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Sunday, July 22, 2012

ccnp route redistribute!!


Note: Route-maps that are applied to redistribution behave the same way as ACLs: if the route does not match any clause in a route-map then the route redistribution is denied as if the route-map contained deny statement at the end.

Notice that RIP metric is based on hop count only, and the maximum valid metric is 15. Anything above 15 is considered infinite. By default, when no metric is assigned when redistributing from EIGRP, OSPF, IS-IS, BGP into RIP, the default metric will be infinite. Therefore we must define a metric that is understandable to the receiving protocol. Usually, we should use a small value (like 1, 2, 3) so that after redistributing, that route can be advertised through many routers (because the limit is 15).

Same as RIP, when redistribute into EIGRP from OSPF, the default metric is infinite -> We must set a seed metric when redistributing into EIGRP. Below lists the default seed metrics when redistributing from a routing protocol into another:

Redistributed Protocol Default Seed Metric
RIP        Infinity
IGRP/EIGRP Infinity
OSPF        20 for all (except for BGP, which is 1)
BGP        is set to IGP metric value

The “passive-interface …” command in EIGRP or OSPF will shut down the neighbor relationship of these two routers (no hello packets are exchanged)

In RIP, this command will not allow sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP speaking neighbors. This means that the router will still be able to receive updates o­n that passive interface and use them in its routing table.

When redistributing into OSPF, the default route type is E2. Notice that the cost of E2 type is always the cost of the external route only.
Also, the default seed metric when redistributing into OSPF is always 20 (except for BGP, which is 1).

When redistributing into EIGRP, the external EIGRP routes have an administrative distance of 170 by default.

These rules apply when using route-map with an access-list:
* If you use an ACL in a route-map permit clause, routes that are permitted by the ACL are redistributed.
* If you use an ACL in a route-map deny clause, routes that are permitted by the ACL are not redistributed.
* If you use an ACL in a route-map permit or deny clause, and the ACL denies a route, then the route-map clause match is not found and the next route-map clause is evaluated.

And in each route-map:
* Multiple match criteria in the same line use a logical OR
* Each vertical match uses a logical AND

Don’t be confused between the route-map clause number (sequence number) and the access-list number. The “match ip address” specifies which access lists are matched. The route-map clause number only uses to specify the order in which the clauses are executed.

If IGRP and EIGRP use the same Autonomous System (AS) then redistribution occurs automatically.

Prefix lists are configured with permit or deny keywords to either permit or deny the prefix based on the matching condition. A prefix list consists of an IP address and a bit mask. The IP address can be a classful network, a subnet, or a single host route. The bit mask is entered as a number from 1 to 32.

Prefix lists are configured to match an exact prefix length or a prefix range. The ge and le keywords are used to specify a range of the prefix lengths to match, providing more flexible configuration that can be configured with just the network/length argument. The prefix list is processed using an exact match when neither ge nor le keyword is entered.

The ASBR accepts RFC 1918 addresses and set these networks to “tag 255″ but when advertising into Area 0, the ABR Area 0 filters out these networks because they match “tag 255″ so the OSPF backbone will not learn any RFC 1918 addresses

By default, all routes redistributed into OSPF will be tagged as external type 2 (E2) with a metric of 20, except for BGP routes (with a metric of 1).

Note: The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route. A type 1 cost is the addition of the external cost and the internal cost used to reach that route.

The set command specifies the action(s) to take on the packets that match the criteria. You can specify any or all of the following:
* precedence: Sets precedence value in the IP header. You can specify either the precedence number or name.
* df: Sets the “Don’t Fragment” (DF) bit in the ip header.
* vrf: Sets the VPN Routing and Forwarding (VRF) instance.
* next-hop: Sets next hop to which to route the packet.
* next-hop recursive: Sets next hop to which to route the packet if the hop is to a router which is not adjacent.
* interface: Sets output interface for the packet.
* default next-hop: Sets next hop to which to route the packet if there is no explicit route for this destination.
* default interface: Sets output interface for the packet if there is no explicit route for this destination.

The “show route-map “route-map name” displays the policy routing match counts so we can learn if PBR reacts to packets

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