Why You Do That BGP? Best Path Reason Command!

Standard

One of the more frustrating things that comes from studying BGP is that it isn’t always obvious why it is selecting one path over another.  Fortunately, I came across a cool new BGP feature in modern IOS-XE that explains exactly that!

This can be particularly useful for CCIE study while your working through crazy practice labs.

Topology / Setup

bgp_topology_001

In this post we will simply have 4 routers connected in a line with two connections per router. We will also add a few loopbacks on each router to give us some flexibility.

I’m using CSR1000v for my routers and they are on 16.11.1a firmware.

R01

R01(config)#interface GigabitEthernet2.12
R01(config-subif)# encapsulation dot1Q 12
R01(config-subif)# ip address 10.1.2.1 255.255.255.0
R01(config-subif)#interface GigabitEthernet2.21
R01(config-subif)# encapsulation dot1Q 21
R01(config-subif)# ip address 10.2.1.1 255.255.255.0

R01(config)#interface Loopback10
R01(config-if)# ip address 192.168.10.1 255.255.255.0
R01(config-if)#interface Loopback11
R01(config-if)# ip address 192.168.11.1 255.255.255.0
R01(config-if)#interface Loopback12
R01(config-if)# ip address 192.168.12.1 255.255.255.0
R01(config-if)#interface Loopback13
R01(config-if)# ip address 192.168.13.1 255.255.255.0
R01(config-if)#interface Loopback14
R01(config-if)# ip address 192.168.14.1 255.255.255.0
R01(config-if)#interface Loopback15
R01(config-if)# ip address 192.168.15.1 255.255.255.0

R01(config)#router bgp 65101
R01(config-router)#neighbor 10.1.2.2 remote 65102
R01(config-router)#neighbor 10.2.1.2 remote 65102
R01(config-router)# redistribute connected

R02

R02(config)#interface GigabitEthernet2.12
R02(config-subif)# encapsulation dot1Q 12
R02(config-subif)# ip address 10.1.2.2 255.255.255.0
R02(config-subif)#interface GigabitEthernet2.21
R02(config-subif)# encapsulation dot1Q 21
R02(config-subif)# ip address 10.2.1.2 255.255.255.0
R02(config-subif)#interface GigabitEthernet2.23
R02(config-subif)# encapsulation dot1Q 23
R02(config-subif)# ip address 10.2.3.2 255.255.255.0
R02(config-subif)#interface GigabitEthernet2.32
R02(config-subif)# encapsulation dot1Q 32
R02(config-subif)# ip address 10.3.2.2 255.255.255.0

R02(config)#interface Loopback20
R02(config-if)# ip address 192.168.20.1 255.255.255.0
R02(config-if)#interface Loopback21
R02(config-if)# ip address 192.168.21.1 255.255.255.0
R02(config-if)#interface Loopback22
R02(config-if)# ip address 192.168.22.1 255.255.255.0
R02(config-if)#interface Loopback23
R02(config-if)# ip address 192.168.23.1 255.255.255.0
R02(config-if)#interface Loopback24
R02(config-if)# ip address 192.168.24.1 255.255.255.0
R02(config-if)#interface Loopback25
R02(config-if)# ip address 192.168.25.1 255.255.255.0

R02(config)# router bgp 65102
R02(config-router)# bgp log-neighbor-changes
R02(config-router)# neighbor 10.1.2.1 remote-as 65101
R02(config-router)# neighbor 10.2.1.1 remote-as 65101
R02(config-router)# neighbor 10.2.3.3 remote-as 65103
R02(config-router)# neighbor 10.3.2.3 remote-as 65103
R02(config-router)# redistribute connected

