Overview

In a traditional Layer 3 network, an IP address should be assigned to each interface to provide connectivity from one link endpoint to another. But in the data center network, with a large number of spine and leaf devices and the number of interfaces interconnecting them could be very large. This leads to a sharp increase in the number of IP addresses required. Such large networks can consume a lot of IP addresses, where each peer requires a separate IP address.

BGP unnumbered is very useful in this kind of scenarios. BGP unnumbered interface is a BGP interface that does not need to specify a unique IP address, it uses the IPv6 link-local address, which is assigned automatically to each interface. BGP unnumbered works by using the Extended Next Hop Encoding (ENHE) as defined in RFC 5549, which provides a way to advertise an IPv4 route with an IPv6 next-hop. The IPv6 next-hop is resolved by using the IPv6 Neighbor Discovery (ND) process. Prior to RFC 5549, an IPv4 route could only be advertised with an IPv4 next-hop.

For BGP unnumbered, the next-hop address for each prefix is an IPv6 link-local address, which is assigned automatically to each interface. Using the IPv6 link-local address as a next-hop instead of an IPv4 unicast address, BGP unnumbered saves you from having to configure IPv4 addresses on each interface.

NOTEs:

BGP can only be used on two switches with point-to-point connection.
BGP unnumbered cannot be used in EVPN underlay.

Configuration Example

Networking Requirements

Figure 1. BGP Unnumbered Configuration Example

As shown in Figure 1, Switch A and Switch B are BGP peers. An EBGP connection is established between Switch A and Switch B. To save IP addresses, BGP unnumbered can be configured between Switch A and Switch B. In this example, routed interfaces are configured for the use of BGP unnumbered interface.

The only difference between a BGP unnumbered configuration and the BGP numbered configuration is that the BGP neighbor is specified as an interface instead of an IP address. The interface between the two peers does not need to have an IP address configured on each side.

NOTE:

The ebgp-requires-policy function needs to be disabled in EBGP, but not in IBGP.

Procedure

Switch A

Step 1 Enable te-1/1/1 as a routed interface and configure the interface name as r1.

admin@SwitchA# set interface gigabit-ethernet te-1/1/1 routed-interface enable true
admin@SwitchA# set interface gigabit-ethernet te-1/1/1 routed-interface name r1
admin@SwitchA# set vlans reserved-vlan 80-90
admin@SwitchA# set l3-interface routed-interface r1

Step 2 Configure EBGP and disable ebgp-requires-policy policy.

admin@SwitchA# set protocols bgp local-as 65001
admin@SwitchA# set protocols bgp router-id 49.49.49.49
admin@SwitchA# set protocols bgp interface r1 remote-as 65002
admin@SwitchA# set protocols bgp interface r1 ipv6-unicast activate true
admin@SwitchA# set protocols bgp interface r1 capability extended-nexthop
admin@SwitchA# set protocols bgp ebgp-requires-policy false

Step 3 Enable IP routing.

admin@SwitchA# set ip routing enable true

Step 4 Advertise the host subnets through BGP.

admin@SwitchA# set protocols bgp ipv4-unicast network 49.49.49.49/32

Step 5 Commit the configuration.

admin@SwitchA# commit

Switch B

Step 1 Enable ge-1/1/5 as a routed interface and configure interface name as r1.

admin@SwitchB# set interface gigabit-ethernet ge-1/1/5 routed-interface enable true
admin@SwitchB# set interface gigabit-ethernet ge-1/1/5 routed-interface name r1
admin@SwitchB# set vlans reserved-vlan 80-90
admin@SwitchB# set l3-interface routed-interface r1

Step 2 Configure EBGP and disable ebgp-requires-policy policy.

admin@SwitchB# set protocols bgp local-as 65002
admin@SwitchB# set protocols bgp router-id 45.45.45.45
admin@SwitchB# set protocols bgp interface r1 remote-as 65001
admin@SwitchB# set protocols bgp interface r1 ipv6-unicast activate true
admin@SwitchB# set protocols bgp interface r1 capability extended-nexthop
admin@SwitchB# set protocols bgp ebgp-requires-policy false

Step 3 Enable IP routing.

admin@SwitchB# set ip routing enable true

Step 4 Advertise the host subnets through BGP.

admin@SwitchB# set protocols bgp ipv4-unicast network 45.45.45.45/32

Step 5 Commit the configuration.

