Example for Configuring BGP Default Route Advertisement

By controlling the advertising of default routes, you can specify traffic from a specific path to enter ASs.

Networking Requirements

In Figure 1, all routers run BGP. To ensure that the traffic that leaves AS 200 is forwarded by Device E and Device F, EBGP connections are established between Device A and Device B, between Device C and Device E, and between Device D and Device F; IBGP connections are established between Device B and Device C, and between Device B and Device D.

Figure 1 Configuring BGP default route advertisement

Interfaces 1 through 3 in this example are GE 0/1/0, GE 0/1/8, GE 0/1/16, respectively.



Device Name

Interface

IP Address

Device A

GE 0/1/0

10.20.1.1/24

Loopback 0

1.1.1.1/32

Device B

GE 0/1/0

10.20.1.2/24

GE 0/1/8

10.0.1.1/24

GE 0/1/16

10.0.3.2/24

Loopback 0

2.2.2.2/32

Device C

GE 0/1/0

10.20.2.2/24

GE 0/1/8

10.0.1.2/24

GE 0/1/16

10.0.2.1/24

Loopback 0

3.3.3.3/32

Device D

GE 0/1/0

10.20.3.2/24

GE 0/1/8

10.0.3.1/24

GE 0/1/16

10.0.2.2/24

Loopback 0

4.4.4.4/32

Device E

GE 0/1/0

10.20.2.1/24

GE 0/1/8

10.21.7.1.1/24

Loopback 0

5.5.5.5/32

Device F

GE 0/1/0

10.20.3.1/24

GE 0/1/8

10.22.8.1.1/24

Loopback 0

6.6.6.6/32

Precautions

When configuring BGP to advertise default routes, note the following rules:

  • Default routes have two functions. They can represent all network routes. For example, in a stub AS, instead of advertising all network routes, you can use only a default route to forward traffic destined outside the stub AS. In addition, they can represent all routes except specific routes; for example, they can be used in the multi-home load balancing scenario.

  • When establishing a peer relationship, if the specified IP address of the peer is a loopback interface address or a sub-interface address, run the peer connect-interface command on both ends to ensure that the two ends are correctly connected.

Configuration Roadmap

The configuration roadmap is as follows:

  1. Configure OSPF on Device B, Device C, and Device D.

  2. Establish EBGP connections between Device A and Device B, between Device C and Device E, and between Device D and Device F.

  3. Establish IBGP connections between Device B and Device C, and between Device B and Device D.

  4. Configure an import routing policy on Device C to accept only default routes.

  5. Configure an import routing policy on Device D to accept default routes and all specific routes, and then set Local_Pref values for the accepted default routes.

Data Preparation

To complete the configuration, you need the following data:

  • Router IDs and AS numbers of Device A, Device B, Device C, Device D, Device E, and Device F

  • Names of the import routing policies to be configured on Device C and Device D

  • Local_Pref values to be set for the accepted default routes on Device D

Procedure

  1. Configure an IP address for each interface. For configuration details, see Configuration Files in this section.
  2. Configure OSPF.

    # Configure Device B.

    [~DeviceB] ospf 1
[*DeviceB-ospf-1] area 0
[*DeviceB-ospf-1-area-0.0.0.0] network 10.0.1.0 0.0.0.255
[*DeviceB-ospf-1-area-0.0.0.0] network 10.0.3.0 0.0.0.255
[*DeviceB-ospf-1-area-0.0.0.0] network 2.2.2.2 0.0.0.0
[*DeviceB-ospf-1-area-0.0.0.0] commit
[~DeviceB-ospf-1-area-0.0.0.0] quit
[~DeviceB-ospf-1] quit

    # Configure Device C.

    [~DeviceC] ospf 1
[*DeviceC-ospf-1] area 0
[*DeviceC-ospf-1-area-0.0.0.0] network 10.0.1.0 0.0.0.255
[*DeviceC-ospf-1-area-0.0.0.0] network 10.0.2.0 0.0.0.255
[*DeviceC-ospf-1-area-0.0.0.0] network 3.3.3.3 0.0.0.0
[*DeviceC-ospf-1-area-0.0.0.0] commit
[~DeviceC-ospf-1-area-0.0.0.0] quit
[~DeviceC-ospf-1] quit