R03

R03(config)#interface GigabitEthernet2.23
R03(config-subif)# encapsulation dot1Q 23
R03(config-subif)# ip address 10.2.3.3 255.255.255.0
R03(config-subif)#interface GigabitEthernet2.32
R03(config-subif)# encapsulation dot1Q 32
R03(config-subif)# ip address 10.3.2.3 255.255.255.0
R03(config-subif)#interface GigabitEthernet2.34
R03(config-subif)# encapsulation dot1Q 34
R03(config-subif)# ip address 10.3.4.3 255.255.255.0
R03(config-subif)#interface GigabitEthernet2.43
R03(config-subif)# encapsulation dot1Q 43
R03(config-subif)# ip address 10.4.3.3 255.255.255.0

R03(config)#interface Loopback30
R03(config-if)# ip address 192.168.30.1 255.255.255.0
R03(config-if)#interface Loopback31
R03(config-if)# ip address 192.168.31.1 255.255.255.0
R03(config-if)#interface Loopback32
R03(config-if)# ip address 192.168.32.1 255.255.255.0
R03(config-if)#interface Loopback33
R03(config-if)# ip address 192.168.33.1 255.255.255.0
R03(config-if)#interface Loopback34
R03(config-if)# ip address 192.168.34.1 255.255.255.0
R03(config-if)#interface Loopback35
R03(config-if)# ip address 192.168.35.1 255.255.255.0

R03(config)#router bgp 65103
R03(config-router)# bgp log-neighbor-changes
R03(config-router)# neighbor 10.2.3.2 remote-as 65102
R03(config-router)# neighbor 10.3.2.2 remote-as 65102
R03(config-router)# neighbor 10.3.4.4 remote-as 65104
R03(config-router)# neighbor 10.4.3.4 remote-as 65104
R03(config-router)# redistribute connected

R04

R04(config)#interface GigabitEthernet2.34
R04(config-subif)# encapsulation dot1Q 34
R04(config-subif)# ip address 10.3.4.4 255.255.255.0
R04(config-subif)#interface GigabitEthernet2.43
R04(config-subif)# encapsulation dot1Q 43
R04(config-subif)# ip address 10.4.3.4 255.255.255.0

R04(config)#interface Loopback40
R04(config-if)# ip address 192.168.40.1 255.255.255.0
R04(config-if)#interface Loopback41
R04(config-if)# ip address 192.168.41.1 255.255.255.0
R04(config-if)#interface Loopback42
R04(config-if)# ip address 192.168.42.1 255.255.255.0
R04(config-if)#interface Loopback43
R04(config-if)# ip address 192.168.43.1 255.255.255.0
R04(config-if)#interface Loopback44
R04(config-if)# ip address 192.168.44.1 255.255.255.0
R04(config-if)#interface Loopback45
R04(config-if)# ip address 192.168.45.1 255.255.255.0

R04(config)# router bgp 65104
R04(config-router)# bgp log-neighbor-changes
R04(config-router)# neighbor 10.3.4.3 remote-as 65103
R04(config-router)# neighbor 10.4.3.3 remote-as 65103
R04(config-router)# redistribute connected

A Reminder on BGP Path Selection

BGP has a rather complex path selection, here is a reminder for how it works while we play with some examples.

  1. Prefer the path with the highest WEIGHT.
  2. Prefer the path with the highest Local Preference
  3. Prefer the path that was locally originated via a network or aggregate BGP subcommand or through redistribution from an IGP.
  4. Prefer the path with the shortest AS_PATH.
  5. Prefer the path with the lowest origin type.
  6. Prefer the path with the lowest Multi-Exit Discriminator
  7. Prefer eBGP over iBGP paths.
  8. Prefer the path with the lowest IGP metric to the BGP next hop.
  9. Determine if multiple paths require installation in the routing table for BGP Multipath. Continue, if bestpath is not yet selected.
  10. When both paths are external, prefer the path that was received first (the oldest one).
  11. Prefer the route that comes from the BGP router with the lowest router ID.
  12. If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.
  13. Prefer the path that comes from the lowest neighbor address.

