IP Routing

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):

This article is a continuation of the basic BGP with Huawei CLI.

Today I want to explain you, why labnarioR5 router cannot ping BGP prefixes advertised by BGP AS50. Do you remember our BGP topology? I have configured BGP protocol as in the picture below:

There are three BGP Autonomous Systems: AS50, AS100 and AS55. All are connected via EBGP peering sessions. AS100 has three routers forming IBGP full mesh. LabnarioR1 router in AS50 advertises three prefixes:

• 11.10.10.0/24
• 12.10.10.0/24
• 13.10.10.0/24

All these prefixes are advertised by the routers in AS100 and pass to AS55. LabnarioR5 router has these prefixes in its BGP and routing tables, but cannot ping them. Why is it like that?

BGP stands for Border Gateway Protocol. It is widely used among Internet Service Providers to make core routing decisions on the Internet. The current BGP version is BGP-4 defined in RFC 4271.

BGP uses TCP port 179 as its underlying delivery mechanism to increase the reliability of the peer connection. BGP is called a path vector routing protocol. It uses a list of AS numbers through which the packet must pass in order to reach the destination. This list of AS numbers is associated with a BGP route and is called the AS_Path attribute. AS_Path is one of several path attributes associated with each BGP route. How does a BGP select best path to a destination network, you can read at how does BGP select routes.

I do not want to describe BGP in details, as this is out of the scope of this article. What I want to do is to show you, how to configure basic BGP features using Huawei CLI. Especially I want to show you how to:

• run BGP protocol
• configure IBGP peering sessions using peer groups
• configure EBGP peering
• advertise networks.