Use of OSI IS-IS for routing in TCP/IP and dual environments
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Extracted elements (31)
Integrated IS-IS uses a single routing protocol for both IP and OSI, avoiding the 'Ships in the Night' (S.I.N.) approach of running OSPF and IS-IS independently. This reduces implementation complexity, CPU/memory usage, network overhead, and simplifies network management with a single coordinated MIB and backbone.
IP addresses are intentionally decoupled from IS-IS area structure and OSI addressing, because dual routers are often installed into environments with pre-assigned IP addresses and the limited IP address space constrains what can be assigned. Hierarchical summarization is encouraged but not required.
IP-only routers must carry OSI-style NSAP addresses solely for use in IS-IS packet formats, because IS-IS packet formats mandate OSI addresses for area membership determination. An algorithmic derivation from AS number and IP address is provided to avoid a separate OSI address allocation process.
The pseudonode LSP deliberately omits IP reachability information because TCP/IP explicitly assigns subnet identifiers per LAN: hosts on one logical subnet cannot directly send to a router on a different logical subnet sharing the same physical LAN, so each router must announce its own reachable subnets independently.
IP-only routers omit the 'End Systems Neighbours' and 'Prefix Neighbours' entries from their LSPs; all other IS-IS packet formats remain unchanged. OSI-only routers will silently ignore any IP-specific TLVs present in packets from dual routers.
Manually configured summary addresses at Level 2 supersede more-specific Level 1 entries based solely on [IP address, subnet mask] match; metric values are not considered for suppression. If a packet matches a summary address but no Level 1 route exists, the packet must be discarded.
All IS-IS routers, including IP-only routers, MUST be able to receive IS-IS packets using the normal OSI encapsulation for each link type, since IS-IS packets are encapsulated as OSI packets.
All IS-IS routers MUST ignore and pass through any TLV codes in received PDUs that are not recognized. This allows IP-specific TLVs to be safely ignored by OSI-only routers and OSI-specific TLVs to be safely ignored by IP-only routers.
All IS-IS routers with point-to-point interfaces MUST be able to transmit and receive ISO 9542 ISH packets on those links. IP-capable routers MUST also include the 'Protocols Supported' field in those ISH packets.
Best-match routing MUST be applied: if a destination IP address matches more than one [IP address, subnet mask] entry, the entry with the most '1' bits in the subnet mask (the most specific match) is preferred.
Each IP-capable router MUST include at least one of its IP addresses in its LSPs. Where a single router operates as both Level 1 and Level 2, the same IP address(es) MUST appear in both its Level 1 and Level 2 LSPs.
For a dual router, the 'Protocols Supported' field value MUST be identical on every link — all Hellos and LSPs from a given router must advertise the same set of supported protocols.
In a dual routing domain, if both IP and OSI traffic are to be routed between areas, all Level 2 routers MUST be dual. Within a dual area, ONLY dual routers may be used.
Routes using only internal metrics MUST always be preferred over routes using external metrics. Default routes are permitted only at Level 2 as external routes; default routes MUST NOT be used at Level 1.
The 'Protocols Supported' field MUST be present in all IS-IS Hello packets and all LSPs with LSP number 0 transmitted by IP-capable routers. If absent, the packet is assumed to originate from an OSI-only router.
When a dual router must forward an IP packet to an OSI-only router (or vice versa), the packet MUST be discarded and an error report (destination unreachable) MAY be returned.
When calculating a Level 2 LSP metric by summing a Level 1 metric and the distance to the Level 1 router, if the result exceeds 63 (the maximum reportable value), the value 63 MUST be used.
For LAN subnetworks, a pseudonode represents the LAN in the link-state database. The elected designated router sends an LSP on behalf of the pseudonode advertising links to all routers on the LAN, reducing n² adjacency announcements to n. The pseudonode LSP does NOT include IP reachability information.
IS-IS PDUs are divided into three classes: Hello packets (Level 1 LAN, Level 2 LAN, and Point-to-Point variants) for adjacency maintenance; Link State Packets (LSPs) for topology distribution; and Sequence Number Packets (Complete and Partial SNPs) for LSP database synchronization.
IS-IS uses two-level hierarchical routing: Level 1 routers know the topology within their area and forward off-area traffic to the nearest Level 2 router; Level 2 routers know the inter-area topology and route based on area addresses. A router may be both Level 1 and Level 2.
Six new IP-specific TLV fields are defined for Integrated IS-IS: (1) Protocols Supported, (2) IP Interface Address, (3) Authentication Information, (4) IP Internal Reachability Information, (5) IP External Reachability Information, and (6) Inter-Domain Routing Protocol Information.
The Inter-Domain Routing Protocol Information field in Level 2 LSPs (not in pseudonode LSPs) allows border routers to exchange external routing information. Each occurrence includes a 'type' field identifying the inter-domain protocol; initial type values are defined in Annex A.
The IP External Reachability Information field in Level 2 LSPs carries routes to destinations outside the routing domain, learned from OSPF, EGP, RIP, or any other external protocol. Each entry carries a metric typed as 'internal' (comparable to internal metrics) or 'external' (not comparable).
TOS routing maps the IP TOS field to IS-IS QOS metrics: low delay (bit 3 set) uses the delay metric; high reliability (bit 5 set) uses the reliability metric; all other combinations use the default (throughput) metric. IP has no cost TOS; the minimum cost metric has no IP mapping.
Authentication Type 1 (Simple Password) is defined in Annex D: a cleartext password is carried in the authentication field. The specification notes that the precise form of authentication mechanisms beyond simple passwords is outside its scope, leaving stronger authentication for future definition.
The Authentication Information field may be included in any IS-IS PDU to authenticate the originator and contents of the entire packet. A packet carrying invalid authentication information must be discarded. When an LSP or SNP is split across multiple packets, each packet is authenticated independently. Authentication is optional; routers that do not implement it silently ignore the field.
Level 1 forwarding decision: if the destination IP address matches an [IP address, subnet mask] reachable within the area, route via Level 1; otherwise forward to the nearest attached Level 2 router. An unattached Level 2 router sets the 'not attached' flag in its Level 1 LSPs so Level 1 routers bypass it.
IP-only routers derive a GOSIP v2.0-compatible NSAP address for IS-IS use: AFI=0x47 (1 octet), ICD=0x0005 (2 octets), DFI (1 octet), AA (3 octets), Reserved=0x0000 (2 octets), RD=AS number (2 octets), Area (2 octets), ID=IEEE 802 address or 0x0200 prepended to IP address (6 octets), SEL (1 octet).
The IP Interface Address field encodes up to 63 IP addresses per interface in Hello packets and one or more router interface addresses in LSPs. Each address is 4 octets (IPv4). The field may be omitted or present with zero entries on interfaces with no IP address.
The IP Internal and External Reachability Information fields encode reachable destinations as [IP address, subnet mask, metric] tuples. The default metric is always present; delay, expense, and error TOS metrics are optional. Metric values in level 2 LSPs are capped at 63.
Variable-length fields in IS-IS PDUs are encoded as TLV triplets: a one-octet code field, a one-octet length field, and a value of 0 to 254 octets. Unrecognized codes must be ignored and passed through unchanged.