InterAS Option B

This chapter explains the different InterAS option B configuration options. The available options are InterAS option B, InterAS option B (with RFC 3107), and InterAS option B lite. The InterAS option B (with RFC 3107) implementation ensures complete IGP isolation between the data centers and WAN. When BGP advertises a particular route to ASBR, it also distributes the label which is mapped to that route.

Information About InterAS

An autonomous system (AS) is a single network or group of networks that is controlled by a common system administration group and using a single, clearly defined protocol. In many cases, virtual private networks (VPNs) extend to different ASes in different geographical areas. Some VPNs must extend across multiple service providers; these VPNs are called overlapping VPNs. The connection between ASes must be seamless to the customer, regardless of the complexity or location of the VPNs.

InterAS and ASBR

Separate ASes from different service providers can communicate by exchanging information in the form of VPN IP addresses. The ASBRs use EBGP to exchange that information. The IBGP distributes the network layer information for IP prefixes throughout each VPN and each AS. The following protocols are used for sharing routing information:

  • Within an AS, routing information is shared using IBGP.

  • Between ASes, routing information is shared using EBGP. EBGP allows service providers to set up an interdomain routing system that guarantees loop-free exchange of routing information between separate ASes.

The primary function of EBGP is to exchange network reachability information between ASes, including information about the list of AS routes. The ASes use EBGP border edge routers to distribute the routes, which includes label-switching information. Each border edge router rewrites the next-hop and MPLS labels.

InterAS configuration supported in this MPLS VPN can include an interprovider VPN, which is MPLS VPNs that include two or more ASes, connected by separate border edge routers. The ASes exchange routes use EBGP, and no IBGP or routing information is exchanged between the ASes.

Exchanging VPN Routing Information

ASes exchange VPN routing information (routes and labels) to establish connections. To control connections between ASes, the PE routers and EBGP border edge routers maintain a label forwarding information base (LFIB). The LFIB manages the labels and routes that the PE routers and EBGP border edge routers receive during the exchange of VPN information.

The ASes use the following guidelines to exchange VPN routing information:

  • Routing information includes:

    • The destination network.

    • The next-hop field associated with the distributing router.

    • A local MPLS label

  • A route distinguisher (RD1) is part of a destination network address. It makes the VPN IP route globally unique in the VPN service provider environment.

The ASBRs are configured to change the next-hop when sending VPN NLRIs to the IBGP neighbors. Therefore, the ASBRs must allocate a new label when they forward the NLRI to the IBGP neighbors.

InterAS Options

Nexus 9508 series switches support the following InterAS options:

  • InterAS option A - In an interAS option A network, autonomous system border router (ASBR) peers are connected by multiple subinterfaces with at least one interface VPN that spans the two ASes. These ASBRs associate each subinterface with a VPN routing and forwarding (VRF) instance and a BGP session to signal unlabeled IP prefixes. As a result, traffic between the back-to-back VRFs is IP. In this scenario, the VPNs are isolated from each other and, because the traffic is IP Quality of Service (QoS) mechanisms that operate on the IP traffic can be maintained. The downside of this configuration is that one BGP session is required for each subinterface (and at least one subinterface is required for each VPN), which causes scalability concerns as the network grows.

  • InterAS option B - In an interAS option B network, ASBR ports are connected by one or more subinterfaces that are enabled to receive MPLS traffic. A Multiprotocol Border Gateway Router (MP-BGP) session distributes labeled VPN prefixes between the ASBRs. As a result, the traffic that flows between the ASBRs is labeled. The downside of this configuration is that, because the traffic is MPLS, QoS mechanisms that are applied only to IP traffic cannot be carried and the VRFs cannot be isolated. InterAS option B provides better scalability than option A because it requires only one BGP session to exchange all VPN prefixes between the ASBRs. Also, this feature provides nonstop forwarding (NSF) and Graceful Restart. The ASBRs must be directly connected in this option.

Some functions of option B are noted below:

  • You can have an IBGP VPNv4/v6 session between Nexus 9508 series switches within an AS and you can have an EBGP VPNv4/v6 session between data center edge routers and WAN routers.

  • There is no requirement for a per VRF IBGP session between data center edge routers, like in the lite version.

  • – LDP distributes IGP labels between ASBRs.

  • InterAS option B (with BGP-3107 or RFC 3107 implementation)

  • You can have an IBGP VPNv4/v6 implementation between Nexus 9508 switches within an AS and you can have an EBGP VPNv4/v6 session between data center edge routers and WAN routers.

  • BGP-3107 enables BGP packets to carry label information without using LDP between ASBRs.

  • The label mapping information for a particular route is piggybacked in the same BGP update message that is used to distribute the route itself.

  • When BGP is used to distribute a particular route, it also distributes an MPLS label which is mapped to that route. Many ISPs prefer this method of configuration since it ensures complete IGP isolation between the data centers.

  • InterAS option B lite – Support for the InterAS option B feature is restricted in the Cisco NX-OS 6.2(2) release. Details are noted in the Configuring InterAS Option B (lite version) section.

Guidelines and Limitations for Configuring InterAS Option B

InterAS Option B has the following guidelines and limitations:

  • InterAS option B is not supported with BGP confederation AS.

