What is null0 in BGP?

Using the null0 route gets the summary of the addresses that match your BGP network statement into the routing table. Once you’ve done this you can enter the network statement for 203.0. 113.0/24 in BGP, and BGP will advertise that network.

What does null0 mean?

So configuring ip route 172.17.0.0 255.255.0.0 null 0 is a way to assure that if the packet can not be routed it will be dropped. There are some other situations where a route may be configured to null 0. One of those is on routers that run BGP.

What are the BGP attributes?

There are four categories of BGP attributes:

  • Well-known mandatory: Recognized by all BGP peers, passed to all peers, and present in all Update messages.
  • Well-known discretionary: Recognized by all routers, passed to all peers, and optionally included in the Update message.
  • Optional transitive:
  • Optional non-transitive:

What is null0 in IP route?

A null interface is a virtual interface that discards IP packets and is used to prevent routing loops from occurring in the network. You do not assign the null interface an IP address. Instead, you create a static route for a network and set the next hop to the null interface (null0).

What is the use of Null0?

The Null0 interface is typically used to prevent routing loops. For example, during route aggregation, a route to the Null0 interface is always created. In the example network shown in Figure 2-11, Device A provides access services for multiple remote nodes.

What is the purpose of a Null0 route in the routing table?

Null0 is a virtual interface that is a route to nowhere and is used to prevent routing loops for destinations that are included in a summary network but do not have a specific entry in the routing table.

What is the purpose of Null0?

What are the two most important BGP attributes?

BGP Attributes

  • Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers.
  • Well-known discretionary: Must be recognized by all BGP routers and passed on to other BGP routers but need not be present in an update, for example, local preference.

What are three well-known mandatory BGP attributes?

BGP Path Attributes

  • Well-Known Mandatory (for example: Origin, AS Path, and Next Hop)
  • Well-Known Discretionary (for example: Local Preference)
  • Optional Transitive (for example: Community)
  • Optional Non-Transitive (for example: Cluster List)

What is Null0 in OSPF?

The summary address command is used only on ASBRs. Since you’re redistributing the connected routes, the router installs the null0 as the discard route, but on the other router you’ll notice that the route is an E2 route.

Can a Blackhole route prevent routing loops?

To prevent the routing loops, you can configure a 32-bit-mask blackhole route bound for addresses in the address pool on the NGFW. The NGFW discards packets whose destination addresses match the blackhole route.

What is the main advantage of multi area OSPF?

Advantages of Multiarea OSPF: Smaller routing tables – Fewer routing table entries as network addresses can be summarized between areas. Reduced link-state update overhead. Reduced frequency of SPF calculations.

What is the as _ PATH attribute in BGP?

The AS_PATH attribute is a well-known mandatory attribute and represents the list of the AS the prefix has crossed. Each AS is added in front of the current AS PATH. In the following example, the route received by R1 from R11 has the AS_PATH 11 111 and the route received from R12 has the AS_PATH 12 22 111.

Why is Med an optional non transitive BGP attribute?

Because all the previous steps of the best path selection algorithm could not select the best path (due to the fact that all the values were equal), the Origin values must be compared. Since Origin IGP is better than Origin Incomplete, the route from R11 is chosen as the best route. MED is an optional non-transitive BGP attribute.

Are there exceptions to the BGP path selection process?

There are some exceptions to the BGP path selection process when you use (advanced) BGP features like confederations, route reflectors, or multipath. Cisco has a detailed list with the BGP best path selection algorithm. I hope this lesson has been useful to understand how BGP selects the best path.

What is the default value for BGP local preference?

The local preference is used within an autonomous system and exchanged between iBGP routers. We prefer the path with the highest local preference. The default value is 100. To learn more, take a look at the BGP local preference attribute lesson.