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

Propagation of default route into EIGRP domain

Two main options exist:
1. Configure static default route and then redistribute it into EIGRP;
2. Configure default-network command that should point toward the network connected to gateway router (for example, it could be it's WAN subnet or just loopback host route). This command could be configured at the Enterprise edge router that have default route or full routing table. All routers in EIGRP domain will mark this newly learnt network as "candidate default". The most easily understandable explanation for me is that we should mark network as default if we know that when packet from anywhere of the inside network reach the router connected to this network it will know how to further forward this packet.

Date of the beginning of my CCIE studies!

October 1st, 2012 I began to study my CCIE Written and Lab Exam. Hope to be ready to pass the Lab by the end of 2013. First attempt is my goal, so I don't want to rush. Since the born of my twin daughters on 17 of January I don't have much time, but hope that this goal can be accomplished.

EIGRP Citation

About routers going Active and querying their neighbors.

From the CCNP ROUTE Cert Guide:

If a router receives an EIGRP Query for a prefix/prefix length, does not have an exactly matching (both prefix and prefix length) route, but does have a summary route that includes the prefix/prefix length, that router immediately sends an EIGRP Reply and does not flood the Query to its own neighbors.

Route filtering by means of distribution lists with ACLs, IP Prefix Lists and Route Maps

When using ACLs with Distribution lists, the most confusing thing for me is that:

1. Deny statement in ACL is used to deny the route from being accepted or sent;
2. If there is no "Permit Any" in the end of ACL then it will "pass" only routes explicitly mentioned with "Permit" statement.

I'm used to know, that to "catch" something, ACL's should do it with "Permit" statement (for example, this is the case when using distribution lists with route maps).

IP Prefix Lists use the same logic:

"Each command has a permit or deny action, but because it is used only for matching routes, and not for packet filtering, the permit or deny keyword just implies whether a route is matched (permit) or not (deny)." - CCNP ROUTE Off Cert Guide.

So, to permit the route in distribution list, "Permit" statement must be used in IP Prefix List and to deny the route "Deny" statement is used. As always, at the end of list is implicit "Deny All" statement.