Select Preferred EBGP Peer with Weights

In the previous lab exercises, you configured EBGP sessions with two routers belonging to upstream ISPs and advertised your IPv4 prefixes to them.

Now, imagine that you want to use one of the uplinks just for backup purposes – it might be either too slow or too expensive for regular use.

Lab topology

In this lab, you’ll modify your BGP configuration to ensure your router always prefers routes advertised by X1 (ISP-1).

Existing BGP Configuration

The routers in your lab use the following BGP AS numbers. Each autonomous system advertises an IPv4 prefix. Upstream routers (x1, x2) also advertise the default route to your router (rtr).

Node/ASN Router ID Advertised prefixes
AS65000
rtr 10.0.0.1 192.168.42.0/24
AS65100
x1 10.0.0.10 192.168.100.0/24
AS65101
x2 10.0.0.11 192.168.101.0/24

Your router has these EBGP neighbors. netlab configures them automatically; if you’re using some other lab infrastructure, you’ll have to manually configure EBGP neighbors and advertised prefixes.

Neighbor Neighbor IPv4 Neighbor AS
x1 10.1.0.2 65100
x2 10.1.0.6 65101

Device Requirements

  • Customer router: use any device supported by the netlab BGP configuration module.
  • netlab can configure default route origination on almost all supported devices. You’ll have to configure BGP default route origination yourself if you want to use an unsupported device for X1 or X2.
  • You can do automated lab validation with Arista EOS or FRRouting running on the customer router.

Start the Lab

You can start the lab on your own lab infrastructure or in GitHub Codespaces (more details):

  • Change directory to policy/1-weights
  • Execute netlab up
  • Log into your device (RTR) with netlab connect rtr and verify IP addresses and BGP configuration.

Note: netlab will configure IP addressing, EBGP sessions, and BGP prefix advertisements on your router. If you’re not using netlab, continue with the configuration you made during the previous exercise.

Configuration Tasks

You want your device to prefer routes advertised by X1 over those by X2. For example, the route for X2’s loopback interface should use X1 as the next hop.

Many BGP implementations use a mechanism called weight (usually applied per neighbor) to prefer routes advertised by one of the BGP peers.

If your device supports BGP weights, use them to prefer routes advertised by X1. Otherwise, you’ll have to use BGP local preference to achieve the same result.

Warning

Applying routing policy parameters to BGP neighbors doesn’t necessarily change the BGP table as the new parameters might be evaluated only on new incoming updates – you might have to use a command similar to clear ip bgp * soft in to tell your router to ask its neighbors to resend their BGP updates.

Verification

You can use the netlab validate command if you use FRRouting or Arista EOS on your router. The validation tests check:

  • The state of the EBGP session between RTR and X1/X2.
  • Whether RTR receives the prefix from X2 (192.168.101.0/24).
  • Whether RTR uses X1 as the next hop for the prefix advertised by X2.

This is the printout you could get when trying to validate an incomplete solution:

You can also examine the BGP table on your router to verify that the routes advertised by X1 (next hop: 10.1.0.2) are the best (active) routes. This is a printout you should get on Arista EOS:

rtr#show ip bgp
BGP routing table information for VRF default
Router identifier 10.0.0.1, local AS number 65000
Route status codes: s - suppressed contributor, * - valid, > - active, E - ECMP head, e - ECMP
                    S - Stale, c - Contributing to ECMP, b - backup, L - labeled-unicast
                    % - Pending BGP convergence
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI Origin Validation codes: V - valid, I - invalid, U - unknown
AS Path Attributes: Or-ID - Originator ID, C-LST - Cluster List, LL Nexthop - Link Local Nexthop

          Network                Next Hop              Metric  AIGP       LocPref Weight  Path
 * >      0.0.0.0/0              10.1.0.2              0       -          100     200     65100 i
 *        0.0.0.0/0              10.1.0.6              0       -          100     100     65101 i
 * >      192.168.42.0/24        -                     -       -          -       0       ?
 * >      192.168.100.0/24       10.1.0.2              0       -          100     200     65100 i
 *        192.168.100.0/24       10.1.0.6              0       -          100     100     65101 65100 i
 * >      192.168.101.0/24       10.1.0.2              0       -          100     200     65100 65101 i
 *        192.168.101.0/24       10.1.0.6              0       -          100     100     65101 i

You could dig deeper and examine the details of an IPv4 prefix that originated in AS 65101 (X2), for example, 192.168.101.0/24. Yet again, the next hop of the best path should be X1 (10.1.0.2)

rtr#show ip bgp 192.168.101.0/24
BGP routing table information for VRF default
Router identifier 10.0.0.1, local AS number 65000
BGP routing table entry for 192.168.101.0/24
 Paths: 2 available
  65100 65101
    10.1.0.2 from 10.1.0.2 (10.0.0.10)
      Origin IGP, metric 0, localpref 100, IGP metric 0, weight 200, tag 0
      Received 00:00:46 ago, valid, external, best
      Rx SAFI: Unicast
  65101
    10.1.0.6 from 10.1.0.6 (10.0.0.11)
      Origin IGP, metric 0, localpref 100, IGP metric 0, weight 100, tag 0
      Received 00:00:46 ago, valid, external
      Rx SAFI: Unicast

Next:

Reference Information

This lab uses a subset of the 4-router lab topology. The following information might help you if you plan to build custom lab infrastructure:

Lab Wiring

Origin Device Origin Port Destination Device Destination Port
rtr Ethernet1 x1 eth1
rtr Ethernet2 x2 eth1
x1 eth2 x2 eth2

Lab Addressing

Node/Interface IPv4 Address IPv6 Address Description
rtr 10.0.0.1/32 Loopback
Ethernet1 10.1.0.1/30 rtr -> x1
Ethernet2 10.1.0.5/30 rtr -> x2
x1 192.168.100.1/24 Loopback
eth1 10.1.0.2/30 x1 -> rtr
eth2 10.1.0.9/30 x1 -> x2
x2 192.168.101.1/24 Loopback
eth1 10.1.0.6/30 x2 -> rtr
eth2 10.1.0.10/30 x2 -> x1