Technical Tip: BGP : 'allowas-in-enable' or 'as-override' when local AS in the AS-path conflicts with the received prefix

[ddabhade]

Description
This article explains how to configure 'allowas-in-enable' or 'as-override' when using MPLS with the same AS in different locations
to avoid routing loops.

e.g. Using AS 65001 at location A and B

If the route advertised by the Location A is rejected by the location B because of the AS path the route from location B will be rejected by the location A.

This can be avoided by configuring 'allowas-in-enable' or 'as-override'.

Solution
BGP Config and route advertisement:

Location A:

#config router bgp

    set as 65001
    set router-id 10.109.19.146
    config neighbor
        edit "10.109.16.172"                             <----- ISP IP
            set soft-reconfiguration enable
            set remote-as 1111
        next
    end
    config redistribute "connected"
        set status enable
    end

Route advertise by Location A to ISP

#Location A # get  router info  bgp neighbors 10.109.16.172 advertised-routes
BGP table version is 2, local router ID is 10.109.19.146
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight RouteTag Path
*> 10.109.16.0/20   10.109.19.146                      32768        0 ?
*> 10.201.0.0/20    10.109.19.146                      32768        0 ?
*> 10.205.0.0/20    10.109.19.146                      32768        0 ?

ISP

#config router bgp
    set as 1111
    config neighbor
        edit "10.109.19.146"                             <----- Location A
            set soft-reconfiguration enable
            set remote-as 65001
        next
        edit "10.109.16.73"                              <----- Location B
            set soft-reconfiguration enable
            set remote-as 65001
        next
    end

Here ISP is advertising the route to the Location B

#ISP # get router info bgp  neighbors 10.109.16.73 advertised-routes
BGP table version is 4, local router ID is 10.201.0.172
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight RouteTag Path
*> 10.109.16.0/20   10.109.19.146                          0        0 65001 ?
*> 10.201.0.0/20    10.109.19.146                          0        0 65001 ?
*> 10.205.0.0/20    10.109.19.146                          0        0 65001 ?

Total number of prefixes 3

Location B

config router bgp
    set as 65001
    config neighbor
        edit "10.109.16.172"                             <----- ISP
            set soft-reconfiguration enable
            set remote-as 1111
        next
    end     

The route advertise is visible by the ISP, but route is not installed in the routing table of location B.

#Location B #  get  router info bgp  neighbors 10.109.16.172 received-route
BGP table version is 1, local router ID is 192.168.100.99
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight RouteTag Path
*> 10.109.16.0/20   10.109.19.146                          0        0 1111 65001 ?
*> 10.201.0.0/20    10.109.19.146                          0        0 1111 65001 ?
*> 10.205.0.0/20    10.109.19.146                          0        0 1111 65001

Total number of prefixes 3

In routing table of location B route is not installed

#Location B # get  router info routing-table details

Routing table for VRF=0
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default

S*      0.0.0.0/0 [10/0] via 10.109.31.254, port1
                  [10/0] via 10.109.63.254, mgmt
C       10.109.16.0/20 is directly connected, port1

SOLUTION:

There are two solutions to overcome this problem:

1) Configure the 'allowas-in-enable' in the bgp config of Location B.

Location B#

#config router bgp
    set as 65001
    config neighbor
        edit "10.109.16.172"                            <----- ISP
            set allowas-in-enable enable   
            set soft-reconfiguration enable
            set remote-as 1111
        next
    end

#Location B # get  router  info routing-table details
Routing table for VRF=0
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default

S*      0.0.0.0/0 [10/0] via 10.109.31.254, port1
                  [10/0] via 10.109.63.254, mgmt
C       10.109.16.0/20 is directly connected, port1
B       10.201.0.0/20 [20/0] via 10.109.19.146, port1, 00:00:38
B       10.205.0.0/20 [20/0] via 10.109.19.146, port1, 00:00:38

#Location B # get  router info bgp network 10.205.0.0/20
BGP routing table entry for 10.205.0.0/20
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  1111 65001
    10.109.19.146 from 10.109.16.172 (10.201.0.172)
      Origin incomplete metric 0, localpref 100, valid, external, best
      Last update: Sun Oct  6 13:13:19 2019

2) Enable the 'as-override' in the bgp config of ISP.

ISP#

#config router bgp
    set as 1111
    config neighbor
        edit "10.109.19.146"
            set soft-reconfiguration enable
            set as-override enable
            set remote-as 65001
        next
        edit "10.109.16.73"
            set soft-reconfiguration enable
            set as-override enable
            set remote-as 65001
        next
    end

After enabling the as-override, the routes are available in the routing table of Location B

#Location B # get  router  info routing-table details

Routing table for VRF=0
Codes: K - kernel, C - connected, S - static, R - RIP, B - BGP
       O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default

S*      0.0.0.0/0 [10/0] via 10.109.31.254, port1
C       10.109.16.0/20 is directly connected, port1
B       10.201.0.0/20 [20/0] via 10.109.19.146, port1, 00:02:23
B       10.205.0.0/20 [20/0] via 10.109.19.146, port1, 00:02:23

#Location B # get  router  info bgp network 10.201.0.0/20
BGP routing table entry for 10.201.0.0/20
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  1111 1111
    10.109.19.146 from 10.109.16.172 (10.201.0.172)
      Origin incomplete metric 0, localpref 100, valid, external, best
      Last update: Sun Oct  6 14:25:57 2019

Note: After enabling the 'allowas-in-enable' or 'as-override' bgp neighbor gets down and came up.