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ME60 V800R010C10SPC500 Feature Description - WAN Access 01

This is ME60 V800R010C10SPC500 Feature Description - WAN Access
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
OSPF IP FRR

OSPF IP FRR

With OSPF IP fast reroute (FRR), a device pre-computes alternate next hops and stores them in the IP routing table. If a primary link fails, the device switches the traffic to a backup link.

Background

As networks develop, voice over IP (VoIP) and online video services pose higher requirements for real-time transmission. Nevertheless, if a primary link fails, OSPF-enabled devices need to perform multiple operations, including detecting the fault, updating the link-state advertisement (LSA), flooding the LSA, calculating routes, and delivering forward information base (FIB) entries before switching traffic to a new link. This process takes a much longer time, the minimum delay to which users are sensitive. As a result, the requirements for real-time transmission cannot be met. OSPF IP FRR can solve this problem. OSPF IP FRR conforms to dynamic IP FRR defined by standard protocols. With OSPF IP FRR, devices can switch traffic from a faulty primary link to a backup link, protecting against a link or node failure.

Major FRR techniques include loop-free alternate (LFA), U-turn, Not-Via, Remote LFA, and MRT, among which OSPF supports only LFA and Remote LFA.

Related Concepts

OSPF IP FRR

OSPF IP FRR refers to a mechanism in which a device uses the loop-free alternate (LFA) algorithm to compute the next hop of a backup link and stores the next hop together with the primary link in the forwarding table. If the primary link fails, the device switches the traffic to the backup link before routes are converged on the control plane. This mechanism keeps the traffic interruption duration and minimizes the impacts.

OSPF IP FRR policy

An OSPF IP FRR policy can be configured to filter alternate next hops. Only the alternate next hops that match the filtering rules of the policy can be added to the IP routing table.

LFA algorithm

A device uses the shortest path first (SPF) algorithm to calculate the shortest path from each neighbor with a backup link to the destination node. The device then uses the inequalities defined in standard protocols and the LFA algorithm to calculate the next hop of the loop-free backup link that has the smallest cost of the available shortest paths.

Remote LFA

LFA FRR cannot be used to calculate alternate links on large-scale networks, especially on ring networks. Remote LFA FRR addresses this problem by calculating a PQ node and establishing a tunnel between the source node of a primary link and the PQ node. If the primary link fails, traffic can be automatically switched to the tunnel, which improves network reliability.

P space

P space consists of the nodes through which the shortest path trees (SPTs) with the source node of a primary link as the root are reachable without passing through the primary link.

Extended P space

Extended P space consists of the nodes through which the SPTs with neighbors of a primary link's source node as the root are reachable without passing through the primary link.

Q space

Q space consists of the nodes through which the SPTs with the destination node of a primary link as the root are reachable without passing through the primary link.

PQ node

A PQ node exists both in the extended P space and Q space and is used by Remote LFA as the destination of a protection tunnel.

OSPF LFA FRR

OSPF IP FRR guarantees protection against either a link failure or a node-and-link failure. The link-and-node protection takes precedence over the link protection.

Link protection

Link protection takes effect when the traffic to be protected flows along a specified link.

In Figure 6-31, traffic flows from Device S to Device D. The primary link is Device S->Device E->Device D, and the backup link is Device S->Device N->Device E->Device D. The link costs meet the inequality: Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D). With OSPF IP FRR, Device S switches the traffic to the backup link if the primary link fails, keeping the traffic interruption duration.

NOTE:
Distance_opt(X, Y) indicates the shortest link from X to Y. S stands for a source node, E for the faulty node, N for a node along a backup link, and D for a destination node.
Figure 6-31 OSPF IP FRR link protection

Node-and-link protection

Node-and-link protection takes effect when the traffic to be protected.

In Figure 6-32, traffic flows from Device S to Device D. The primary link is Device S->Device E->Device D, and the backup link is Device S->Device N->Device D. The preceding inequalities are met. With OSPF IP FRR, Device S switches the traffic to the backup link if the primary link fails, keeping the traffic interruption duration.

The traffic to be protected flows along a specified link and node and the following conditions are met:

  • The link costs meet the inequality: Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D).
  • The interface costs meet the inequality: Distance_opt(N, D) < Distance_opt(N, E) + Distance_opt(E, D).
NOTE:
Distance_opt(X, Y) indicates the shortest link from X to Y. S stands for a source node, E for the faulty node, N for a node along a backup link, and D for a destination node.
Figure 6-32 OSPF IP FRR node-and-link protection

OSPF Remote LFA FRR

Similar to LFA FRR, Remote LFA protects against both link and node-and-link failures. The following example shows how Remote LFA works to protect against link failures:

In Figure 6-33, traffic flows through PE1 -> P1 -> P2 -> PE2, and the primary link is between P1 and P2. Remote LFA calculates a PQ node (P4) and establishes a Label Distribution Protocol (LDP) tunnel between P1 and P4. If P1 detects a failure on the primary link, P1 encapsulates packets into MPLS packets and forwards MPLS packets to P4. After receiving the packets, P4 removes the MPLS label from them and searches the IP routing table for a next hop to forward the packets to PE2. Remote LFA ensures uninterrupted traffic forwarding.

