Tag Archives: ISIS

Some time ago I wrote about local PBR and interface PBR.

It’s time to talk about routing policy, that is a different mechanism. Routing policy is applied to routing information and it is combined with routing protocols to form policies. PBR mechanism is applied to data flows and and packets are forwarded according to the configured policy.

Routing policy is a tool which can be used to filter routes and set route attributes, when importing routing information into OSPF, RIP, ISIS or BGP protocols. BGP can use routing policy to filter advertising routes as well. Routing policy defines which of the routes from the specific routing protocol are allowed to be imported into the target routing protocol. It can be also used to match routes or certain route attributes and to change these attributes when the matching rules are met.

Routing policy command syntax:

route-policy route-policy-name { permit | deny } node node

A route-policy may consists of multiple nodes, for example:

route-policy LABNARIO-POLICY permit node 10
route-policy LABNARIO-POLICY deny node 20

Some time ago we talked about a basic configuration of BGP/MPLS VPNs. Let’s go on with hub&spoke networking today. Such solution can be adopted to control the mutual access of users, when an access control device is set. In this case no direct route exists between spoke sites. A spoke site advertises routes to a hub site and then the hub site advertises the routes to other spoke sites. Thus, communication between spoke sites is controlled by hub site.

Let’s look at our topology:

Let’s check today how a Designated Intermediate System (DIS) is elected in broadcast network.

Unlike in OSPF, in ISIS, routers of the same level, in a network segment, set up adjacencies (including non-DIS routers). DIS, as a pseudo node also generates LSPs, to describe available routers in the network. A pseudo node is indentified by the system ID of the DIS and the 1-byte circuit ID, which is always not 0. The main task of DIS is to reduce the number of generated LSPs. Even though all routers set up adjacencies in an ISIS broadcast network, LSDBs are synchronized by the DISs. One router can be DIS either for Level -1 or Level 2 routers. It depends on DIS priority configured. You can configure different DIS priority for different levels. The highest priority means the router is elected as DIS. In case that all routers have the same DIS priority, the one with the highest MAC address is chosen as DIS. By default DIS priority is set to 64 and can be changed manually. Comparing to OSFP DR, the router with the priority 0 also takes part in DIS election. Each time you connect a new router with higher DIS priority, the new router is elected as the new DIS, which causes LSPs flooding.

Let’s leave theory and carry out a test of DIS election in ISIS broadcast network.

Based on the below topology configure, IP addresses for physical and loopback interfaces (it is omitted here).

Let’s keep going and try to configure ISIS route aggregation based on the following topology:

If you want to recall how to configure ISIS adjacency on Huawei routers, just go to ‘ISIS on Huawei routers‘.

To avoid DIS election, configure all physical interfaces as ISIS point-to-point (p2p) links (Router 1 as an example):

[1-Ethernet0/0/8]isis circuit-type p2p

[1]dis isis interface 

                       Interface information for ISIS(1)
                       ---------------------------------
 Interface       Id      IPV4.State          IPV6.State      MTU  Type  DIS   
 Eth0/0/8        003         Up                 Down         1497 L1/L2 -- 
 Loop0           001         Up                 Down         1500 L1/L2 -- 
 Loop100         002         Up                 Down         1500 L1/L2 -- 

ISIS is a link-state IGP protocol. It gathers routing information from adjacent neighbors and uses the SPF algorithm to determine the best paths to destinations.

I wouldn’t like to focus on the theory because you can find it in many sources.

Let’s configure ISIS protocol based on the following topology:

Configure IP addresses of physical and loopback interfaces on all routers (it is omitted here):