The Fun Stuff

Let’s have a look at the BGP table and focus on the 192.168.4x.0/24 routes

R01#show bgp ipv4 unicast 192.168.40.0/21 longer-prefixes 
BGP table version is 42, local router ID is 192.168.255.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
x best-external, a additional-path, c RIB-compressed, 
t secondary path, L long-lived-stale,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

Network Next Hop Metric LocPrf Weight Path
* 192.168.40.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?
* 192.168.41.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?
* 192.168.42.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?
* 192.168.43.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?
* 192.168.44.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?
* 192.168.45.0 10.2.1.2 0 65102 65103 65104 ?
*> 10.1.2.2 0 65102 65103 65104 ?

To use the fancy best-path-reason feature, we simply need to add it to the end of the bgp command, yes you can also use it if you are still using show ip bgp!

To try this out we’ll look at the 192.168.40.0/24 route

We can see that 10.1.2.2 is the best path, if we look at the 10.1.2.1 we can see it lost because the  route had a higher peer address which is the 13th step in the path selection.

R01#show bgp ipv4 unicast 192.168.40.0/24 best-path-reason 
BGP routing table entry for 192.168.40.0/24, version 33
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 1
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 05:35:52 UTC
Best Path Evaluation: Higher peer address
Refresh Epoch 1
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 05:35:52 UTC
Best Path Evaluation: Overall best path

We’ll work through a few examples so we can see how the feature works!

Weight

We might as well start with the HEAVIEST metric (not sorry!)

This will require a route-map that matches a loopback, I’ll use 192.168.40.0/24 and apply it to the 10.1.2.2 neighbor on R01

R01(config)#ip prefix-list PL_NET40 permit 192.168.40.0/24

R01(config)#route-map RM_BGP_IN permit 
R01(config-route-map)#match ip address prefix PL_NET40
R01(config-route-map)#set weight 500
R01(config-route-map)#route-map RM_BGP_IN permit 20 
R01(config-route-map)#exit

R01(config)#router bgp 65101
R01(config-router)#neighbor 10.1.2.2 route-map RM_BGP_IN in
R01(config-router)#end
R01#
*Jun 4 06:04:57.876: %SYS-5-CONFIG_I: Configured from console by admin on vty0 (192.168.106.1)
R01#clear ip bgp * soft

Now when we look at the BGP output again, we can see that 10.2.1.2 is losing because it has a lower weight!

R01#show bgp ipv4 unicast 192.168.40.0/24 best-path-reason
BGP routing table entry for 192.168.40.0/24, version 43
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 2
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:05:26 UTC
Best Path Evaluation: Lower weight
Refresh Epoch 2
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, localpref 100, weight 500, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:05:26 UTC
Best Path Evaluation: Overall best path

Local Preference

Next up is my personal preference (ok sorry!)

R01(config)#ip prefix-list PL_NET_41 permit 192.168.41.0/24 

R01(config)#no route-map RM_BGP_IN

R01(config)#route-map RM_BGP_IN permit 10
R01(config-route-map)#match ip address prefix PL_NET_41
R01(config-route-map)#set local
R01(config-route-map)#set local-preference 5000
R01(config-route-map)#route-map RM_BGP_IN permit 20 
R01(config-route-map)#exit

R01(config)#do clear ip bgp * soft
R01(config)#^Z
R01#
*Jun 4 06:16:10.026: %SYS-5-CONFIG_I: Configured from console by admin on vty0 (192.168.106.1)

This time we can see 10.2.1.2 is losing because of a lower local preference.

R01#show bgp ipv4 unicast 192.168.41.0/24 best-path-reason
BGP routing table entry for 192.168.41.0/24, version 45
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 3
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:16:05 UTC
Best Path Evaluation: Lower local preference
Refresh Epoch 4
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, localpref 5000, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:16:05 UTC
Best Path Evaluation: Overall best path

Origin

Maybe I should have done this one first!

