Tag: router ID

BGP Path Attributes: The BGP Path Selection Process

BGP Path Attributes

BGP has many attributes in choosing the best path. It is like an ice cream. It has many flavors. I bought Gianduia flavor from Gelato Messina while I was preparing this topic. I think I need loads of sugar to feed my brain as this BGP topic is robust and every attribute can be well-explained if we are going to lab it.

BGP’s attributes are mainly for path manipulation and these can influence either outbound or inbound traffic. It has a systematic process that it uses to choose the best path in the network.

BGP Path Selection Algorithm

The first thing that BGP checks is whether the WEIGHT is configured or not. WEIGHT is Cisco Proprietary so it is obvious that it prioritizes Cisco devices which has BGP WEIGHT configured. In short, if you are using Cisco devices, WEIGHT is the first thing it checks before it goes on with the series of standard BGP attributes. Keep in mind that WEIGHT is local to the router and doesn’t pass to other routers. The higher the value is more preferred. (more…)

OSPF Packet Header Format

Have you ever wondered what is inside the OSPF packet header and how they communicate with other routers in the domain? The OSPF packet header varies depending upon the message type. Below is the common header format that the routers in the OSPF domain use to process packets.

Version NumberTypePacket length
Router ID
Area ID
ChecksumAuthentication Type
Message Body


OSPF Link State Advertisement (LSA) Types

Link State Advertisement (LSA) is the building block of OSPF. You will hear this LSA over and over again when studying or working with OSPF. There are so many LSA Types that are very confusing at first sight. These LSA types are very important pieces to the OSPF LSDB puzzle most especially when configuring OSPF area types.

LSA Type 1Router LSA
LSA Type 2Network LSA
LSA Type 3Summary LSA
LSA Type 4ASBR Summary LSA
LSA Type 5External LSA
LSA Type 6Multicast LSA (MOSPF)
LSA Type 7NSSA External LSA
LSA Type 8 Link Local LSA for OSPFv3

LSA Type 1 or called Router LSA, as the name implies, represents the router. Each router within an OSPF area will flood LSA Type 1 and it stays within an area.

OSPF LSA Types: show ip ospf database
show ip ospf database