Figure 6-33 Networking for Remote LFA
On the network shown in Figure 6-33, Remote LFA calculates the PQ node as follows:
  1. Calculates the SPTs with all neighbors of P1 as roots. The nodes through which the SPTs are reachable without passing through the primary link form an extended P space. The extended P space in this example is {PE1, P1, P3, P4}.

  2. Calculates the SPTs with P2 as the root and obtains the Q space {PE2, P4}.

  3. Selects the PQ node (P4) that exists both in the extended P space and Q space.

OSPF anti-microloop

In Figure 6-33, OSPF remote LFA FRR is enabled, the primary link is PE1 -> P1 -> P2 -> PE2, and the backup link is PE1 -> P1 -> P3 -> P4 -> P2 -> PE2, and the link P1 -> P3 -> P4 is an LDP tunnel. If the primary link fails, traffic is switched to the backup link, and then another round of the new primary link calculation begins. Specifically, after P1 completes route convergence, its next hop becomes P3. However, the route convergence on P3 is slower than that on P1, and P3's next hop is still P1. As a result, a temporary loop occurs between P1 and P3. OSPF anti-microloop can address this problem by delaying P1 from switching its next hop until the next hop of P3 becomes P4. Then traffic is switched to the new primary link (PE1 -> P1 -> P3 -> P4 -> P2 -> PE2), and on the link P1 -> P3 -> P4, traffic is forwarded based on IP routes.

NOTE:
OSPF anti-microloop applies only to OSPF remote LFA FRR.

OSPF FRR in a Multi-Source Routing Scenario

Both OSPF LFA FRR and OSPF remote LFA FRR use the SPF algorithm to calculate the shortest path from each neighbor (root node) that provides a backup link to the destination node and store the node-based backup next hop, which applies to single-source routing scenarios. As networks are increasingly diversified, two ABRs or ASBRs are deployed to improve network reliability. In this case, OSPF FRR in a multi-source routing scenario is needed.

NOTE:
In a multi-source routing scenario, OSPF FRR is implemented by calculating the Type 3 LSAs advertised by ABRs of an area for intra-area, inter-area, ASE, or NSSA routing. Inter-area routing is used as an example to describe how OSPF FRR in a multi-source routing scenario works.
Figure 6-34 OSPF FRR in a multi-route source scenario

In Figure 6-34, Device B and Device C function as ABRs to forward area 0 and area 1 routes. Device E advertises an intra-area route. Upon receipt of the route, Device B and Device C translate it to a Type 3 LSA and flood the LSA to area 0. After OSPF FRR is enabled on Device A, Device A considers Device B and Device C as its neighbors. Without a fixed neighbor as the root node, Device A fails to calculate FRR backup next hop. To address this problem, a virtual node is simulated between Device B and Device C and used as the root node of Device A, and Device A uses the LFA or remote LFA algorithm to calculate the backup next hop. This solution converts multi-source routing into single-source routing.

For example, both Device B and Device C advertise the 100.1.1.0/24 route. After Device A receives the route, it fails to calculate a backup next hop for the route due to a lack of a fixed root node. To address this problem, a virtual node is simulated between Device B and Device C and used as the root node of Device A. The cost of the Device B-virtual node link is 0, and the cost of the Device C-virtual node link is 5. The cost of the virtual node-Device B or Device C link is the maximum value (65535). If the virtual node advertises the 100.1.1.0/24 route, it will use the smaller cost of the routes advertised by Device B and Device C as the cost of the route. Device A is configured to consider Device B and Device C as invalid sources of the 100.1.1.0/24 route and use the LFA or remote LFA algorithm to calculate the backup next hop for the route, with the virtual node as the root node.

In a multi-source routing scenario, OSPF FRR can use the LFA or remote LFA algorithm. When OSPF FRR uses the remote LFA algorithm, PQ node selection has the following restrictions:

  • An LDP LSP will be established between a faulty node and a PQ node, and a virtual node in a multi-source routing scenario cannot transmit traffic through LDP LSPs. As a result, the virtual node cannot be selected as a PQ node.
  • The destination node is not used as a PQ node. After a virtual node is added to a multi-source routing scenario, the destination node becomes the virtual node. As a result, the nodes directly connected to the virtual node cannot be selected as PQ nodes.

Derivative Functions

If you bind a Bidirectional Forwarding Detection (BFD) session with OSPF IP FRR, the BFD session goes Down if BFD detects a link fault. If the BFD session goes Down, OSPF IP FRR is triggered to switch traffic from the faulty link to the backup link, which minimizes the loss of traffic.

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Updated: 2019-01-04

Document ID: EDOC1100059473

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