    # Configure Device D.

    [~DeviceD] ospf 1
[*DeviceD-ospf-1] area 0
[*DeviceD-ospf-1-area-0.0.0.0] network 10.0.2.0 0.0.0.255
[*DeviceD-ospf-1-area-0.0.0.0] network 10.0.3.0 0.0.0.255
[*DeviceD-ospf-1-area-0.0.0.0] network 4.4.4.4 0.0.0.0
[*DeviceD-ospf-1-area-0.0.0.0] commit
[~DeviceD-ospf-1-area-0.0.0.0] quit
[~DeviceD-ospf-1] quit

  3. Configure BGP connections.

    # Configure Device A.

    [~DeviceA] bgp 100
[*DeviceA-bgp] router-id 1.1.1.1
[*DeviceA-bgp] peer 10.20.1.2 as-number 200
[*DeviceA-bgp] commit
[~DeviceA-bgp] quit

    # Configure Device B.

    [~DeviceB] bgp 200
[*DeviceB-bgp] router-id 2.2.2.2
[*DeviceB-bgp] peer 10.20.1.1 as-number 100
[*DeviceB-bgp] network 10.20.1.0 24
[*DeviceB-bgp] peer 3.3.3.3 as-number 200
[*DeviceB-bgp] peer 3.3.3.3 connect-interface LoopBack0
[*DeviceB-bgp] peer 4.4.4.4 as-number 200
[*DeviceB-bgp] peer 4.4.4.4 connect-interface LoopBack0
[*DeviceB-bgp] commit
[~DeviceB-bgp] quit

    # Configure Device C.

    [~DeviceC] bgp 200
[*DeviceC-bgp] router-id 3.3.3.3
[*DeviceC-bgp] peer 10.20.2.1 as-number 300
[*DeviceC-bgp] network 10.20.2.0 24
[*DeviceC-bgp] peer 2.2.2.2 as-number 200
[*DeviceC-bgp] peer 2.2.2.2 connect-interface LoopBack0
[*DeviceC-bgp] commit
[~DeviceC-bgp] quit

    # Configure Device D.

    [~DeviceD] bgp 200
[*DeviceD-bgp] router-id 4.4.4.4
[*DeviceD-bgp] peer 10.20.3.1 as-number 400
[*DeviceD-bgp] network 10.20.3.0 24
[*DeviceD-bgp] peer 2.2.2.2 as-number 200
[*DeviceD-bgp] peer 2.2.2.2 connect-interface LoopBack0
[*DeviceD-bgp] commit
[~DeviceD-bgp] quit

    # Configure Device E.

    [~DeviceE] bgp 300
[*DeviceE-bgp] router-id 5.5.5.5
[*DeviceE-bgp] peer 10.20.2.2 as-number 200
[*DeviceE-bgp] network 10.1.1.0 24
[*DeviceE-bgp] commit
[~DeviceE-bgp] quit

    # Configure Device F.

    [~DeviceF] bgp 400
[*DeviceF-bgp] router-id 6.6.6.6
[*DeviceF-bgp] peer 10.20.3.2 as-number 200
[*DeviceF-bgp] network 10.2.1.0 24
[*DeviceF-bgp] commit
[~DeviceF-bgp] quit

  4. Configure Device E and Device F to advertise default routes.

    # Configure Device E to advertise default routes.

    [~DeviceE-bgp] ipv4-family unicast
[*DeviceE-bgp-af-ipv4] peer 10.20.2.2 default-route-advertise
[*DeviceE-bgp-af-ipv4] commit

    # Configure Device F to advertise default routes.

    [~DeviceF-bgp] ipv4-family unicast
[*DeviceF-bgp-af-ipv4] peer 10.20.3.2 default-route-advertise
[*DeviceF-bgp-af-ipv4] commit

