corey_costello's profile

27 Messages

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410 Points

Mon, Jan 11, 2021 9:05 PM

How do I check for ECMP with ebgp?

Hi all, 

I've enabled ip load sharing globally as well as maximum-paths in ebgp itself. 

Checking my route table, I see only one route to my destination. 

If I disable the ports, the secondary route does insert itself in the table. 

Is this expected behavior for ECMP over ebgp? 

I would think that the route table would show both hops for a destination with the same MED or weight, etc.

27 Messages

 • 

410 Points

1 y ago

So, oddly enough.

 Removing

- maximum-paths ebgp 2 from router bgp

and enabling ip-load sharing 2

with maximum paths use-load-sharing works.

maybe a bug.

Official Rep

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210 Messages

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3.1K Points

1 y ago

Hi Corey,

IP load sharing kicks in when IP route table contains more than one path to a destination and the paths each have the lowest cost.
IP load sharing uses a hashing algorithm based on the source IP address, destination IP address, and protocol field in the IP header, TCP, and UDP information to distribute the traffic.
In BGP, When multiple paths to the same destination is recieved, BGP selects the best path based on a list of attributes. This is to have granular control over how you forward your traffic and to which autonomous systems instead of just going for the shortest path based on a metric. Only the best path gets into the routing table and remaining will show up in the bgp table.

(edited)

Official Rep

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210 Messages

 • 

3.1K Points

1 y ago

The best path is selected based on following attributes and priority.

Priority    Attribute
1              Weight
2              Local Preference
3              Originate
4              AS path length
5              Origin code
6              MED
7              eBGP path over iBGP path
8              Shortest IGP path to BGP next hop
9              Oldest path
10            Router ID
11            Neighbor IP address

(edited)

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