SRv6 Flex-Algo Route Leaking
As shown in Figure 1, IS-IS divides a domain into two areas through two levels. PE2, P5, and P6 belong to the Level-2 area; P3 and P4 belong to both the Level-1 and Level-2 areas; PE1, P1, and P2 belong to the Level-1 area. In area 1 and area 2, set Flex-Algo to 128 and Metric-Type to the minimum link delay.
SRv6 Flex-Algo route leaking involves the following two directions:
- When Level-2 to Level-1 route leaking is configured on P3 and P4, according to IS-IS, the D flag in the SRv6 Locator TLV needs to be set, and the Metric field needs to be set to the link delay values of the links from P3 and P4 to PE2. The link delay values are then leaked to each node in the Level-1 area through LSPs. Currently, even if route leaking is configured on a Huawei device, the SRv6 Locator TLV will not be leaked to the Level-1 area. Instead, only the SRv6 locator prefix route is leaked.
After SRv6 Flex-Algo route leaking is supported, P3 extracts SRv6 locator prefix information from the received IS-IS Level-2 LSP that carries the SRv6 Locator TLV. P3 then floods the information to the Level-1 area through the IS-IS Level-1 LSP, which carries the SRv6 locator prefix route. The Prefix Reachability TLV 236/237 does not carry algorithm information. As such, if the Flex-Algo field in the SRv6 Locator TLV is not 0, the Prefix Reachability TLV 236/237 is not advertised. Conversely, if this field is 0, the SRv6 locator prefix route is advertised through the Prefix Reachability TLV 236/237.
- SRv6 locator prefix route calculation: After collecting topology information, PE1 uses the flexible algorithm definition (FAD) to calculate an SRv6 locator prefix route and calculates an SPF path based on the minimum link delay. If a node in area 2 does not have the Flex-Algo 128 capability, it cannot calculate a route.
- According to IS-IS, routes in the Level-1 area, including the SRv6 locator prefix route, are leaked to the Level-2 area by default. When P3 and P4 receive an IS-IS Level-1 LSP, they do not set the D flag in the SRv6 Locator TLV. Instead, they set the Metric field to the link delay values of the links from P3 and P4 to PE1 and flood the IS-IS LSP to the Level-2 area. The Prefix Reachability TLV 236/237 does not carry algorithm information. As such, if the Flex-Algo field in the SRv6 Locator TLV is not 0, the Prefix Reachability TLV 236/237 is not advertised. Conversely, if this field is 0, the SRv6 locator prefix route is advertised through the Prefix Reachability TLV 236/237.
- SRv6 locator prefix route calculation: After collecting topology information, PE2 uses the FAD to calculate an SRv6 locator prefix route and calculates an SPF path based on the minimum link delay. If a node in area 2 does not have the Flex-Algo 128 capability, it cannot calculate a prefix route.