BGP Route Selection

Different BGP speakers can advertise the vpnv4 route when, for instance, a customer site is dual homed to two PE routers. The receiving BGP speaker must then choose one BGP route as the best one. The process for selecting the best vpnv4 route is the same as the one for regular IPv4 BGP routes. The only difference is that now the BGP routes are not 32-bit IPv4 prefixes but 96-bit vpnv4 prefixes. Therefore, if a customer site is dual homed to two PE routers, the ingress PE router receives the vpnv4 route with two different BGP next hops—namely, the two egress PE routers. The ingress PE router applies the BGP best path selection process and installs one of the two BGP paths in the VRF routing table.

NOTE Refer to www.Cisco.com for details on the BGP best path selection process.

BGP Multipath

BGP selects only one best path for each prefix it receives. This means that only one BGP path is installed in the IP routing table, which excludes the possibility of load balancing. BGP Multipath is a BGP feature whereby the selection process still selects one BGP path as the best but allows multiple BGP paths to be installed in the routing table. BGP Multipath comes in three types: iBGP, eBGP, and eiBGP. iBGP Multipath is the installation of two or more internal BGP paths. eBGP Multipath is the installation of two or more external BGP paths. eiBGP Multipath is the installation of one or more internal and one or more external BGP paths. Not just any BGP path can be chosen for Multipath together with the best path. The following criteria must be fulfilled for the BGP path to be a candidate for Multipath. The following BGP attributes of the alternate BGP paths must be identical to the ones of the best path for the alternative paths to be used in parallel:

■ Local preference

■ Multi-exit discriminator (MED)

Neighboring autonomous system (AS) or sub-AS (before the eiBGP Multipath feature was added)

AS-PATH (after the eiBGP Multipath feature was added) ■ IGP metric to the BGP next hop

Table 7-3 shows what command is used under the specific address family of BGP to configure BGP Multipath.

Table 7-3 BGP Multipath Commands

BGP Multipath

BGP Address Family Command

eBGP

maximum-paths n

iBGP

maximum-paths ibgp n

eiBGP

maximum-paths eiBGP n

The n in the BGP address family maximum-paths command indicates how many paths can be installed in the IP routing table. The default value of n is 1, so BGP Multipath is disabled by default.

Look at Figure 7-13 to see an example of eiBGP Multipath in an MPLS VPN network. Figure 7-13 Example of a MPLS VPN with eiBGP Multipath

Bgp Maximum Paths

PE1 receives a route from site 2 either directly from the eBGP peering across the VRF interface toward CE2 or indirectly from the iBGP peering to PE2. If eiBGP Multipath is enabled on PE1 under address family ipv4 vrf, PE1 can install both BGP paths into the IP VRF routing table.

If BGP Multipath is used for MPLS VPN, you can configure an additional import keyword. The import keyword indicates how many routes can be imported from one VRF into another. This occurs when the RDs are different in the source VRF and the destination VRF. This was true in Example 7-6, where 1:1:10.10.100.1/32 was imported into the VRF with RD 1:2. Therefore, the prefix became 1:2:10.10.100.1/32 in the VRF with RD 1:2.

Figure 7-14 shows iBGP Multipath in an MPLS VPN network with the import keyword. Figure 7-14 Example of a MPLS VPN with iBGP Multipath and the Import Keyword

Router BGP 1

Address-family IPv4 VRF Cust-two

Maximum-paths iBGP 2 Import 2 Exit-address-family

Router BGP 1

Address-family IPv4 VRF Cust-two

Maximum-paths iBGP 2 Import 2 Exit-address-family

PE1 and PE2 advertise a vpnv4 prefix with RD 1:1. However, the VRF on PE3 has RD 1:2. Importing the appropriate RTs controls the importing of the routes into the VRF cust-two. The RDs of the vpnv4 routes that are received on PE3 and the configured RD of VRF cust-two differ, though, so the import keyword for iBGP Multipath is needed here.

Using Multiple RDs

When a CE router is dual homed to two PE routers and RRs are used for vpnv4 routes, you will have a problem when trying to use BGP Multipath on the PE routers. BGP Multipath should install multiple paths for the same destination in the routing table. However, when RRs are used, they use the BGP best path selection process to choose the best vpnv4 route. The RRs advertise or reflect only this best path further. The ingress PE router gets only one route to install instead of two. Look at Figure 7-15. The prefix 10.1.1.0/24 is advertised from two PE routers. On each PE router, the same RD 1:1 is attached to the IP prefix. The RR receives two BGP prefix advertisements for this vpnv4 prefix. The RR chooses the best path as the one with PE1 as the next hop. This is the only path advertised from the RR to its clients (the PE routers).

Figure 7-15 RR Advertises Only the Best BGP Path

Figure 7-15 RR Advertises Only the Best BGP Path

A simple workaround for this problem is to make the vpnv4 prefixes that are advertised by both egress PE routers different. If the two routes are different, the RRs will advertise both of them. The vpnv4 routes can be made different from each other by assigning different RDs to the VRFs on both egress PE routers. By having a different RD under the VRF configuration on the egress PE routers, the routes become unique. In that case, the RRs advertise both routes, and the ingress PE routers can install both paths in the VRF routing table if BGP Multipath is used. Look at Figure 7-16 for the same example as in Figure 7-15, but with two RDs used for the same IPv4 route from the VRF on the egress PE routers. The result is that the RR can forward two vpnv4 routes to the PE routers. PE3 can install both paths if it has iBGP Multipath configured.

Figure 7-16 Usage of Multiple RDs

Figure 7-16 Usage of Multiple RDs

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