This document compares the characteristics of IS-IS and OSPF and analyzes the background of generating default routes in a single-protocol network. When IS-IS and OSPF generate two default routes on the border router at the same time,
Similarities between IS-IS and OSPF
a. is a link state protocol. Routers in an area exchange path information and link status information. Link status information is collected in the link state database and is an abstract description of the regional network topology.
b. Routing Information Exchange Similar Mechanism, Diffusion
c. Use specified routers to control the flooding and reduce the number of system resources of many-to-many adjacencies in such media.
d. Based on information in a link state database, an optimal route is calculated by using an almost identical algorithm, that is, a shortest path first algorithm
e. Supporting Hierarchical Route Selection at Two Layers
f. Both IP prefix and protocol packet authentication are supported.
g. Supporting Hierarchical Route Selection at Two Layers
Differences Between IS-IS and OSPF
a.ISIS runs directly at the data link layer. OSPF runs directly at the IP layer. IS-IS cannot run on ATM virtual links but can prevent IP packets from being spoofed.
b.ISIS The packet loads the information to be transmitted through the TLV. OSPF loads the information to be transmitted through different LSAs. Compared with the different LSAs of OSPF,
Different TLVs of IS-IS are short and exquisite. Indicates the province bandwidth.
c.ISIS Only point-to-point and broadcast links are expended. OSPF also supports point-to-multipoint, NBMA, and on-demand links.
d.ISIS is in the link division area. A router belongs to only one special area. OSPF is divided into areas on routers. A router can belong to multiple areas.
e.ISIS is classified into Level-1 and level-2. The Level-1 area is equivalent to the stub in OSPF and maintains only the LSDB of the local area.
Analysis on the generation of IS-IS and OSPF default routes
What Are the Types of OSPF Default Routes?
(1) External Default Route (Generated by Type 5 LSAs)
Run the following commands: Default-route-advertise default-route-advertise always generates a route of category 5.
(2) Internal Default Route (Class 3 LSA))
Stub totally stub totally nssa area
The ABR connected to the backbone network generates a default route of the LSA 3 type. The ABR generates a summary LSA whose link state ID is 0.0.0.0 and advertises it to the entire NSSA.
Generation of IS-IS default routes
It is invalid to import the default route in the Level-1 area. It is valid only when the default route is imported to the level-2 router.
Display isis lsdb
When importing an external default route, you need to run the default-route-advertise command to generate a default route and flood the route to the area. OSPF uses default-route-advertise.
The routing table must have an active default route that is not generated by OSPF. If there is no active non-OSPF default route in the routing table, run the default-route-advertise always command.
If the default-route-advertise command is run on an IS-IS interface, there is no other condition. This command generates an IS-IS default route, which is the same as the default-route-advertis command.