admin@SwitchB# commit

Verify the Configuration

Check the BGP neighbor by running the following command.

admin@SwitchA# run show bgp neighbor
BGP neighbor on r1: fe80::1a5a:5810:83c:42a1, remote AS 65002, local AS 65001, external link
Hostname: SwitchB
  BGP version 4, remote router ID 45.45.45.45, local router ID 49.49.49.49
  BGP state = Established, up for 00:03:38
  Last read 00:00:38, Last write 00:00:38
  Hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
    4 Byte AS: advertised and received
    Extended Message: advertised and received
    AddPath:
      IPv4 Unicast: RX advertised IPv4 Unicast and received
      IPv6 Unicast: RX advertised IPv6 Unicast and received
    Extended nexthop: advertised and received
      Address families by peer:
                   IPv4 Unicast
    Route refresh: advertised and received(old & new)
    Enhanced Route Refresh: advertised and received
    Address Family IPv4 Unicast: advertised and received
    Address Family IPv6 Unicast: advertised and received
    Hostname Capability: advertised (name: 49sw,domain name: n/a) received (name: 45sw,domain name: n/a)
    Graceful Restart Capability: advertised and received
      Remote Restart timer is 120 seconds
      Address families by peer:
        none
  Graceful restart information:
    End-of-RIB send: IPv4 Unicast, IPv6 Unicast
    End-of-RIB received: IPv4 Unicast, IPv6 Unicast
    Local GR Mode: Helper*
    Remote GR Mode: Helper
    R bit: True
    Timers:
      Configured Restart Time(sec): 120
      Received Restart Time(sec): 120
    IPv4 Unicast:
      F bit: False
      End-of-RIB sent: Yes
      End-of-RIB sent after update: No
      End-of-RIB received: Yes
      Timers:
        Configured Stale Path Time(sec): 360
    IPv6 Unicast:
      F bit: False
      End-of-RIB sent: Yes
      End-of-RIB sent after update: Yes
      End-of-RIB received: Yes
      Timers:
        Configured Stale Path Time(sec): 360
  Message statistics:
    Inq depth is 0
    Outq depth is 0
                         Sent       Rcvd
    Opens:                  3          2
    Notifications:          0          2
    Updates:                9          9
    Keepalives:             5          5
    Route Refresh:          0          0
    Capability:             0          0
    Total:                 17         18
  Minimum time between advertisement runs is 0 seconds 

 For address family: IPv4 Unicast
  Update group 5, subgroup 5
  Packet Queue length 0
  Community attribute sent to this neighbor(all)
  1 accepted prefixes 

 For address family: IPv6 Unicast
  Update group 6, subgroup 6
  Packet Queue length 0
  Community attribute sent to this neighbor(all)
  0 accepted prefixes 

  Connections established 1; dropped 0
  Last reset 00:03:41,  No AFI/SAFI activated for peer
Local host: fe80::1a5a:5820:61f:63a1, Local port: 179
Foreign host: fe80::1a5a:5810:83c:42a1, Foreign port: 57988
Nexthop local: fe80::1a5a:5820:61f:63a1
BGP connection: shared network
BGP Connect Retry Timer in Seconds: 120
Estimated round trip time: 2 ms
Read thread: on  Write thread: on  FD used: 25

After completing the configuration, check BGP route on the switches.

admin@SwitchA# run show route bgp
RIB entry for bgp
=================
Codes: K - kernel route, C - connected, S - static, R - RIP,
       O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, A - Babel, F - PBR, f - OpenFabric,
       > - selected route, * - FIB route, q - queued, r - rejected, b - backup
       t - trapped, o - offload failure 

B>* 45.45.45.45/32 [20/0] via fe80::1a5a:5810:83c:42a1, r1, weight 1, 02:03:58

Show the brief information for BGP.

admin@SwitchA# run show bgp
show bgp ipv4 unicast
=====================
BGP table version is 10, local router ID is 49.49.49.49, vrf id 0
Default local pref 100, local AS 65001
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
 
   Network          Next Hop            Metric LocPrf Weight Path
*> 45.45.45.45/32   r1                       0             0 65002 i 

Displayed 1 routes and 1 total paths

show bgp ipv6 unicast
=====================
No BGP prefixes displayed, 0 exist