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Example: Configuring a Layer 3 VPN with Route Reflection and AS Override

Requirements

No special configuration beyond device initialization is required before configuring this example.

Overview

This example has two CE devices, two provider edge (PE) devices, and several provider core devices. The provider network is also using IS-IS to support LDP and BGP loopback reachability Device P2 is acting as a route reflector (RR). Both CE devices are in autonomous system (AS) 64512. The provider network is in AS 65534.

The as-override statement is applied to the PE devices, thus replacing the CE device's AS number with that of the PE device. This prevents the customer AS number from appearing more than once in the AS path attribute.

Figure 1 shows the topology used in this example.

Figure 1: AS Override Topology

CLI Quick Configuration shows the configuration for all of the devices in Figure 1. The section Step-by-Step Procedure describes the steps on Device PE1.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Step-by-Step Procedure

1. Configure the interfaces.

   To enable MPLS, include the protocol family on the interface so that the interface does not discard incoming MPLS traffic.

   [edit interfaces]user@PE1# set ge-1/2/0 unit 0 family inet address 10.0.0.2/30user@PE1# set ge-1/2/0 unit 0 family isouser@PE1# set ge-1/2/0 unit 0 family mplsuser@PE1# set ge-1/2/1 unit 0 family inet address 10.0.0.5/30user@PE1# set ge-1/2/1 unit 0 family isouser@PE1# set ge-1/2/1 unit 0 family mplsuser@PE1# set ge-1/2/2 unit 0 family inet address 10.0.0.21/30user@PE1# set ge-1/2/2 unit 0 family isouser@PE1# set ge-1/2/2 unit 0 family mplsuser@PE1# set lo0 unit 0 family inet address 10.255.2.2/32user@PE1# set lo0 unit 0 family iso address 49.0001.0010.0000.0202.00
2. Add the interface to the MPLS protocol to establish the control plane level connectivity.

   Set up the IGP so that the provider devices can communicate with each other.

   To establish a mechanism to distribute MPLS labels, enable LDP. Optionally, for LDP, enable forwarding equivalence class (FEC) deaggregation, which results in faster global convergence.

   [edit protocols]user@PE1# set mpls interface ge-1/2/2.0user@PE1# set mpls interface ge-1/2/1.0user@PE1# set mpls interface lo0.0user@PE1# set mpls interface fxp0.0 disableuser@PE1# set isis interface ge-1/2/1.0 level 2 metric 10user@PE1# set isis interface ge-1/2/1.0 level 1 disableuser@PE1# set isis interface ge-1/2/2.0 level 2 metric 10user@PE1# set isis interface ge-1/2/2.0 level 1 disableuser@PE1# set isis interface fxp0.0 disableuser@PE1# set isis interface lo0.0 level 2 metric 0user@PE1# set ldp deaggregateuser@PE1# set ldp interface ge-1/2/1.0user@PE1# set ldp interface ge-1/2/2.0user@PE1# set ldp interface fxp0.0 disableuser@PE1# set ldp interface lo0.0
3. Enable the internal BGP (IBGP) connection to peer with the RR only, using the IPv4 VPN unicast address family.
   [edit protocols bgp group l3vpn]user@PE1# set type internaluser@PE1# set local-address 10.255.2.2user@PE1# set family inet-vpn unicastuser@PE1# set peer-as 65534user@PE1# set local-as 65534user@PE1# set neighbor 10.255.4.4
4. Configure the routing instance, including the as-override statement.

   Create the routing-Instance (VRF) on the PE device, setting up the BGP configuration to peer with Device CE1.

   [edit routing-instances VPN-A]user@PE1# set instance-type vrfuser@PE1# set interface ge-1/2/0.0user@PE1# set route-distinguisher 65534:1234user@PE1# set vrf-target target:65534:1234user@PE1# set protocols bgp group CE type externaluser@PE1# set protocols bgp group CE family inet unicastuser@PE1# set protocols bgp group CE neighbor 10.0.0.1 peer-as 64512user@PE1# set protocols bgp group CE neighbor 10.0.0.1 as-override
5. Configure the router ID and the AS number.
   [edit routing-options]user@PE1# set router-id 10.255.2.2user@PE1# set autonomous-system 65534

Results

From configuration mode, confirm your configuration by entering the show interfaces, show protocols, show routing-instances, and show routing-options commands. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

• Checking AS Path to the CE Devices
• Checking How the Route to Device CE2 Is Advertised
• Checking the Route on Device CE1

Checking AS Path to the CE Devices

Purpose

Display information on Device PE1 about the AS path attribute for the route to Device CE2’s loopback interface.

Action

On Device PE1, from operational mode, enter the show route table VPN-A.inet.0 10.255.6.6 command.

 

VPN-A.inet.0: 5 destinations, 6 routes (5 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.255.6.6/32      *[BGP/170] 02:19:35, localpref 100, from 10.255.4.4
                      AS path: 64512 I, validation-state: unverified
                    > to 10.0.0.22 via ge-1/2/2.0, Push 300032, Push 299776(top)

Meaning

The output shows that Device PE1 has an AS path for 10.255.6.6/32 as coming from AS 64512.

Checking How the Route to Device CE2 Is Advertised

Purpose

Make sure the route to Device CE2 is advertised to Device CE1 as if it is coming from the MPLS core.

Action

On Device PE1, from operational mode, enter the show route advertising-protocol bgp 10.0.0.1 command.

 
VPN-A.inet.0: 5 destinations, 6 routes (5 active, 0 holddown, 0 hidden)
  Prefix                  Nexthop              MED     Lclpref    AS path
* 10.0.0.16/30            Self                                    I
* 10.255.1.1/32           10.0.0.1                                65534 I
* 10.255.6.6/32           Self                                    65534 I

Meaning

The output indicates that Device PE1 is advertising only its own AS number in the AS path.

Checking the Route on Device CE1

Purpose

Make sure that Device CE1 contains only the provider AS number in the AS path for the route to Device CE2.

Action

From operational mode, enter the show route table inet.0 terse 10.255.6.6 command.

inet.0: 5 destinations, 6 routes (5 active, 0 holddown, 1 hidden)
+ = Active Route, - = Last Active, * = Both

A V Destination        P Prf   Metric 1   Metric 2  Next hop        AS path
* ? 10.255.6.6/32      B 170        100                             65534 65534 I
  unverified                                       >10.0.0.2

Meaning

The output indicates that Device CE1 has a route to Device CE2. The loop issue is resolved with the use of the as-override statement.

One route is hidden on the CE device. This is because Junos OS does not perform a BGP split horizon. Generally, split horizon in BGP is unnecessary, because any routes that might be received back by the originator are less preferred due to AS path length (for EBGP), AS path loop detection (IBGP), or other BGP metrics. Advertising routes back to the neighbor from which they were learned has a negligible effect on the router's performance, and is the correct thing to do.