Domain-wide Prefix Distribution with Two-Level IS-IS

IS-IS Route Preference for Extended IP and IPv6 Reachability

“…So long as we confine ourselves to a single IS-IS instance and the current number of supported levels (two), it is impossible to have a prefix advertised in a Level 2 LSP and have the up/down bit set to 1. However, because [RFC5302] anticipated a future extension to IS-IS that might support additional levels, it allowed for the possibility that the up/down bit might be set in a Level 2 LSP and supported easy migration in the event such an extension was introduced. Section 3.3 of [RFC5302] states:…”

“…However, because [RFC5302] anticipated a future extension to IS-IS that might support additional levels, it allowed for the possibility that the up/down bit might be set in a Level 2 LSP and supported easy migration in the event such an extension was introduced. Section 3.3 of [RFC5302] states:…”

“…[RFC5305] introduced the IP Extended Reachability TLV 135 but did not explicitly adapt the route preference rules defined in [RFC5302] for the new TLV. [RFC5308] defines the IPv6 Reachability TLV 236 and does include an explicit statement regarding route preference --but the statement introduces use of the up/down bit in advertisements that appear in Level 2 LSPs, which is inconsistent with statements made in [RFC5302] and [RFC5305]. This document defines explicit route preference rules for TLV 135, revises the route preference rules for TLV 236, and clarifies the usage of the up/down bit when it appears in TLVs in Level 2 LSPs.…”

“…This document defines explicit route preference rules for TLV 135, revises the route preference rules for TLV 236, and clarifies the usage of the up/down bit when it appears in TLVs in Level 2 LSPs. This document is a clarification (NOT a correction) of [RFC5302] and [RFC5305]; it is a correction of the route preference rules defined in [RFC5308] to be consistent with the rules for IPv4. It also makes explicit that the same rules apply to the Multi-Topology (MT) equivalent TLVs 235 and 237.…”

On the interaction of multiple routing algorithms

“…IS-IS computes three distinct types of internal routes -L1 routes, L2 or L1→L2 routes, and L2→L1 routesand prefers L1 routes over L2 or L1→L2 routes which are preferred over L2→L1 [13].…”