Configure BGP to Advertise a Default Route on Nexus Switches

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Updated:June 15, 2023
Document ID:213952

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    Introduction

    This document describes the configuration Border Gateway Protocol to advertise Default Route to BGP neighbors on Cisco Nexus NX-OS based Switches.

    Prerequisites

    Requirements

    Cisco recommends prior knowledge of these topics:

    • Nexus NX-OS Software.
    • Routing Protocols, specifically Border Gateway Protocol (BGP).

    Components Used

    The information in this document is based on Cisco Nexus 7000 with NX-OS version 7.3(0)D1(1).

    The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

    Configure

    Network Command

    The network 0.0.0.0/0 command injects the default route in the BGP RIB (BGP Routing Information Base).

    The prerequisite is to have the default route in the Routing Table via any other Routing Protocol or manually configured with a Static Route.

    Once in the BGP RIB, the default route is advertised to all BGP neighbors unless specifically denied by an outbound filter configured per neighbor.


    BGP configuration as seen in the show running-config output.

    Nexus BGP Configuration 
    Nexus# show running-config bgp

!Command: show running-config bgp
!Time: Tue Dec  4 01:27:43 2018

version 7.3(0)D1(1)
feature bgp

router bgp 64512
  address-family ipv4 unicast
    network 0.0.0.0/0
  neighbor 10.1.3.3
    remote-as 64512
    address-family ipv4 unicast


    In this example, Nexus receives the default route from EIGRP protocol. Therefore, the Nexus Routing Table shows the default route is available via EIGRP.

    EIGRP route to BGP

    Nexus Routing Table 
    Nexus# show ip route 0.0.0.0
    IP Route Table for VRF "default"
    '*' denotes best ucast next-hop
    '**' denotes best mcast next-hop
    '[x/y]' denotes [preference/metric]
    '%<string>' in via output denotes VRF <string>

    0.0.0.0/0, ubest/mbest: 1/0
        *via 10.1.2.2, Eth2/1, [170/2816], 00:00:50, eigrp-1, external
    Nexus#


    BGP RIB shows 0.0.0.0/0 as valid and best path since prerequisites are met. Default route is in the Routing Table, and the network 0.0.0.0/0 command is configured in BGP configuration section.

    Nexus BGP RIB (BGP table) 
    Nexus# show bgp ipv4 unicast
BGP routing table information for VRF default, address family IPv4 Unicast
BGP table version is 13, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>l0.0.0.0/0          0.0.0.0                           100      32768 i


    The advertised-routes parameter shows the default-route is advertised to BGP peer 10.1.3.3

    Nexus BGP Routes Advertised to a Specific BGP Peer 
    Nexus# show bgp ipv4 unicast neighbors 10.1.3.3 advertised-routes 

Peer 10.1.3.3 routes for address family IPv4 Unicast:
BGP table version is 13, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>l0.0.0.0/0          0.0.0.0                           100      32768 i


    Redistribute and Default-Information Originate Commands

    The redistribute command configured under the BGP process injects all the routes that exist in the Routing Table by a specific source routing protocol (and permitted by a route-map) in the BGP RIB with the exception of the default route.

    To additionally allow the default route to be installed from the source routing protocol in the BGP RIB, the command default-information originate is required.

    Once in the BGP RIB, the default route is advertised to all BGP neighbors unless specifically denied by an outbound filter configured per neighbor.


    BGP configuration section shows redistribute EIGRP and default-information originate in place.

    Nexus BGP Configuration 
    Nexus# show running-config bgp

!Command: show running-config bgp
!Time: Tue Dec  4 01:33:41 2018

version 7.3(0)D1(1)
feature bgp

router bgp 64512
  address-family ipv4 unicast
    redistribute eigrp 1 route-map PERMIT-ALL
    default-information originate
  neighbor 10.1.3.3
    remote-as 64512
    address-family ipv4 unicast

    A route map with no match entry permits all routes.

route-map PERMIT-ALL permit 10


    In this example, Nexus receives the default route from EIGRP protocol. Therefore, the Nexus Routing Table shows the default route is available via EIGRP.

    Nexus Routing Table 
    Nexus# show ip route eigrp
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

0.0.0.0/0, ubest/mbest: 1/0
    *via 10.1.2.2, Eth2/1, [170/2816], 00:07:19, eigrp-1, external
192.168.2.0/24, ubest/mbest: 1/0
    *via 10.1.2.2, Eth2/1, [170/2816], 00:00:07, eigrp-1, external


    BGP RIB shows 0.0.0.0/0 as valid and the best path since prerequisites are met. The default route has been redistributed from source routing protocol in BGP, and default-information originate command is configured in BGP configuration section.

    Nexus BGP RIB (BGP table) 
    Nexus# show bgp ipv4 unicast 
BGP routing table information for VRF default, address family IPv4 Unicast
BGP table version is 17, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>r0.0.0.0/0          0.0.0.0               2816        100      32768 ?
*>r192.168.2.0/24     0.0.0.0               2816        100      32768 ?


    The advertised-routes parameter shows the default-route is advertised to BGP peer 10.1.3.3

    Nexus BGP Routes Advertised to a Specific BGP Peer 
    Nexus# show bgp ipv4 unicast neighbors 10.1.3.3 advertised-routes 

Peer 10.1.3.3 routes for address family IPv4 Unicast:
BGP table version is 17, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>r0.0.0.0/0          0.0.0.0               2816        100      32768 ?
*>r192.168.2.0/24     0.0.0.0               2816        100      32768 ?


    In the absence of the default-information originate command from BGP configuration, the BGP RIB does not install the default route.

    BGP configuration section next shows only redistribute EIGRP in place.

