BGP loop prevention

Written by admin. Posted in BGP

BGP loop prevention

When BGP updates travel through different Autonomous Systems (AS), EBGP routers prepend their AS to AS PATH attribute.  BGP routers use this information to check through which Autonomous Systems certain updates passed. If a EBGP speaking router detects its own AS in AS PATH attribute update, the router will ignore the update and will not advertise it further to IBGP neighbors, because it is a routing information loop. This is a built in mechanism for loop prevention in BGP.


OSPF summary LSA (Type 3 LSA)

Written by admin. Posted in OSPF

Type-3 LSA

Summary LSA can be of two flavors: type 3 and type 4. In this tutorial we will focus on type 3 LSA. Type 3 LSAs are generated by ABR. They represent networks from an area and are sent to the rest of the areas in OSPF domain. Type 1 LSAs don’t cross area boundary, so Area Border Router (ABR) uses type 3 LSA to inform other areas about networks learned in its area. Type 3 LSA uses network address as Link ID and router id of advertising router as ADV Router in OSPF database. 


OSPF Network LSA (Type 2 LSA)

Written by admin. Posted in OSPF

OSPF network LSAs (or type 2 LSA) are generated for NBMA (non broadcast multiaccess) and transit broadcast networks that resides in an area. As a transit broadcast network could serve an Ethernet network where two or more routers are connected via ethernet links and share the same broadcast domain like in the picture below.


OSPF Router LSA (Type 1 LSA)

Written by admin. Posted in OSPF

Type 1 LSA (also called Router LSA) contains information about directed connected links in the area to which the router belong. They are flooded to all routers in that area. If the router is an ABR (Area Border Router), it generates type 1 LSAs for all the areas to which it is connected and send those LSAs to all neighbors in corresponding areas.


OSPF LSAs introduction

Written by admin. Posted in OSPF

OSPF is a link-state routing protocol, which in contrast to distance vector routing protocols enable the router to have a map of the whole OSPF domain. This is done by collecting routing information from the rest of the routers in OSPF domain. Routing information is exchanged between routers by using LSAs (Link State Advertisements).