Used by: iBGP
   • Modified by: eBGP
   • Description: When advertising routes to iBGP neighbors, the Next Hop is typically not changed. However, when advertising to eBGP neighbors, the next hop is changed to the advertising router's IP address.
   • Example: An iBGP router will pass the route with the original next-hop, while an eBGP neighbor will set its own IP address as the next hop.

Attributes Considered by eBGP Neighbors Only:

These attributes are either propagated or considered only by eBGP neighbors and may be ignored or handled differently by iBGP neighbors:

1. Multi-Exit Discriminator (MED):

   • Used by: eBGP
   • Ignored by: iBGP unless explicitly configured
   • Description: The MED is used to influence inbound traffic by suggesting a preferred path into an AS. By default, MED is not propagated to iBGP neighbors. If not explicitly configured to pass through iBGP peers, only eBGP neighbors will consider this value.
   • Example: When an eBGP neighbor receives a route with a lower MED, it will prefer that route. However, iBGP peers generally don’t propagate the MED attribute.

2. AS Path:

   • Used by: eBGP
   • Ignored by: iBGP (in terms of AS path prepending within the same AS)
   • Description: AS Path length is a key factor for eBGP neighbors in path selection. The AS Path is modified when advertised to eBGP neighbors (the advertising router adds its own AS to the AS Path), but it remains unchanged for iBGP neighbors.
   • Example: eBGP neighbors use the AS Path length to prefer shorter paths, while iBGP peers ignore the internal AS numbers when considering the path within the same AS.

Attributes Considered by Both iBGP and eBGP Neighbors:

These attributes are relevant to both iBGP and eBGP neighbors, although there may be differences in how they are handled:

1. Weight (Cisco-specific):

   • Used by: Both iBGP and eBGP (but only locally significant)
   • Description: The Weight attribute is used for path selection, but it is not propagated to other routers. It is locally significant to the router and influences the router’s decision on the best path.
   • Example: Both iBGP and eBGP routers use the Weight attribute, but it applies only to the router where it is configured.

2. Origin:

   • Used by: Both iBGP and eBGP
   • Description: The Origin attribute helps indicate where a route originated from (IGP, EGP, or Incomplete) and is considered by both iBGP and eBGP neighbors for path selection.
   • Example: Both iBGP and eBGP routers prefer routes with the IGP origin type over EGP or Incomplete.

3. BGP Community:

   • Used by: Both iBGP and eBGP
   • Description: BGP Community attributes are used for tagging routes and are propagated to both iBGP and eBGP neighbors, depending on policies.
   • Example: Both iBGP and eBGP neighbors can act on policies based on community tags, like controlling route redistribution or preference.

4. Next Hop:

   • Used by: Both iBGP and eBGP
   • Description: The Next Hop attribute is considered by both iBGP and eBGP neighbors, but eBGP routers typically change the next hop to themselves when advertising a route.
   • Example: eBGP routers modify the next hop to themselves, while iBGP routers maintain the next hop unless an iBGP peering requires next-hop-self.