6PE: Crossing Multiple AS

6PE: Crossing Multiple AS

RFC 4798 describes three approaches to provide communication between two IPv6 islands connected to two different BGP Autonomous Systems (AS).

These approaches are-

eBGP redistribution of IPv6 routes from AS to neighboring AS
eBGP redistribution of labeled IPv6 routes from AS to neighboring AS
Multihop eBGP redistribution of labeled IPv6 routes between source and destination ASes, with eBGP redistribution of labeled IPv4 routes from AS to neighboring AS.

Network topology:

Approach 2:

The 6PE1 router has iBGP peering with ASBR-1 router in AS 100 and redistribute labeled IPv6 prefixes to ASBR-1 router. The ASBR-1 router then uses eBGP to redistribute non-labeled IPv6 prefixes to neighboring ASBR-2 router in AS 200, which in turn distributes them to 6PE2 router.

The inter-ASBR link is IPv6 enabled and each connected interface has an IPv6 address. There is no LSP between ASBRs.

MP-BGP Configuration on 6PE routers:

6PE1 and 6PE2 routers run eBGP session with CE1 and CE2 routers respectively. 6PE1 router also runs iBGP session with ASBR-1 router in AS 100. 6PE2 router runs iBGP session with ASBR-2 router in AS 200. ASBR-1 and ASBR-2 router also run an eBGP connection between them.

BGP Configuration on 6PE1

ipv6 unicast-routing

ipv6 cef

mpls label protocol ldp

mpls ldp router-id Loopback 0 force

mpls ipv6 source-interface Loopback 0

interface Loopback 0

 ip address 1.1.1.1 255.255.255.255

 ipv6 address ABCD::1/128

router bgp 100

 neighbor 2001:1::1 remote-as 65101

 neighbor 2001:1::1 description eBGP with CE1 router

 neighbor 3.3.3.3 remote-as 100

 neighbor 3.3.3.3 description iBGP with ASBR-1 router

 neighbor 3.3.3.3 update-source Loopback 0

 address-family ipv6

 neighbor 2001:1::1 activate

 neighbor 3.3.3.3 activate

 neighbor 3.3.3.3 send-label

 redistribute connected

 exit-address-family

BGP Configuration on ASBRs

ASBR-1 router:

ipv6 unicast-routing

ipv6 cef

mpls label protocol ldp

mpls ldp router-id Loopback 0 force

mpls ipv6 source-interface Loopback 0

interface Loopback 0

 ip address 3.3.3.3 255.255.255.255

 ipv6 address ABCD::3/128

router bgp 100

 neighbor 1.1.1.1 remote-as 100

 neighbor 1.1.1.1 description iBGP with 6PE1 router

 neighbor 2001:3::2 remote-as 200

 neighbor 2001:3::2 description eBGP with ASBR-2 router

 address-family ipv6

 neighbor 1.1.1.1 activate

 neighbor 1.1.1.1 send-label

 neighbor 2001:3::2 activate

 redistribute connected

 exit-address-family

ASBR-2 router:

ipv6 unicast-routing

ipv6 cef

mpls label protocol ldp

mpls ldp router-id Loopback 0 force

mpls ipv6 source-interface Loopback 0

interface Loopback 0

 ip address 4.4.4.4 255.255.255.255

 ipv6 address ABCD::4/128

router bgp 200

 neighbor 2001:3::1 remote-as 100

 neighbor 2001:3::1 description eBGP with ASBR-1 router

 neighbor 6.6.6.6 remote-as 200

 neighbor 6.6.6.6 description iBGP with 6PE2 router

 neighbor 6.6.6.6 update-source Loopback 0

 address-family ipv6

 neighbor 2001:3::1 activate

 neighbor 6.6.6.6 activate

 neighbor 6.6.6.6 send-label

 redistribute connected

 exit-address-family

BGP Configuration on 6PE2

ipv6 unicast-routing

ipv6 cef

mpls label protocol ldp

mpls ldp router-id Loopback 0 force

mpls ipv6 source-interface Loopback 0

interface Loopback 0

 ip address 6.6.6.6 255.255.255.255

 ipv6 address ABCD::6/128

router bgp 200

 neighbor 4.4.4.4 remote-as 200

 neighbor 4.4.4.4 description iBGP with ASBR-2 router

 neighbor 4.4.4.4 update-source Loopback 0

 neighbor 2001:2::1 remote-as 65102

 neighbor 2001:2::1 description eBGP with CE2 router

 address-family ipv6

 neighbor 4.4.4.4 activate

 neighbor 4.4.4.4 send-label

 neighbor 2001:2::1 activate

 redistribute connected

 exit-address-family

6PE1 router advertises a label for its connected IPv6 networks to ASBR-1 router via BGP. The following output shows IPv6 prefixes with their labels. IPv6 prefixes 2001:1::/124 and ABCD::1/128 are advertised by 6PE1 router. The next-hop for these IPv6 prefixes is the IPv4-mapped IPv6 address ::FFFF:1.1.1.1 of 6PE1 router. The In Label/Out Label for IPv6 prefix 2001:1::/124 is No Label/20. This indicates that there is no LSP between ASBR-1 and ASBR-2 router, because ASBR-1 router imposes label 20 if packets are destined for 2001:1::/124 network with next-hop as 6PE1 router.

IPv6 Prefixes and labels on ASBR-1

ASBR-1# show bgp ipv6 unicast labels

   Network          Next Hop      In label/Out label

   2001:1::/124     ::FFFF:1.1.1.1  nolabel/20

   2001:3::/124     2001:3::2       21/nolabel

                    ::              21/nolabel

   ABCD::1/128      ::FFFF:1.1.1.1  nolabel/19

   ABCD::3/128      ::              19/nolabel

   ABCD::4/128      2001:3::2       20/nolabel

   ABCD::6/128      2001:3::2       22/nolabel

Similarly, on ASBR-2 router, the In Label/Out Label for IPv6 prefix 2001:1::/124 is 23/No Label. This also indicates that there is no LSP between ASBR-2 and ASBR-1 router as the Out Label is No Label. However, the In Label is Label 23 which it will advertise to 6PE2 router via BGP.

IPv6 Prefixes and labels on ASBR-2

ASBR-2# show bgp ipv6 unicast labels

   Network          Next Hop      In label/Out label

   2001:1::/124     2001:3::1       23/nolabel

   2001:3::/124     ::              20/nolabel

                    2001:3::1       20/nolabel

   ABCD::1/128      2001:3::1       22/nolabel

   ABCD::3/128      2001:3::1       21/nolabel

   ABCD::4/128      ::              19/nolabel

   ABCD::6/128      ::FFFF:6.6.6.6  nolabel/19

The following output shows the labels that will be imposed by 6PE2 router to reach IPv6 prefix 2001:1::/124 connected to 6PE1 and CE1 routers. The top label Label 16 is the IGP label while the bottom Label 23 is the BGP VPNv4 label advertised by ASBR-2 router.

Labels imposed by 6PE2

6PE2# show bgp ipv6 unicast labels

   Network          Next Hop      In label/Out label

   2001:1::/124     ::FFFF:4.4.4.4  nolabel/23

   2001:3::/124     ::FFFF:4.4.4.4  nolabel/20

   ABCD::1/128      ::FFFF:4.4.4.4  nolabel/22

   ABCD::3/128      ::FFFF:4.4.4.4  nolabel/21

   ABCD::4/128      ::FFFF:4.4.4.4  nolabel/19

   ABCD::6/128      ::              19/nolabel

6PE2# show ip cef 4.4.4.4

4.4.4.4/32, version 12, epoch 0, cached adjacency 10.2.2.5

0 packets, 0 bytes

  tag information set

    local tag: 17

    fast tag rewrite with Fa0/0, 10.2.2.5, tags imposed: {16}

  via 10.2.2.5, FastEthernet0/0, 0 dependencies

    next hop 10.2.2.5, FastEthernet0/0

    valid cached adjacency

    tag rewrite with Fa0/0, 10.2.2.5, tags imposed: {16}

6PE2# show ipv6 cef 2001:1::

2001:1::/124

     nexthop ::FFFF:4.4.4.4

    fast tag rewrite with Fa0/0, 10.2.2.5, tags imposed: {16 23}

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