CCNP ENARSI v8 Chapters 15 – 17: Conditional Forwarding and Route Redistribution Test Online

Hint

ACLs are processed in a top down manner. When an ACL is inspected, if the information in a packet header and an ACL statement match, the remaining statements are not examined, and the packet is either denied or permitted through as specified by the ACL. If a packet header does not match an ACL statement, the packet is tested against the next statement in the list. This matching process continues until the end of the list is reached. Every ACL has an implied deny at the end of the list. This implied deny statement is applied to all packets for which conditions did not test true.

Hint

To match the criterion of the need for a prefix match pattern, a prefix must have the same 13 high-order bits as 10.168.0.0/13 and also have a network mask that is equal to or greater than /24. The two prefixes that meet both requirements are 10.168.0.0/24 and 10.173.1.0/28.

Hint

Packets that match access-list 2 will have route-map 20 applied. This route map specifies that matching packets will have a policy applied that sends these packets to the next hop 172.17.1.2.

Hint

To match the criteria in the prefix list, a prefix must have the same 8 high-order bits as 10.0.0.0/8 and also have a network mask that is equal to or greater than /22 but also less than or equal to /26. The prefix that meets both requirements is 10.0.0.0/24.

Hint

PBR has the following characteristics and capabilities:

• It examines packets as they enter a router interface.
• Local PBR can identify packets that originate from a router.
• It can perform conditional forwarding based on packet characteristics in addition to a source or destination IP address.
• It can forward packets to next-hop addresses that are not in the routing table by using the set ip default next-hop command.
• The next-hop addresses defined in set statements are not placed in the RIB.

Hint

Route maps are applied to inbound interfaces. Therefore, in this example the route map should be placed on the G1/0 interface of router RTB so that it can forward packets destined for the internet through router R2.

Hint

Route maps statements have rules as to how they are applied. If no processing action is provided, the default value is permit . If no sequence number is provided, the number increments by 10 automatically. If there is no matching statement, an implied all prefixes statement is applied. Processing of route map statements stops after a match criterion is matched.

Hint

If a sequence number is not provided in the route map command, the sequence number is auto-incremented by 10.

Hint

When redistributing into OSPF, if the optional subnets keyword is not included, only classful networks are redistributed.

Hint

The default seed metric for routes redistributed into EIGRP is infinity. However, since a seed metric is defined by the default-metric command, this new metric is applied to both RIP and OSPF routes when redistributed into EIGRP.

Hint

The redistribution configuration exists under the destination protocol and identifies the source protocol. The source protocol, EIGRP in this case, provides the network prefixes that are to be redistributed into OSPF. The option for the metric-type keyword is either 1 or 2. Type 2 is the default if not specified. Redistributed routes appear in the routing table with a code of O E1 or OE 2.

Hint

BGP assigns a default weight metric of 32,768 to routes redistributed from IGPs.

Hint

For a route to be redistributed, it must exist in the RIB. Redistribution is sequential but not transitive. By default, BGP redistributes only EBGP routes into IGP protocols. Although EBGP can be redistributed into an IGP, it is not always recommended.

Hint

In redistribution between two protocols, the redistribution configuration is issued under the destination protocol and identifies the source protocol. In redistribution between two or more routing protocols on a single router, redistribution is not transitive.

Hint

The default seed metric for EIGRP is infinity. In this example a new seed metric for OSPF routes is defined but not for RIP routes. Therefore, RIP routes will have a metric of infinity when redistributed into EIGRP.

Hint

External EIGRP routes are assigned an administrative distance of 170 and a default seed metric of infinity when they are redistributed into EIGRP.

Hint

There are two types of OSPF external routes, type 1 and type 2. Type 2 is the default setting if not otherwise specified in the redistribute command. The metric for type 1 routes equals the redistribution metric plus the total path metric. For type 2 routes the metric stays the same as the redistribution metric throughout the OSPF domain. The seed metric for BGP-sourced routes redistributed into OSPF is 1.

Hint

The redistribution configuration exists under the destination protocol and identifies the source protocol. The source protocol, EIGRP in this case, provides the network prefixes that are to be redistributed. Routes redistributed into OSPF from another IGP receive a default seed metric of 20 and a default route type of type 2.

Hint

When routes are being redistributed from one routing source into another routing source, the information from the original routing source is lost when the seed metric is injected at the redistribution point.

Hint

When implementing mutual redistribution on a network with links using a variety of speeds, it is common for suboptimal routing to occur.

Hint

Router BR1 is configured to set a tag of 10 for the routes identified by the prefix list of 10.44.100.0. However, the prefix list being referenced is incorrect and does not exist.

Hint

The default seed metric is used by specific routing protocols to provide a default metric value for a destination routing process. This ensures that a metric is available and a route can become reachable after the redistribution process between varying routing protocols.

Hint

The EIGRP configuration command distance eigrp ad-internal ad-external is used to modify the default administrative distance on IOS routers. The valid values for the AD are between 1 and 255; a value of 255 stops the installation of the route into the routing information base (RIB).

Hint

The redistribution process does not include external OSPF routes by default. In order to allow the external OSPF routes to be implemented with redistribution, the match keyword should be used.

Hint

The subnets keyword is extremely important in the redistribution of routes into the OSPF routing process. Without the subnets keyword, only classful networks will be included in the redistribution process.

Hint

In order to have an external EIGRP route with a default AD of 170 preferred over an OSPF route with a default AD of 110 to the same destination network, the administrative distance value should be modified to a lower value.

Hint

In order to include the networks associated with the local interfaces participating in the routing process that is being redistributed, the include-connected keyword must be used.

Hint

When working with dissimilar routing protocols, a network technician must remember that a seed metric has the following order of preference:

• A metric defined in the route map that was applied to the redistribute command
• A metric parameter defined with the redistribute command
• A metric defined with the default-metric command

Hint

Default redistribution behavior is as follows:

• BGP only redistributes EBGP routes into IGP protocols.
• Routes redistributed into OSPF from BGP have a seed metric of 1.
• Routes redistributed into EIGRP have an AD of 170.
• Routes redistributed into OSPF have an AD of 110.
• The default BGP weight for redistributed routes is 32,768.

Hint

Routes learned through another routing protocol are treated as external routes by OSPF. Routes learned from BGP are given a default seed metric of 1. Routes learned from another IGP are given a default seed metric of 20.

Hint

The metric keyword is required when redistributing OSPF routes into an EIGRP routing process.

Hint

When OSPF is being redistributed into BGP, the match keyword must be included during redistribution in order to include external OSPF routes. Without the match keyword, only internal OSPF routes are included in the redistribution process.