Skip to main content

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.

Comments

Popular posts from this blog

How to import Putty Saved Connections to mRemoteNG

Just started using mRemoteNG and its being very cool to connect to different remote connection with different protocols e.g Window Remote Desktop, VNC to Linux, SSH, HTTP connection etc. from a single application. As new user I configured some remote desktop connection which was quite easy to figure out. But when I wanted to add SSH connections, it came in my mind to import all of the saved connections in the putty. But I couldn't figure it out how can it be done, though it was quite easy and here are the steps. Open your mRemoteNG Create a folder if you want segregation of multiple networks Create a new connection Enter the IP address of remote server under connection in Config pane Under the config pane, select protocol " SSH version 2 ".  Once you select protocol to SSH version 2 you are given option to import putty sessions, as shown in the snap below. In the above snap, I have imported CSR-AWS session from my saved sessions in Putty.

BGP Local Preference Controlling Outbound Traffic in BGP

In BGP, Local Preference is used to control the outbound traffic path. It helps you decide which egress point (exit point) should be used when you have multiple connections to external networks, such as ISPs. Local Preference is an attribute that is local to your AS and is shared with all iBGP peers but not with eBGP neighbors. Higher Local Preference = More preferred outbound path. Example Scenario : You have two external links: ISP1 (via CE1) and ISP2 (via CE2). You want traffic to prefer ISP1 for all outbound traffic. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP2): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Higher Local Preference) : Create a route map to set the local preference to 200 for routes learned from CE1: route-map SET_LOCAL_PREF permit 10 set local-preference 200 In the BGP configuration for CE1, apply this route map to the neighbor: router bgp 65001 ne...

BGP MED: Managing Inbound Traffic with Multi-Exit Discriminator

The Multi-Exit Discriminator (MED) is used in BGP to control inbound traffic into your AS. It tells a neighboring AS which entry point into your network it should prefer when there are multiple links between your AS and the neighboring AS. The lower the MED value , the more preferred the path. MED is only honored between the same neighboring AS . Example Scenario : You are connected to ISP1 via two routers, CE1 and CE2 , and want to control which router ISP1 uses to send traffic into your AS. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP1): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Lower MED, More Preferred) : Create a route map to set the MED to 50 for CE1: route-map SET_MED permit 10 set metric 50 Apply this route map to the neighbor in the BGP configuration for CE1: router bgp 65001 neighbor 10.0.1.1 remote-as 65000 neighbor 10.0.1.1 route-map SET_MED out Configuratio...