LFA FRR P,Q & PQ Space

wissal · Created Oct 10, 2021 20:45:02

Hello,

TI-LFA FRR

Topology-independent loop-free alternate (TI-LFA) fast reroute (FRR) protects links and nodes on segment routing tunnels. If a link or node fails, TI-LFA FRR rapidly switches traffic to a backup path, minimizing traffic loss.

Related Concepts

Table 2-24 TI-LFA FRR related concepts

Concept	Definition
P Space	The P space contains a set of nodes reachable to the root node on links, not the protected link, along the SPF tree that originates from the protected link's source node functioning as the root node.
Extended P space	The extended P space contains nodes reachable to the root nodes on links, not the protected link, along the SPF trees originating from the root nodes that are neighbors of protected link's source node.
Q Space	The Q space contains nodes reachable to the root node on links, not the protected link, along the reverse SPF tree originating from the protected link's destination node functioning as the root node.
PQ node	A PQ node resides in both the extended P space and Q space. The PQ node functions as the destination node of a protected tunnel.
LFA	The loop-free alternate (LFA) algorithm computes a standby link. A root node that can provide a standby link runs the Shortest Path First (SPF) protocol to compute the shortest path to a destination node. The root node then computes a loop-free standby link with the smallest cost. For more information about LFA, see IS-IS Auto FRR.
RLFA	Remote LFA (RLFA) computes a PQ node based on a protected path and establishes a tunnel between the source and PQ nodes to provide next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability. For more information about RLFA, see IS-IS Auto FRR.
TI-LFA	In some LFA or RLFA scenarios, the P space and Q space do not share nodes or have directly connected neighbors. Consequently, no backup path can be calculated, which does not meet reliability requirements. In this situation, TI-LFA can be used. The TI-LFA algorithm computes the P space and Q space based on a protected path, a shortest path tree (also called a post-convergence tree), and a repair list. The algorithm establishes a segment routing tunnel between the source node and PQ node to provide backup next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability.

Background

Conventional LFA requires that at least one neighbor be a loop-free next hop to a destination. RLFA requires that there be at least one node that connects to the source and destination nodes along links without passing through any faulty node. Unlike LFA or RLFA, TI-LFA uses an explicit path to represent a backup path, which poses no requirements on topology constraints and provides more reliable FRR.

In Figure 2-53, there are packets that need to be sent from Device A to Device F. If the P space and Q space do not intersect, RLFA requirements fail to be fulfilled, and RLFA cannot compute a backup path, that is, the Remote LDP LSP. If a fault occurs on the link between Device B and Device E, Device B forwards data packets to Device C. Device C is not the Q node and doe not have the destination IP address directly to the destination IP address. In this situation, Device C has to recompute a path. The cost of the link between Device C and Device D is 1000. Device C considers that the optimal path to Device F passes through Device B. Device C loops the packet to Device B, leading to a loop and resulting in a forwarding failure.

Figure 2-53 RLFA networking
download?uuid=e7414a79d3ea47d08dcd28835bed0edf

TI-LFA can be used to solve this problem. In Figure 2-54, if a fault occurs on the link between Device B and Device E, Device B enables TI-LFA FRR backup entries and adds new path information (node label of Device C and adjacency label for the C-D adjacency) to the packets to ensure that the data packets can be forwarded along the backup path.

Figure 2-54 TI-LFA networking

download?uuid=3932f2b081164a758acb6c1f2b0bbbad

You can find all the detail in the link below:

https://support.huawei.com/enterprise/en/doc/EDOC1100059465/2149ad9d/ti-lfa-frr

Hope this can help you

Thanks

wissal · Created Oct 10, 2021 20:45:02

Hello,

TI-LFA FRR

Topology-independent loop-free alternate (TI-LFA) fast reroute (FRR) protects links and nodes on segment routing tunnels. If a link or node fails, TI-LFA FRR rapidly switches traffic to a backup path, minimizing traffic loss.

Related Concepts

Table 2-24 TI-LFA FRR related concepts

Concept	Definition
P Space	The P space contains a set of nodes reachable to the root node on links, not the protected link, along the SPF tree that originates from the protected link's source node functioning as the root node.
Extended P space	The extended P space contains nodes reachable to the root nodes on links, not the protected link, along the SPF trees originating from the root nodes that are neighbors of protected link's source node.
Q Space	The Q space contains nodes reachable to the root node on links, not the protected link, along the reverse SPF tree originating from the protected link's destination node functioning as the root node.
PQ node	A PQ node resides in both the extended P space and Q space. The PQ node functions as the destination node of a protected tunnel.
LFA	The loop-free alternate (LFA) algorithm computes a standby link. A root node that can provide a standby link runs the Shortest Path First (SPF) protocol to compute the shortest path to a destination node. The root node then computes a loop-free standby link with the smallest cost. For more information about LFA, see IS-IS Auto FRR.
RLFA	Remote LFA (RLFA) computes a PQ node based on a protected path and establishes a tunnel between the source and PQ nodes to provide next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability. For more information about RLFA, see IS-IS Auto FRR.
TI-LFA	In some LFA or RLFA scenarios, the P space and Q space do not share nodes or have directly connected neighbors. Consequently, no backup path can be calculated, which does not meet reliability requirements. In this situation, TI-LFA can be used. The TI-LFA algorithm computes the P space and Q space based on a protected path, a shortest path tree (also called a post-convergence tree), and a repair list. The algorithm establishes a segment routing tunnel between the source node and PQ node to provide backup next hop protection. If the protected link fails, traffic automatically switches to the backup path, which improves network reliability.

Background

Conventional LFA requires that at least one neighbor be a loop-free next hop to a destination. RLFA requires that there be at least one node that connects to the source and destination nodes along links without passing through any faulty node. Unlike LFA or RLFA, TI-LFA uses an explicit path to represent a backup path, which poses no requirements on topology constraints and provides more reliable FRR.

In Figure 2-53, there are packets that need to be sent from Device A to Device F. If the P space and Q space do not intersect, RLFA requirements fail to be fulfilled, and RLFA cannot compute a backup path, that is, the Remote LDP LSP. If a fault occurs on the link between Device B and Device E, Device B forwards data packets to Device C. Device C is not the Q node and doe not have the destination IP address directly to the destination IP address. In this situation, Device C has to recompute a path. The cost of the link between Device C and Device D is 1000. Device C considers that the optimal path to Device F passes through Device B. Device C loops the packet to Device B, leading to a loop and resulting in a forwarding failure.

Figure 2-53 RLFA networking
download?uuid=e7414a79d3ea47d08dcd28835bed0edf

TI-LFA can be used to solve this problem. In Figure 2-54, if a fault occurs on the link between Device B and Device E, Device B enables TI-LFA FRR backup entries and adds new path information (node label of Device C and adjacency label for the C-D adjacency) to the packets to ensure that the data packets can be forwarded along the backup path.

Figure 2-54 TI-LFA networking

download?uuid=3932f2b081164a758acb6c1f2b0bbbad

You can find all the detail in the link below:

https://support.huawei.com/enterprise/en/doc/EDOC1100059465/2149ad9d/ti-lfa-frr

Hope this can help you

Thanks

hpl_Panda · Created Oct 11, 2021 11:12:08

Good

Unicef · Created Oct 12, 2021 13:29:42

Well done answer

LFA FRR P,Q & PQ Space

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