1.5b Path Selection

To decide which route is the best, each BGP router has a “best path selection” algorithm, where the information from two similar paths are compared. This is because it is not unusual for a BGP speaker to receive the same route from multiple peers. In fact, this is quite normal when you have multiple upstreams or peers.

Before we cover how the BGP best path selection algorithm works, however, it’s important to know that not all of the received BGP routes are candidates for being selected as the best route. There are many reasons for this, a common one being that the next-hop advertised as an attribute for the route is inaccessible.

1.5b i Attributes

Now, let’s take a look at the factors that govern selecting a best route candidate (and the order they are considered in) using the BGP selection algorithm on a Cisco router:

Priority	Attribute	Description
1	Weight	Prefer the path with the highest weight.  This is a value that is local to the router and is Cisco proprietary. Default weight is 0.
2	Local Preference	The local preference is use within an AS and exchanged between iBGP routers. Highest local preference is preferred, default value is 100.
3	Originate	Prefer the path that the local router originated. In the BGP table you will see next hop 0.0.0.0.  Will prefer routes that are installed into BGP locally, over a route installed into BGP on another router.
4	AS Path Length	Prefer the path with the shortest AS Path Length. E.g. AS Path 1 2 3 is preferred over AS Path 1 2 3 4 5.
5	Origin Code	Prefer the lowest Origin Code.  The three Origin Codes are IGP < EGP < Incomplete
6	MED	Prefer the path with the lowest MED.  The MED is exchanged between autonomous systems.
7	eBGP > iBGP	As it says on the tin, prefer the eBGP path over an iBGP path.
8	Shortest IGP Path	Prefer the path within the AS with the lowest IGP metric to the BGP next-hop.
9	Oldest Path	AS it says on the tin, prefer the path received first (oldest path).
10	Router ID	Prefer the path of the BGP neighbor that has the lowest Router ID.
11	Neighbor IP Address	Prefer the path of the BGP neighbour that has the lowest IP address.

1.5.b ii Best path selection algorithm

In order to find the best route, BGP will go thru each attribute listed above. Higher priority wins.

Details can be found here: BGP Best Path Selection Algorithm

1.5.b iii Load balancing

By default, BGP operates by selecting a single best path to each destination prefix, however Equal Cost Multi-Path Routing is configurable.  In order for this to occur, the following details must match: 

• Weight 
• Local Preference 
• AS Path (Both AS numbers and AS Path Length) 
• Origin Code 
• MED 
• IGP metric 

The next hop address for each path must be different. 

To configure this, use the command maximum-paths <number of paths>.

router bgp 65000
 maximum-paths 2

This is useful if you are doing multipath to the same AS but will not work when your paths are to different AS’. If your multipath is to different AS’ but have the same AS Path Length, we can use an additional command: bgp bestpath as-path multipath-relax, which only checks AS Path Length. 

router bgp 100
 bgp bestpath as-path multipath-relax

BGP unequal cost load sharing

By default, traffic sharing on multipath in BGP is 1:1 or Equal-cost, but just like with EIGRP, we can manipulate this to do unequal-cost multi-path.

The below example is taken from https://aboutnetworks.net/bgp-load-sharing/

Unequal cost load sharing for outgoing traffic from AS-4.

Now we have 20Mbps between AS-4 and AS-2 and 40Mbps between AS-4 and AS3.

For this, on R4 we have first to configure the real bandwidth on the physical interfaces to R2 and R3:

interface GigabitEthernet0/1
 description to R2
 bandwidth 20000
 ip address 10.0.0.14 255.255.255.252
 duplex full
 speed auto
 media-type rj45
!
interface GigabitEthernet0/3
 description to R3
 bandwidth 40000
 ip address 10.0.0.18 255.255.255.252
 duplex full
 speed auto
 media-type rj45!

And then, we need to configure BGP dmzlink-bw globally and for each uplink:

router bgp 4
 bgp log-neighbor-changes
 bgp bestpath as-path multipath-relax
 bgp dmzlink-bw
 network 4.4.4.4 mask 255.255.255.255
 neighbor 10.0.0.13 remote-as 2
 neighbor 10.0.0.13 next-hop-self
 neighbor 10.0.0.13 dmzlink-bw
 neighbor 10.0.0.17 remote-as 3
 neighbor 10.0.0.17 next-hop-self
 neighbor 10.0.0.17 dmzlink-bw
 neighbor 10.0.0.22 remote-as 5
 neighbor 10.0.0.22 next-hop-self
 maximum-paths 4
R4#

Now, let’s look at the routing table for 0.0.0.0/0:

R4#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "bgp 4", distance 20, metric 0, candidate default path
  Tag 2, type external
  Last update from 10.0.0.17 00:03:14 ago
  Routing Descriptor Blocks:
    10.0.0.17, from 10.0.0.17, 00:03:14 ago
      Route metric is 0, traffic share count is 2
      AS Hops 1
      Route tag 2
      MPLS label: none
  * 10.0.0.13, from 10.0.0.13, 00:03:14 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 2
      MPLS label: none
R4#

Now you can see we have a share count 2:1 between the two interfaces, based on the bandwidth configured.