Multicast RPF Table
Level 2 Partial Sequence Number PDU (PSNP): Used to request one or more level 2 LSPs that
were detected to be missing from a level 2 CSNP. The local router sends a level 2 PSNP to the neighbor that transmitted the incomplete level 2 CSNP. That router, in turn, forwards the missing level 2 LSPs to the requesting router.
8.2.2 IS-IS Neighbor State Machine on Point- to-Point Links
To establish adjacencies on point-to-point links, each side declares the other side to be reachable if a Hello packet is heard. Once this occurs, each side then sends a CSNP to trigger database
8.2.3 IS-IS on Multiaccess Networks
On broadcast networks, a single router is elected as the designated intermediate system (DIS). A DIS is elected independently for each level. The DIS on a LAN that has both level 1 and level 2
adjacencies can be the same router, but it is not required.
An IS-IS DIS is functionally similar to an OSPF designated router (DR). Like the OSPF DR, the DIS acts as the spokesperson for the LAN. The DIS is the only router that builds an LSP for the LAN itself, which is known as the pseudonode. Unlike OSPF, IS-IS does not provide a backup DIS capability. A backup is not necessary because all IS-IS routers on a LAN become adjacent with one another. When the DIS becomes unavailable, a new DIS is elected. In OSPF, by contrast, all routers on a LAN become adjacent only with the DR and backup DR.
DIS priority is transmitted in the Hello packets. If a new router appears on a LAN with a higher DIS priority, it becomes the new DIS. This also differs from OSPF, where DR-ship is "sticky." In OSPF, once a router is elected DR, it remains so unless it disappears. In OSPF, the DR is typically the first router that comes up. IS-IS's preemptive DIS behavior, on the other hand, provides deterministic DIS selection.
8.2.4 Exchanging Link-State Information with Neighbors
Each side of an IS-IS adjacency sends a CSNP when the adjacency first comes up and when a change is detected in the link-state database. The CSNP contains a summary of each LSP in the database
(essentially the LSP's name, a checksum, and an age), but it does not carry any network layer reachability information. When a router receives a CSNP, it checks to see whether the information contained within corresponds to its own link-state database. If the local router recognizes that its
neighbor is missing or has an older version of an LSP, the local router sends the latest version of the LSP to the neighbor.
If the local router notices that it is missing an LSP or its neighbor has a newer version of an LSP, the local router sends a PSNP to its neighbor requesting that LSP. The neighbor responds to the PSNP with the appropriate LSP.
This procedure happens throughout the network and eventually all L1 routers in each area have the exact same level 1 link-state database. Likewise, all L2 routers have the exact same level 2 link-state database.
8.2.5 Interarea Leaking
An L1/L2 router sets the ATTACHED bit (ATT) in its L1 LSP. Setting this bit informs all L1 routers in this area that the L1/L2 router can be used to reach other areas. L1 routers install a default route toward the closest router that generated an LSP with the ATT bit set. The operation of the ATT bit provides similar functionality to OSPF totally stubby areas, in that only a default route is injected into the area and more specific routes are suppressed.
RFC 2966, "Domain-wide Prefix Distribution with Two-Level IS-IS," introduces a way to enable IS-IS areas to behave similarly to OSPF stub and not-so-stubby areas. Like an OSPF ABR, an IS-IS L1/L2 router that supports RFC 2966 can inject route information from other areas into its own area. With interarea leaking, an L1 router no longer has to send traffic destined for other areas to its own closest L1/L2 router. Instead, an L1 router can send interarea traffic to the L1/L2 router closest to the
8.2.6 Extending TLVs
The flexibility of TLVs makes IS-IS much easier to extend than OSPF for such added capabilities as multiple topologies, IPv6, and traffic engineering. Enabling IS-IS to carry a new type of information is as simple as adding and defining a new TLV.
M-ISIS introduces new TLVs, but no new PDU types are required. This can be compared to adding new attributes to the existing BGP Update message, which also did not require any new message formats.
Table 8-1 shows the TLVs applicable to each PDU.
In the rest of this section, we discuss the IS-IS TLVs upon which the new M-ISIS TLVs build. The new M- ISIS TLVs are discussed in section 8.4.2.
The Extended IS Reachability (TLV 22) is defined in an Internet-Draft titled "IS-IS Extensions for Traffic Engineering" (draft-ietf-isis-traffic-04.txt). TLV 22 adds the ability for IS-IS to carry information, such as link color and bandwidth, that is used in traffic engineering. The format of TLV 22 is as follows:
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID and Pseudonode Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| System ID and Pseudonode Number (continued) |Default Metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Default Metric (continued) |Sub-TLVs Length| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| 0 - 244 octets of sub-TLVs |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Table 8-1. TLVs for PDUs