OSPF uses the Type5 external link LSAs to advertise external routes originated from an ASBR. They are flooded through the whole OSPF domain (at least into the parts where they are accepted) and point packets for those external addresses to the ASBR. There is a field called Forward Address within that Type5 LSA, which is usually set to 0.0.0.0 which means that the packets should be sent directly to the ASBR.
Ah well during my changes to our VPN deployment I got into a nice pitfall with HSRP. To define our default router, we use HSRP Tracking which works quite fine but on one location I accidentally forgot to change the tracking onto the new VPN tunnel while I deleted the old one. Not that bad since the traffic took the second path we have. So I went onto the router to change the tracking and well here we go.
Basically the configuration was like this:
R6#sh run int fa 0/0 ! interface FastEthernet0/0 ip address 126.96.36.199 255.255.255.0 duplex auto speed auto standby 1 track Tunnel10 25 standby 1 ip 188.8.131.52 standby 1 priority 120
Category: Well-known discretionary
Preference: Highest value
Lately I got asked how to find the remote AS number for a BPG peering, especially if you do not have access to that remote router. In real life I’d say you should know which AS’ you peer with but in the Lab it might be a question to find out the remote AS.
The OSPF network command is used to activate OSPF on the specified interfaces. Specifying an interface with the command will include the interface into the OSPF routing process. On broadcast and point-to-point network types, the router will automatically start the neighbor discovery process by sending hello packets to the AllSPFRouters multicast address (184.108.40.206) after an interface has been integrated into the OSPF process.
If there’s another OSPF router on that link and their Hello packets match, they will try to form an adjacency but that’s another story.
Category: well-known mandatory
Preference: shortest path or IGP metric
The BGP path attributes are the characteristics for all BGP advertised routes. Additionally to the necessary informations to perform basic routing , the path attributes allow BGP to set routing policies and advertise them. Each attribute is a member of one of the following categories:
- Well-known mandatory – This attribute has to be known and implemented by all BGP implementations
- Well-known discretionary – This attribute has to be known by all BGP implementations, but does not have to be sent in an update message.
- Optional transitive – This attribute does not have to be known by all BGP implementations but the BGP prozess has to accept the path within the attribute is found and forward it to other BGP peers, even though the BGP process doesnt know the attribute.
- Optional non-transitive – An update with an attribute from this category can be ignored. The router does not have to advertise this route nor the path to its peers.
All examples in this document will use the following diagram
With BGP you have two different ways to create aggregate Routes:
- Aggregate addresses with static routes
- The command aggregate-address
The whole document will be using the following configuration:
The examples will mainly use the loopback addresses from R1
If you want to have a good read about understanding redistribution, you should check out the Internetwork Experts Blog. Petr Lapukhov wrote an excellent guide about the topic providing rules of thumb, informations about problems with redistribution and on how to solve them. The document is split in three parts linked below. Have fun