Formulación del modelo lineal mixto

When tightly tuning the interface failure-detection mechanisms, a best practice is to con- figure IP event dampening on every routed interface. IP event dampening provides a mechanism for controlling the rate at which interface state changes are propagated to the routing protocols if a link flaps. IP event dampening operates like other dampening mech- anisms, providing a penalty and penalty decay mechanism on link-state transitions. If a rapid series of link-status changes occurs, the penalty value for an interface increases until it exceeds a threshold. Then, no additional interface state changes are propagated to the routing protocols until the penalty value associated with the interface drops below the reuse threshold.

Example 3-2 shows IP event dampening configuration with default values for suppress, reuse, and maximum penalty.

ptg

interface GigabitEthernet1/1 description Uplink to Core1 dampening

ip address 10.120.0.205 255.255.255.254

ip ospf dead-interval minimal hello-multiplier 4 logging event link-status

load-interval 30 carrier-delay msec 0

To display dampened interfaces on a router, use the show interface dampening command in EXEC mode, as demonstrated in Example 3-3.

Example 3-3 Summary of the Dampening Parameters and Status

lon-n-pe1# show interfaces dampening

TenGigabitEthernet2/0/0 used for QinQ ES20 card facing lon-agg-pe1 (member of MEC toward VSS)

Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart

0 0 FALSE 0 5 1000 2000 20 16000 0

TenGigabitEthernet2/0/1 used for QinQ ES20 card facing lon-agg-pe2 (member of MEC toward VSS)

Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart

1 0 FALSE 0 5 1000 2000 20 16000 0

TenGigabitEthernet3/2 connected to lon-c1 T1/2 for PE/CE connection with sub-ints Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart

0 0 FALSE 0 5 1000 2000 20 16000 0

TenGigabitEthernet4/0/0 L3 connection to N-PE2

Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart

0 0 FALSE 0 5 1000 2000 20 16000 0

TenGigabitEthernet4/0/1 L3 connection to CRS1-P1 TE0/0/0/0

Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart

0 0 FALSE 0 5 1000 2000 20 16000 0

lon-n-pe1#

For more information about IP event dampening, refer to the IP event dampening docu- mentation available at

http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fsipevdp.html.

BFD

BFD is a detection protocol that provides fast forwarding-path failure detection for all media types, encapsulations, topologies, and the routing protocols (Border Gateway Protocol [BGP], Enhanced Interior Gateway Routing Protocol [EIGRP], Intermediate System-to-Intermediate System [IS-IS], and Open Shortest Path First [OSPF]). BFD pro-

ptg 172.18.0.1 172.16.10.2 172.16.10.1 172.17.0.1 OSPF OSPF Router A Router B BFD BFD Neighbors OSPF Neighbors BFD 3 1 2 2

Figure 3-7 Establishing a BFD neighbor relationship.

vides a low-overhead, short-duration method of detecting failures in the forwarding path between two adjacent routers, including the interfaces, data links, and forwarding planes. By sending rapid failure-detection notices to the routing protocols in the local router to initiate the routing table recalculation process, BFD greatly reduces overall network con- vergence time. BFD can be enabled at the interface and routing protocol levels.

In addition to fast forwarding-path failure detection, BFD provides a consistent failure- detection method regardless of the media type. This feature allows network administra- tors to detect forwarding-path failures at a uniform rate, rather than the variable rates detected by different routing protocol hello mechanisms. This feature makes network profiling and planning easier, and provides consistent and predictable convergence times. Cisco supports BFD asynchronous mode, which depends on the exchange of BFD con- trol packets between two systems to activate and maintain BFD neighbor sessions between routers. For a BFD session to be created, you must configure BFD on both peers. After a BFD session is created, BFD timers are negotiated, and BFD peers begin to send BFD control packets to each other at the negotiated interval.

Figure 3-7 shows a simple network with two routers running OSPF and BFD. The steps for establishing a BFD neighbor relationship, as depicted in the figure, are as follows:

1. OSPF discovers a neighbor.

2. OSPF sends a request to the local BFD process to initiate a BFD neighbor session with the OSPF neighbor router.

3. The BFD neighbor session with the OSPF neighbor router is established. Figure 3-8 shows what happens when a failure occurs in the network: 1. A failure occurs on a link between two BFD neighbors.

2. The BFD neighbor session with the OSPF neighbor router is torn down.

3. BFD notifies the local OSPF process that the BFD neighbor is no longer reachable. 4. The local OSPF process tears down the OSPF neighbor relationship.

ptg 172.18.0.1 172.16.10.2 172.16.10.1 172.17.0.1 OSPF OSPF Router A Router B BFD BFD Neighbors OSPF Neighbors BFD 2 1 4 3 3

Figure 3-8 Tearing down an OSPF neighbor relationship.

After a BFD session is established and timer negations are complete, BFD peers send BFD control packets that act in the same manner as an IGP hello protocol to detect liveli- ness, except at an accelerated rate. When using BFD, be aware of the following:

■ BFD is a forwarding-path failure-detection protocol. BFD detects a failure, but the routing protocol must take action to bypass a failed peer.

■ Typically, BFD can be used at any protocol layer. However, the Cisco implementa- tion of BFD for Cisco IOS Software Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T supports only Layer 3 clients, in particular, the BGP, EIGRP, IS-IS, and OSPF routing protocols.

■ Cisco IOS Software Release 12.2(33)SRC and later support BFD for static routing.

■ Cisco devices use one BFD session for multiple client protocols in the Cisco imple- mentation of BFD for Cisco IOS Releases 12.2(18)SXE, 12.0(31)S, and 12.4(4)T. For example, if a network runs OSPF and EIGRP across the same link to the same peer, only one BFD session is established, and BFD will share session information with both routing protocols.

For more information about BDF, refer to the document Bidirectional Forwarding Detection at www. http://tinyurl.com/55q8mw.