Using BFD to reduce BGP convergence

Overview

Given the following topology:

By enabling bfd on an eBGP session between the edge-2 and isp-1 routers, BGP convergence (route withdrawl) on a failed link is reduced from 150 seconds to less then 1 second.

Details

To automate counting the number of seconds before the routes are withdrawn, I run a tcl script (because Cisco only supports TCL scripting even though it is 2021) on the router. Run the script by typing tclsh and pasting the script below:

set x 0
while "1" {
    set x [expr {$x + 1}]
    puts "x is $x"
    set results [show ip route 1.1.243.1]
    puts "$results"
    after 1000
}

Configuration

Enable BFD on the edge-2 interface to isp-1. Note, BFD must be configured on a BGP neighbor (see below) before it will establish an active session with the neighbor:

interface GigabitEthernet4
 description to isp1 gi1
 ip address 192.168.0.0 255.255.255.254
 negotiation auto
 bfd interval 50 min_rx 50 multiplier 5
 no mop enabled
 no mop sysid
end

Enable BFD in the BGP instance on edge-2:

router bgp 65000
 bgp router-id interface Loopback0
 bgp log-neighbor-changes
 neighbor 192.168.0.1 remote-as 65200
 neighbor 192.168.0.1 fall-over bfd
 !
 address-family ipv4
  network 192.168.0.0 mask 255.255.0.0
  neighbor 192.168.0.1 activate
 exit-address-family

Enable BFD on the isp-1 interface to edge-2. Note, BFD must be configured on a BGP neighbor (see below) before it will establish an active session with the neighbor:

interface GigabitEthernet1
 description to GigabitEthernet4.edge-2
 ip address 192.168.0.1 255.255.255.254
 shutdown
 negotiation auto
 bfd interval 50 min_rx 50 multiplier 5
 no mop enabled
 no mop sysid
end

Enable BFD in the BGP instance on isp-1:

router bgp 65200
 bgp router-id interface Loopback0
 bgp log-neighbor-changes
 neighbor 172.16.200.2 remote-as 65300
 neighbor 192.168.0.0 remote-as 65000
 neighbor 192.168.0.0 fall-over bfd
 !
 address-family ipv4
  network 172.16.0.0 mask 255.240.0.0
  neighbor 172.16.200.2 activate
  neighbor 192.168.0.0 activate
  neighbor 192.168.0.0 default-originate
 exit-address-family

Testing / Validation

Validate a bfd session is established between the routers:

edge-2#show bfd neighbors 

IPv4 Sessions
NeighAddr                              LD/RD         RH/RS     State     Int
192.168.0.1                          4097/4097       Up        Up        Gi4

Run the test script, then shut the gi2 interface on isp-1:

edge-2#tclsh
edge-2(tcl)#set x 0
0
edge-2(tcl)#while "1" {
+>    set x [expr {$x + 1}]
+>    puts "x is $x"
+>    set results [show ip route 1.1.243.1]
+>    puts "$results"
+>    after 1000
+>}
x is 1

Routing entry for 1.1.243.0/24
  Known via "bgp 65000", distance 200, metric 0
  Tag 65200, type internal
  Last update from 192.168.0.3 00:00:03 ago
  Routing Descriptor Blocks:
  * 192.168.0.3, from 192.168.200.1, 00:00:03 ago
      Route metric is 0, traffic share count is 1
      AS Hops 5
      Route tag 65200
      MPLS label: none
x is 2

% Network not in table

Summary

In a production environments run bfd on all links.