Updates for EOL Ubuntu releases

When release reaches it's EOL, the repositories are no longer available at old location, so we have to change their addresses like this:

Edit /etc/apt/sources.list and change 'archive.ubuntu.com' to 'old-releases.ubuntu.com'

or

We can do this with 

sudo sed -i -e 's/archive.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

and then update with 

sudo apt-get update && sudo apt-get dist-upgrade

Thanks to fossfreedom for providing the info on this topic.

Direction of VLAN interface ACL

Tricks of the "distribute-list" OSPF subcommand

The citation is from the Cisco Press ROUTE Official Cert Guide.

The mechanics of the distribute-list router subcommand has a few surprises, which are
summarized in this list:

• The command requires either an in or out direction. Only the in direction works for filtering routes as described in this section.

• The command must refer to either a numbered ACL, named ACL, prefix list, or route map. Regardless, routes matched with a permit action are allowed into the routing table, and routes matched with a deny action are filtered.

• Optionally, the command can include the interface interface-name-and-number parameters. The router compares these parameters to the route’s outgoing interface.

Choice of the best route from the ABR perspective

When choosing the best route to reach any given subnet, ABRs follow the next two rules:

1. Intra-area route is always better, than a competing inter-area route, regardless of metric;
2. Ignore Type 3 LSA learnt in non-backbone area.

Meaning of LSID for different OSPF LSA types

Type 1 LSA's list RID as a LSID
Type 2 LSA's list DR's interface IP address for the subnet as a LSID
Type 3 LSA's list subnet number being advertised as a LSID