The local leaf can be the actual leaf node that is connected to the host, or an ASBR node that acts as the local leaf for the LSP in that AS, as illustrated in Figure 1.
Two types of MoFRR can exist in a unique AS:
IGP MoFRR — When the mcast-upstream-frr command is enabled for LDP, the local leaf selects a single local root, either ASBR or actual, and creates a FEC toward two different upstream LSRs using LFA/ECMP for the ASBR route. If there are multiple ASBRs directed toward the actual root, the local leaf only selects a single ASBR; for example, ASBR-1 in Figure 2. In this example, LSPs are not set up for ASBR-2. The local root ASBR-1 is selected by the local leaf and the primary path is set up to ASBR-1, while the backup path is set up through ASBR-2.
For more information, see Multicast LDP Fast Upstream Switchover.
Figure 2. IGP MoFRR
ASBR MoFRR — When the mcast-upstream-asbr-frr command is enabled for LDP, and the mcast-upstream-frr command is not enabled, the local leaf selects a single ASBR as the primary ASBR and another ASBR as the backup ASBR. The primary and backup LSPs are set to these two ASBRs, as shown in Figure 3. Because the mcast-upstream-frr command is not configured, IGP MoFRR is not enabled in the AS2, and therefore none of the P routers perform local IGP MoFRR.
BGP neighboring and sessions can be used to detect BGP peer failure from the local leaf to the ASBR, and can cause a MoFRR switch from the primary LSP to the backup LSP. Multihop BFD can be used between BGP neighbors to detect failure more quickly and remove the primary BGP peer (ASBR-1 in Figure 3) and its routes from the routing table so that the leaf can switch to the backup LSP and backup ASBR.
Figure 3. ASBR MoFRR
The mcast-upstream-frr and mcast-upstream-asbr-frr commands can be configured together on the local leaf of each AS to create a high-resilience MoFRR solution. When both commands are enabled, the local leaf sets up ASBR MoFRR first and sets up a primary LSP to one ASBR (ASBR-1 in Figure 4) and a backup LSP to another ASBR (ASBR-2 in Figure 4). In addition, the local leaf protects each LSP using IGP MoFRR through the P routers in that AS.
Enabling both the mcast-upstream-frr and mcast-upstream-asbr-frr commands can cause extra multicast traffic to be created. Ensure that the network is designed and the appropriate commands are enabled to meet network resiliency needs.
At each AS, either command can be configured; for example, in Figure 4, the leaf is configured with mcast-upstream-asbr-frr enabled and sets up a primary LSP to ASBR-1 and a backup LSP to ASBR-2. ASBR-1 and ASBR-2 are configured with mcast-upstream-frr enabled, and both perform IGP MoFRR to ASBR-3 only. ASBR-2 can select ASBR-3 or ASBR-4 as its local root for IGP MoFRR; in this example, ASBR-2 has selected ASBR-3 as its local root.
There are no ASBRs in the root AS (AS-1), so IGP MoFRR is performed if mcast-upstream-frr is enabled on ASBR-3.
The mcast-upstream-frr and mcast-upstream-asbr-frr commands work separately depending on the needed behavior. If there is more than one local root, then mcast-upstream-asbr-frr can provide extra resiliency between the local ASBRs, and mcast-upstream-frr can provide extra redundancy between the local leaf and the local root by creating a disjointed LSP for each ASBR.
If the mcast-upstream-asbr-frr command is disabled and mcast-upstream-frr is enabled, and there is more than one local root, only a single local root is selected and IGP MoFRR can provide local AS resiliency.
In the actual root AS, only the mcast-upstream-frr command needs to be configured.