  • InterAS option B is supported on Cisco Nexus 9500 platform switches with -R line cards.

Configuring BGP for InterAS Option B

Configure DC Edge switches with IBGP & EBGP VPNv4/v6 with the following steps:

Before you begin

To configure BGP for InterAS option B, you need to enable this configuration on both the IBGP and EBGP sides. Refer to Figure 1 for reference.

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

router bgp as-number

Example:

 switch(config)# router bgp 100

Enters the router BGP configuration mode and assigns an autonomous system (AS) number to the local BGP speaker device.

Step 3

neighbor ip-address

Example:

 switch(config-router)# neighbor 10.0.0.2

Adds an entry to the BGP or multiprotocol BGP neighbor table, and enters router BGP neighbor configuration mode.

Step 4

remote-as as-number

Example:

 switch(config-router-neighbor)# remote-as 200

The as-number argument specifies the autonomous system to which the neighbor belongs.

Step 5

address-family {vpnv4 | vpnv6} unicast

Example:

switch(config-router-neighbor)# address-family vpnv4 unicast

Enters address family configuration mode for configuring IP VPN sessions.

Step 6

send-community {both | extended}

Example:

switch(config-router-neighbor-af)# send-community both

Specifies that a communities attribute should be sent to both BGP neighbors.

Step 7

retain route-target all

Example:

switch(config-router-neighbor-af)# retain route-target all

(Optional). Retains VPNv4/v6 address configuration on the ASBR without VRF configuration.

Note

 

If you have a VRF configuration on the ASBR, this command is not required.

Step 8

vrf vrf-name

Example:

switch(config-router-neighbor-af)# vrf VPN1

Associates the BGP process with a VRF.

Step 9

address-family {ipv4 | ipv6} unicast

Example:

switch(config-router-vrf)# address-family ipv4 unicast

Specifies the IPv4 or IPv6 address family and enters address family configuration mode.

Step 10

exit

Example:

switch(config-vrf-af)# exit

Exits IPv4 address family.

Step 11

copy running-config startup-config

Example:

switch(config-router-vrf)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Configuring BGP for InterAS Option B (with RFC 3107 implementation)

Configure DC Edge switches with IBGP & EBGP VPNv4/v6 along with BGP labeled unicast family with following steps:

Procedure

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

router bgp as-number

Example:

 switch(config)# router bgp 100

Enters the router BGP configuration mode and assigns an autonomous system (AS) number to the local BGP speaker device.

Step 3

address-family {vpnv4 | vpnv6} unicast

Example:

switch(config-router-neighbor)# address-family vpnv4 unicast

Enters address family configuration mode for configuring IP VPN sessions.

Step 4

redistribute direct route-map tag

Example:

 switch(config-router-af)# redistribute direct route-map loopback

Redistributes directly connected routes using the Border Gateway Protocol.

Step 5

allocate-label all

Example:

 switch(config-router-af)# allocate-label all

Configures ASBRs with the BGP labeled unicast address family to advertise labels for the connected interface.

Step 6

exit

Example:

switch(config-router-af)# exit

Exits address family router configuration mode and enters router BGP configuration mode.

Step 7

neighbor ip-address

Example:

switch(config-router)# neighbor 10.1.1.1

Configures the BGP neighbor’s IP address, and enters router BGP neighbor configuration mode.

Step 8

remote-as as-number

Example:

switch(config-router-neighbor)# remote-as 100

Specifies the BGP neighbor’s AS number.

Step 9

address-family {ipv4|ipv6} labeled-unicast

Example:

switch(config-router-neighbor)# address-family ipv4 labeled-unicast

Configures the ASBR with the BGP labeled unicast address family to advertise labels for the connected interface.

Note

 
This is the command that implements RFC 3107.

Step 10

retain route-target all

Example:

switch(config-router-neighbor-af)# retain route-target all

(Optional). Retains VPNv4/v6 address configuration on the ASBR without VRF configuration.

Note

 
If you have a VRF configuration on the ASBR, this command is not required.

Step 11

exit

Example:

Switch(config-router-neighbor-af)# exit

Exits router BGP neighbor address family configuration mode and returns to router BGP configuration mode.

Step 12

neighbor ip-address

Example:

switch(config-router)# neighbor 10.1.1.1

Configures a loopback IP address, and enters router BGP neighbor configuration mode.

Step 13

remote-as as-number

Example:

switch(config-router-neighbor)# remote-as 100

Specifies the BGP neighbor’s AS number.

Step 14

address-family {vpnv4|vpnv6} unicast

Example:

switch(config-router-vrf)# address-family ipv4 unicast

Configures the ASBR with the BGP VPNv4 unicast address family.

Step 15

exit

Example:

switch(config-vrf-af)# exit

Exits IPv4 address family.

Step 16

address-family {vpnv4|vpnv6} unicast

Example:

switch(config-router-vrf)# address-family ipv4 unicast

Configures the ASBR with the BGP VPNv4 unicast address family.

Step 17

Repeat the process with ASBR2

Configures ASBR2 with option B (RFC 3107) settings and implements complete IGP isolation between the two data centers DC1 and DC2.

Step 18

copy running-config startup-config

Example:

switch(config-router-vrf)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.