    # Check the routing table of Device B.

    [~DeviceB] display bgp routing-table
 BGP Local router ID is 2.2.2.2
 Status codes: * - valid, > - best, d - damped, x - best external, a - add path,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete
 RPKI validation codes: V - valid, I - invalid, N - not-found

Total Number of Routes: 7
     Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>i  0.0.0.0            10.20.2.1       0          100        0      300i
 * i                     10.20.3.1       0          100        0      400i
 *>i  10.1.1.0/24        10.20.2.1       0          100        0      300i
 *>i  10.2.1.0/24        10.20.3.1       0          100        0      400i
 *>   10.20.1.0          0.0.0.0         0                     0      i
 *>i  10.20.2.0          3.3.3.3         0          100        0      i
 *>i  10.20.3.0          4.4.4.4         0          100        0      i

    The command output shows that Device B has received the default routes and all specific routes of AS 300 and AS 400.

  5. Configure import routing policies.

    # Configure an IP prefix list named default on Device C to accept only default routes.

    [~DeviceC] ip ip-prefix default permit 0.0.0.0 0
[*DeviceC] commit
[*DeviceC] bgp 200
[*DeviceC-bgp] peer 10.20.2.1 ip-prefix default import
[*DeviceC-bgp] commit

    # Configure a route-policy named set-default-low on Device D to accept default routes and all specific routes, and set Local_Pref values for the accepted default routes.

    [~DeviceD] ip as-path-filter 10 permit ^(400_)+$
[*DeviceD] ip as-path-filter 10 permit ^(400_)+_[0-9]+$
[*DeviceD] ip ip-prefix default permit 0.0.0.0 0
[*DeviceD] route-policy set-default-low permit node 10
[*DeviceD-route-policy] if-match ip-prefix default
[*DeviceD-route-policy] apply local-preference 80
[*DeviceD-route-policy] quit
[*DeviceD] route-policy set-default-low permit node 20
[*DeviceD-route-policy] quit
[*DeviceD] commit
[~DeviceD] bgp 200
[*DeviceD-bgp] peer 10.20.3.1 as-path-filter 10 import
[*DeviceD-bgp] peer 10.20.3.1 route-policy set-default-low import
[*DeviceD-bgp] commit

    # Check the routing table of Device B.

    [~DeviceB] display bgp routing-table
 BGP Local router ID is 2.2.2.2
 Status codes: * - valid, > - best, d - damped, x - best external, a - add path,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete
 RPKI validation codes: V - valid, I - invalid, N - not-found

 Total Number of Routes: 6
     Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>i  0.0.0.0            10.20.2.1       0          100        0      300i
 * i                     10.20.3.1       0          80         0      400i
 *>i  10.2.1.0/24        10.20.3.1       0          100        0      400i
 *>   10.20.1.0          0.0.0.0         0                     0      i
 *>i  10.20.2.0          3.3.3.3         0          100        0      i
 *>i  10.20.3.0          4.4.4.4         0          100        0      i

    The command output shows that Device B has received only the default routes of AS 300 and the default routes and all specific routes of AS 400 and that the Local_Pref of the accepted default routes destined of AS 400 has been set to 80.

Configuration Files

  • Device A configuration file

    #
sysname DeviceA
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 10.20.1.1 255.255.255.0
#
 interface LoopBack0
 ip address 1.1.1.1 255.255.255.255
#
bgp 100
 peer 10.20.1.2 as-number 200
 #
 ipv4-family unicast
  peer 10.20.1.2 enable
#
return