    Nexus BGP Configuration 
    Nexus# show run bgp

!Command: show running-config bgp
!Time: Tue Dec  4 01:39:30 2018

version 7.3(0)D1(1)
feature bgp

router bgp 64512
  address-family ipv4 unicast
    redistribute eigrp 1 route-map PERMIT-ALL
  neighbor 10.1.3.3
    remote-as 64512
    address-family ipv4 unicast

    A route-map with no match entry permits all routes.

    route-map PERMIT-ALL permit 10


    Routing Table output shows the default route is available from EIGRP protocol.

    Nexus Routing Table 
    Nexus# show ip route eigrp
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

0.0.0.0/0, ubest/mbest: 1/0
    *via 10.1.2.2, Eth2/1, [170/2816], 00:07:19, eigrp-1, external
192.168.2.0/24, ubest/mbest: 1/0
    *via 10.1.2.2, Eth2/1, [170/2816], 00:00:07, eigrp-1, external


    With no default-information originate command, BGP RIB does not install the 0.0.0.0/0 network prefix.

    Nexus BGP RIB (BGP table) 
    Nexus# show bgp ipv4 unicast 
BGP routing table information for VRF default, address family IPv4 Unicast
BGP table version is 18, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>r192.168.2.0/24     0.0.0.0               2816        100      32768 ?


    Note: Before NX-OS version 6.2(2) for Nexus 7000 series and version 6.0(2)N3(1) for Nexus 5000 series, the command default-information originate was not required. Default route used to be also injected in BGP RIB only by the redistribute command.



    Default-Originate Command

    The default-originate command is configured on a per BGP neighbor basis. This command artificially generates and advertises a default route only to the specific BGP peer.

    The default route does not need to exist in the Routing Table, and it is not created in the BGP RIB.

    The default-originate command is configured on a per neighbor basis.

    Nexus BGP Configuration 
    Nexus# show running-config bgp

    !Command: show running-config bgp
    !Time: Tue Dec  4 02:22:43 2018

    version 7.3(0)D1(1)
    feature bgp

    router bgp 64512
      address-family ipv4 unicast
        network 192.168.1.0/24
      neighbor 10.1.3.3
        remote-as 64512
        address-family ipv4 unicast
          default-originate


    There is no need to have a default route in the Routing Table.

    Nexus Routing Table 
    Nexus# show ip route 0.0.0.0
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

Nexus#


    No 0.0.0.0/0 entry is created in the BGP table.

    Nexus BGP RIB (BGP table) 
    Nexus# show bgp ipv4 unicast 
BGP routing table information for VRF default, address family IPv4 Unicast
BGP table version is 20, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>l192.168.1.0/24     0.0.0.0                           100      32768 i


    The entry Originating default network 0.0.0.0/0 can be found in the routes advertised to BGP peer 10.1.3.3

    Nexus BGP Routes Advertised to a Specific BGP Peer 
    Nexus# show bgp ipv4 unicast neighbors 10.1.3.3 advertised-routes 

Peer 10.1.3.3 routes for address family IPv4 Unicast:
BGP table version is 20, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

Originating default network 0.0.0.0/0

   Network            Next Hop            Metric     LocPrf     Weight Path
*>l192.168.1.0/24     0.0.0.0                           100      32768 i


    Optionally, the default-originate command can use a route-map as a parameter to conditionally advertise the default-route.

    Optional default-originate route-map 
    Nexus(config-router-neighbor-af)# default-originate ?
  <CR>       
  route-map  Route-map to specify criteria for originating default


     The route-map needs to match a route installed in the Routing Table in order to make default-originate advertise the default-route to the BGP peer.

    Nexus Routing Table 
    Nexus# show ip route 192.168.3.0
IP Route Table for VRF "default"
'*' denotes best ucast next-hop
'**' denotes best mcast next-hop
'[x/y]' denotes [preference/metric]
'%<string>' in via output denotes VRF <string>

Route not found


    In this case, the route-map named ADVERTISE-DEFAULT-IF references a prefix-list named NETWORKS and the latter 192.168.3.0/24 network prefix, which must exist in the Routing Table in order to advertise the default-route.

    Nexus BGP Configuration 
    ip prefix-list NETWORKS seq 5 permit 192.168.3.0/24
    !
route-map ADVERTISE-DEFAULT-IF permit 10
  match ip address prefix-list NETWORKS
    !
    router bgp 64512
      address-family ipv4 unicast
        network 192.168.1.0/24
      neighbor 10.1.3.3
        remote-as 64512
        address-family ipv4 unicast
          default-originate route-map ADVERTISE-DEFAULT-IF


    Since 192.168.3.0/24 is not in the Routing Table, the default-route is not advertised.

    Nexus BGP Routes Advertised to a Specific BGP Peer 
    Nexus# show bgp ipv4 unicast neighbors 10.1.3.3 advertised-routes 

Peer 10.1.3.3 routes for address family IPv4 Unicast:
BGP table version is 20, local router ID is 10.255.255.1
Status: s-suppressed, x-deleted, S-stale, d-dampened, h-history, *-valid, >-best
Path type: i-internal, e-external, c-confed, l-local, a-aggregate, r-redist, I-i
njected
Origin codes: i - IGP, e - EGP, ? - incomplete, | - multipath, & - backup

   Network            Next Hop            Metric     LocPrf     Weight Path
*>l192.168.1.0/24     0.0.0.0                           100      32768 i

    Revision History

    Revision Publish Date Comments
    2.0
    15-Jun-2023
    Edited title Edited introduction section Updated formatting to current style guide standards Added alt text to figure
    1.0
    06-Dec-2018
    Initial Release
    TAC Authored

    Contributed by Cisco Engineers

    • Hector Gustavo Serrano Gutierrez
      Cisco TAC Engineer
    • kradubey
      Specialist Level I

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