A Networker Blog

Es usado para informar vecinos externos acerca de como prefiero que el trafico para los prefijos anunciados regrese. Un Valor de MED menor es preferido..
Cuando un update entra al AS con un valor X en el MED. este valor es usado por el proceso de decision dentro del SA que recibio el UPDATE. Cuando este update pasa a otro SA, el Valor de Med es resetiado a 0
(al menos claro que el no se configure con un valor)

es decir algo asi

Rack1R2(config-router)#ip prefix-list 3.0 permit 164.1.3.0/24
Rack1R2(config)#route-map SETMETRIC
Rack1R2(config-route-map)#ma ip add pre 3.0
Rack1R2(config-route-map)#set metric 20
Rack1R2(config-route-map)#router bgp 200
Rack1R2(config-router)#neigh 164.1.12.1 route-map SETMETRIC out
Rack1R2(config-router)#do clear ip bgp * out

Cuando una ruta es originada por el Sistema autonomo por si, el valor de la metric sigue los valores de la metrica de IGP para la ruta. Sabiendo esto podemos entonces reflejar el valor de la metrica en auncios a externos, ejemplo un cliente conectadose al mismo proveedor de servicios con 2 sessiones BGP (Caso R1).

Cuando la metrica de IGP  se translada a MED, el trafico entrado por el SA puede entrar por un link mas cercano al destino, ya que a menor metric es preferido..  Los ISP por lo regular lo usan para balancear el trafico sobre multiples links entre los sistemas autonomos..

Por defecto, el router compara el atributo de metrica para caminos recibidos por vecinos que estan el el mismo AS
Caso R2, R3 en R1. La metrica para Distintos SA no es comparada por que la metrica esta asociada con una ruta. Comparar la metrica de disitintos Sistemas Autonomos,  es como comparar Manzanas y Narajas,. Pero si se encientra alguna razon para hacerlo cisco ofrece bgp always-compare-med

This is a new feature in 12.3 that enables the router to perform a reboot using the IOS image currently running in DRAM. Having this capability greatly reduces boot time (no decompression or copying from slow flash) and reduces down-time when the router is returned to ROM by a bug. It also enables us to reboot the router even if we don’t have a valid IOS image in flash.

However, for warm-reboot to be active the router has to be cold-rebooted at least once after it’s configured.

With this command “warm-boot”, comes two options. The first is “count” which configures the maximum number of warm-reboots after which the router will perform a cold reboot. The second keyword is “uptime” which configures the time after which warm-reboot will be safe in case of a crash.

R1(config)#warm-reboot ?
count   Set max number of continuous warm reboots
uptime  Set the uptime after which warm reboot is safe in case of a crash
<cr>

To confirm if warm-boot is active, use the command “show warm-reboot”. You can also test it by issuing a “reload warm”.  See below:

R1#show warm-reboot
Warm Reboot is enabled
Maximum warm reboot count is 5
Uptime after which warm reboot is safe in case of a crash is 5 (min)

Warm reboot can take place only after the next power cycle or reload.
R1#
So we reload the routers, after that,
R1#show warm-reboot
Warm Reboot is enabled
Maximum warm reboot count is 5
Uptime after which warm reboot is safe in case of a crash is 5 (min)

Statistics:

0 warm reboots due to crashes and 0 warm reboots due to requests have taken
place since the last cold reboot
2422 KB taken up by warm reboot storage

R1#
R1#reload warm
Proceed with reload? [confirm]

*Jun 24 18:09:35.135: %SYS-5-RELOAD: Reload requested by console. Reload Reason: Reload Command.
Smart Init is disabled. IOMEM set to: 10

Using iomem percentage: 10

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Cisco IOS Software, 2800 Software (C2800NM-ADVENTERPRISEK9-M), Version 12.4(7a), RELEASE SOFTWARE (fc3)
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Cisco 2811 (revision 49.46) with 237568K/24576K bytes of memory.
Processor board ID FTX1009C5KL
2 FastEthernet interfaces
4 Serial(sync/async) interfaces
1 Virtual Private Network (VPN) Module
DRAM configuration is 64 bits wide with parity enabled.
239K bytes of non-volatile configuration memory.
62720K bytes of ATA CompactFlash (Read/Write)
Press RETURN to get started!

By default, if the router detects corruption in memory block headers, it initiates a software-forced reload. If a software problem is detected in the network and we decide it’s safe for us to rely on the router correcting this corruption, we can configure the router to do so.

Starting 12.3(7)T, IOS gives you this ability a long with options to control the maximum frequency at which corruptions are detected and the absolute number of times the router is allowed to correct it and not initiate a software-forced reload.

Below is the command to do it.

R1(config)#exception memory ignore overflow processor frequency 300 provided this is needed every 5 minutes or more.

By default, if the router detects corruption in memory block headers, it initiates a software-forced reload. If a software problem is detected in the network and we decide it’s safe for us to rely on the router correcting this corruption, we can configure the router to do so.

Starting 12.3(7)T, IOS gives you this ability a long with options to control the maximum frequency at which corruptions are detected and the absolute number of times the router is allowed to correct it and not initiate a software-forced reload.

Below is the command to do it.

R1(config)#exception memory ignore overflow processor frequency 300