  • Device B configuration file

    #
sysname DeviceB
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 10.20.1.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 10.0.1.1 255.255.255.0
#
interface GigabitEthernet0/1/16
 undo shutdown
 ip address 10.0.3.2 255.255.255.0
#
interface LoopBack0
 ip address 2.2.2.2 255.255.255.255
#
bgp 200
 peer 3.3.3.3 as-number 200
 peer 3.3.3.3 connect-interface LoopBack0
 peer 4.4.4.4 as-number 200
 peer 4.4.4.4 connect-interface LoopBack0
 peer 10.20.1.1 as-number 100
 #
 ipv4-family unicast
  network 10.20.1.0 255.255.255.0
  peer 3.3.3.3 enable
  peer 4.4.4.4 enable
  peer 10.20.1.1 enable
#
ospf 1
 area 0.0.0.0
  network 2.2.2.2 0.0.0.0
  network 10.0.1.0 0.0.0.255
  network 10.0.3.0 0.0.0.255
#
return

  • Device C configuration file

    #
sysname DeviceC
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 10.20.2.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 10.0.1.2 255.255.255.0
#
interface GigabitEthernet0/1/16
 undo shutdown
 ip address 10.0.2.1 255.255.255.0
#
interface LoopBack0
 ip address 3.3.3.3 255.255.255.255
#
bgp 200
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 peer 10.20.2.1 as-number 300
 #
 ipv4-family unicast
  network 10.20.2.0 255.255.255.0
  peer 2.2.2.2 enable
  peer 10.20.2.1 enable
  peer 10.20.2.1 ip-prefix default import
#
ospf 1
 area 0.0.0.0
  network 3.3.3.3 0.0.0.0
  network 10.0.1.0 0.0.0.255
  network 10.0.2.0 0.0.0.255
#
ip ip-prefix default index 10 permit 0.0.0.0 0
#
return

  • Device D configuration file

    #
sysname DeviceD
#
interface GigabitEthernet0/1/0
 undo shutdown
  ip address 10.20.3.2 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 10.0.3.1 255.255.255.0
#
interface GigabitEthernet0/1/16
 undo shutdown
 ip address 10.0.2.2 255.255.255.0
#
interface LoopBack0
 ip address 4.4.4.4 255.255.255.255
#
bgp 200
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 peer 10.20.3.1 as-number 400
 #
 ipv4-family unicast
  network 10.20.3.0 255.255.255.0
  peer 2.2.2.2 enable
  peer 10.20.3.1 enable
  peer 10.20.3.1 as-path-filter 10 import
  peer 10.20.3.1 route-policy set-default-low import
#
ospf 1
 area 0.0.0.0
  network 4.4.4.4 0.0.0.0
  network 10.0.2.0 0.0.0.255
  network 10.0.3.0 0.0.0.255
#
route-policy set-default-low permit node 10
 if-match ip-prefix default
 apply local-preference 80
#
route-policy set-default-low permit node 20
#
ip ip-prefix default index 10 permit 0.0.0.0 0
#
ip as-path-filter 10 permit ^(400_)+$
ip as-path-filter 10 permit ^(400_)+_[0-9]+$
#
return

  • Device E configuration file

    #
sysname DeviceE
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 10.20.2.1 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 10.21.7.1.1 255.255.255.0
#
 interface LoopBack0
 ip address 5.5.5.5 255.255.255.255
#
bgp 300
 peer 10.20.2.2 as-number 200
 #
 ipv4-family unicast
  network 10.1.1.0 255.255.255.0
  peer 10.20.2.2 enable
  peer 10.20.2.2 default-route-advertise
#
return

  • Device F configuration file

    #
sysname DeviceF
#
interface GigabitEthernet0/1/0
 undo shutdown
 ip address 10.20.3.1 255.255.255.0
#
interface GigabitEthernet0/1/8
 undo shutdown
 ip address 10.22.8.1.1 255.255.255.0
#
 interface LoopBack0
 ip address 6.6.6.6 255.255.255.255
#
bgp 400
 peer 10.20.3.2 as-number 200
 #
 ipv4-family unicast
  network 10.2.1.0 255.255.255.0
  peer 10.20.3.2 enable
  peer 10.20.3.2 default-route-advertise
#
return
Parent Topic: Configuration Examples for BGP
Copyright © Huawei Technologies Co., Ltd.
Copyright © Huawei Technologies Co., Ltd.
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