MP-BGP and Avenue Distinguishers
Now that PE2 has abstruse routes from both CE-A2 and CE-B2, PE2 needs to acquaint those routes
to the added PEs, in adjustment for the added PEs to apperceive how to advanced packets to the anew learned
subnets. MPLS VPN protocols ascertain the use of IBGP to acquaint the routes—all the routes, from
all the altered VRFs. However, the aboriginal BGP blueprint did not accommodate a way to accord with
the actuality that altered barter may use overlapping prefixes.
MPLS deals with the overlapping prefix botheration by abacus addition cardinal in advanced of the original
BGP NLRI (prefix). Anniversary altered cardinal can represent a altered customer, authoritative the NLRI
values unique. To do this, MPLS took advantage of a BGP RFC, alleged MP-BGP (RFC 4760),
which allows for the re-definition of the NLRI acreage in BGP Updates. This re-definition allows for
an added variable-length number, alleged an abode family, to be added in advanced of the prefix.
MPLS RFC 4364, “BGP/MPLS IP Virtual Private Networks (VPNs),” defines a specific new
address ancestors to abutment IPv4 MPLS VPNs—namely, an MP-BGP abode ancestors alleged Route
Distinguishers (RDs).
RDs acquiesce BGP to acquaint and analyze amid alike IPv4 prefixes. The abstraction is
simple: acquaint anniversary NLRI (prefix) as the acceptable IPv4 prefix, but add addition cardinal (the
RD) that abnormally identifies the route. In particular, the new NLRI format, alleged VPN-V4, has the
following two parts:
■ A 64-bit RD
■ A 32-bit IPv4 prefix
For example, Amount 19-15 continues the adventure from Amount 19-14, with router PE2 application MP-BGP
to acquaint its two routes for IPv4 prefix 10.3.3.0/24 to PE1—one from VRF-A and one from
VRF-B. The BGP Amend shows the new VPN-V4 abode ancestors architecture for the NLRI information,
using RD 1:111 to represent VPN-A, and 2:222 to represent VPN-B.
722 Chapter 19: Multiprotocol Label Switching
Figure 19-15 Authoritative Prefixes Different Application an RD
Without the RD as allotment of the VPN-V4 NLRI, PE1 would accept abstruse about two identical BGP
prefixes (10.3.3.0/24) and would accept had to accept one of the two as the best route—giving PE1
reachability to alone one of the two chump 10.3.3.0/24 subnets. With VPN-V4 NLRI, IBGP
advertises two different NLRI—a 1:111:10.3.3.0 (from VRF-A) and 2:222:10.3.3.0 (from VRF-B).
As a result, PE1 keeps both NLRI in its BGP table. The specific accomplish apparent in the amount are
explained as follows:
1. PE2 redistributes from anniversary of the corresponding per-VRF acquisition agreement instances (RIP-2 in
this case) into BGP.
2. The redistribution action pulls the RD from anniversary corresponding VRF and includes that RD with
all routes redistributed from the VRF’s acquisition table.
3. PE3 uses IBGP to acquaint these routes to PE1, causing PE1 to apperceive both routes for 10.3.3.0/
24, anniversary with the differing RD values.
NOTE PE3 uses next-hop-self, and an amend antecedent of a loopback interface with IP address
3.3.3.3.
NOTE Every VRF charge be configured with an RD; the IOS rd VRF subcommand configures
the value.
PE1
NLRI
1:111:10.3.3.0/24
2:222:10.3.3.0/24
BGP Table
Router PE2
Next-Hop
3.3.3.3
3.3.3.3
Label
41
42
RIB—VRF-A—RD 1:111
Next-Hop
192.168.37.7
Prefix
10.3.3.0/24
Label
S0/1/0
RIB—VRF-B—RD 2:222
Source
RIP
Next-Hop
192.168.38.8
Prefix
10.3.3.0/24
Label
S0/1/1
NLRI
1:111:10:3.3.0/24
2:222:10.3.3.0/24
Next-Hop
3.3.3.3
3.3.3.3
BGP Table
Label
41
42
3
2 1
IBGP BGP
Process
Source
RIP
Redistribute
RIP into BGP
Redistribute 1
RIP into BGP
The RD itself is 8 bytes with some appropriate formatting conventions. The aboriginal 2 bytes identify
which of the three formats is followed. Incidentally, because IOS can acquaint which of the three
formats is acclimated based on the value, the IOS rd VRF subcommand alone requires that you blazon the
integer ethics for the aftermost 6 bytes, with IOS answer the aboriginal 2 bytes (the type) based on the value.
The aftermost 6 bytes, as typed in the rd command and apparent in appearance commands, chase one of these
formats:
■ 2-byte-integer:4-byte-integer
■ 4-byte-integer:2-byte-integer
■ 4-byte-dotted-decimal:2-byte-integer
In all three cases, the aboriginal amount (before the colon) should be either an ASN or an IPv4 address.
The additional value, afterwards the colon, can be any amount you wish. For example, you ability accept an
RD that lists an LSR’s BGP ID application the third format, like 3.3.3.3:100, or you may use the BGP
ASN, for example, 432:1.
At this point in the advancing example, PE1 has abstruse about the two routes for 10.3.3.0/24—one
for VPN-A and one for VPN-B—and the routes are in the BGP table. The abutting area describes
how PE1 again chooses the VRFs into which to add these routes, based on the abstraction of a Route
Target.