R01(config)#ip prefix-list PL_NET_43 permit 192.168.43.0/24 
R01(config)#no route-map RM_BGP_IN
R01(config)#route-map RM_BGP_IN 
R01(config-route-map)#match ip address prefix-list PL_NET_43 
R01(config-route-map)# set origin igp
R01(config-route-map)#route-map RM_BGP_IN permit 20 
R01(config-route-map)#do clear ip bgp * soft

This time round, 10.2.1.2 loses because the origin is incomplete since I redistributed connected routes.

R01(config-route-map)#do show bgp ipv4 unicast 192.168.43.0/24 best-path-reason
BGP routing table entry for 192.168.43.0/24, version 52
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 13
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:30:54 UTC
Best Path Evaluation: Less favorable origin
Refresh Epoch 18
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin IGP, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:30:54 UTC
Best Path Evaluation: Overall best path

AS-Path

At this point we are on the path to working through all the steps!

R01(config)#ip prefix-list PL_NET_42 seq 5 permit 192.168.42.0/24
R01(config)#no route-map RM_BGP_IN 
R01(config)#route-map RM_BGP_IN 
R01(config-route-map)#match ip address prefix-list PL_NET_42
R01(config-route-map)#set as-path prepend last-as 3
R01(config)#route-map RM_BGP_IN permit 20
R01(config-route-map)#do clear ip bgp * soft

This time it loses because of the longer AS path.

R01(config-route-map)#do show bgp ipv4 unicast 192.168.42.0/24 best-path-reason
BGP routing table entry for 192.168.42.0/24, version 76
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 21
65102 65102 65102 65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:43:20 UTC
Best Path Evaluation: Longer AS path
Refresh Epoch 14
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:43:20 UTC
Best Path Evaluation: Overall best path

MED

I’m gonna have to go on meds after all these puns!

R01(config)#ip prefix-list PL_NET_44 seq 5 permit 192.168.44.0/24 
R01(config)#no route-map RM_BGP_IN
R01(config)#route-map RM_BGP_IN 
R01(config-route-map)#match ip address prefix PL_NET_44
R01(config-route-map)#set metric 50
R01(config-route-map)#route-map RM_BGP_IN permit 20 
R01(config-route-map)#do clear ip bgp * soft

By default, a missing metric is the best metric so us adding a low metric in the route-map actually did the opposite of what you might have expected.

R01(config-route-map)#do show bgp ipv4 unicast 192.168.44.0/24 best-path-reason
BGP routing table entry for 192.168.44.0/24, version 109
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1 
Refresh Epoch 24
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, metric 50, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:50:05 UTC
Best Path Evaluation: Higher MED
Refresh Epoch 15
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:44:04 UTC
Best Path Evaluation: Overall best path

We can flip it around by setting med to consider no value as the worst metric.

R01(config-route-map)#router bgp 65101
R01(config-router)#bgp bestpath med missing-as-worst 
R01(config-router)#
R01(config-router)#do show bgp ipv4 unicast 192.168.44.0/24 best-path-reason
BGP routing table entry for 192.168.44.0/24, version 156
BGP Bestpath: med
Paths: (2 available, best #1, table default)
Advertised to update-groups:
1 
Refresh Epoch 25
65102 65103 65104
10.1.2.2 from 10.1.2.2 (192.168.255.2)
Origin incomplete, metric 50, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Updated on Jun 4 2019 06:50:47 UTC
Best Path Evaluation: Overall best path
Refresh Epoch 16
65102 65103 65104
10.2.1.2 from 10.2.1.2 (192.168.255.2)
Origin incomplete, metric 4294967295, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Updated on Jun 4 2019 06:50:47 UTC
Best Path Evaluation: Higher MED

The Rest?

After MED, BGP really just wants to pick a winner such as the peer IP we saw in the first example.

Wrapping up

For this post we looked at the best-path-reason feature and it is pretty neat!

 

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