Blog Posts in February 2007
The versatile more command
Note: IOS documentation claims that the show running command is obsolete and that you should use more system:running-config. This is not true, as the show running command has a number of interesting options that are not implemented with the more command.
Filter sections of your running configuration
IOS release 12.3(2)T (integrated in 12.4) brought us a few new filters, among them the section filter that includes or excludes whole sections (start of section being defined by a line with no leading space). You can use this filter to focus on parts of your router configuration. For example, to display only the routing protocols configuration, use show running | section include router command.
Of course, you can go a step further and define an alias, for example alias exec events show running | include ^event manager configuration command defines the exec-mode events command that lists all EEM applets.
Store your EEM Tcl policies in NVRAM
Simulate interface counters with QoS policy-map
policy-map CountThe service policy counters are then inspected with the show policy-map interface name command:
class class-default
!
interface Serial0/0/0.100 point-to-point
service-policy input Count
service-policy output Count
a1#show policy-map interface Serial 0/0/0.100
Serial0/0/0.100
Service-policy input: Count
Class-map: class-default (match-any)
10 packets, 840 bytes
1 minute offered rate 0 bps, drop rate 0 bps
Match: any
Service-policy output: Count
Class-map: class-default (match-any)
61 packets, 7084 bytes
1 minute offered rate 0 bps, drop rate 0 bps
Match: any
Minimum SDM configuration
Not All Static Routes Were Created Equal
In his excellent question, Matt reminded me of (almost prehistoric) days when static routes pointing to a connected interface (not IP next-hop) had administrative distance zero. I also remembered that we've had interesting routing problems as those static routes actually behaved like connected routes (and were redistributed into routing protocols with redistribute connected command).
Reliable static routing
IOS release 12.3T introduced an interesting concept: static routes that are used only if an object a router is tracking (for example, next-hop router) is available. Named reliable static routing, this feature allows you to constantly ping next-hop router (or any other IP address) and use the static routes only if the tested destination is reachable.While the reliable static routes are easy to understand and configure, a careless implementation can lead to interesting routing loops or other instability problems. You'll find the needed in-depth design and implementation guidelines in my IP Corner article Small Site Multi-homing (which is one of the prime applications for reliable static routing) in sections Not-so-Very-Static-Routes and End-to-End Connectivity Test.
Embedded Event Manager scripting center
Frame Relay local switching
Unequal load split with static routes
Unequal load-sharing with static routes is almost impossible as there is no configuration command to assign non-default traffic share count to a static route. For example, if you configure two default routes, one pointing to a low-speed interface and another one pointing to a high-speed interface, there is no mechanism to force majority of the traffic onto the high-speed link (IOS ignores interface bandwidth when calculating load sharing ratios).
You can, howerer, use a workaround: if you configure multiple routes for the same prefix pointing to the same interface, that interface will attract proportionally more outbound traffic.
Log IP SLA failures
Reporting a state of an IP route
Unequal Cost Load-Sharing with MPLS TE
One of the most commonly asked load-sharing-related questions is “can I load-share traffic across unequal-cost links?”. In general, the answer is no. In order to load-share the traffic, you need more than one path to the destination and the only way to get multiple routes toward a destination in the IP routing table is to make them equal-cost (the only notable exception being EIGRP that supports unequal-cost load-sharing with the variance parameter).
There are, however, two cases where you can force unequal traffic split across equal-cost paths toward a destination: when using inter-AS BGP with the link bandwidth parameter, and when using unequal-bandwidth traffic-engineering tunnels.
Weird VTY numbers in IOS release 12.4T
What is the sl_def_acl access list
Recenty, a lot of people were looking for information on the sl_def_acl access list. Here's the whole story: if you've configured IOS login enhancements on your router, the router generates an access list named sl_def_acl (unless you specify your own with the login quiet-mode access-class command) the first time it has to enter the quiet mode. This access-list is then applied to the VTY lines whenever the router enters the quiet mode and removed from the after the quiet period is over. The access list itself is left in the running configuration.
Default Action in Firewall Policy Maps
Marko asked a very interesting question: What is the default class policy in a firewall policy-map (policy-map type inspect)? Or, using his original wording, "is it mandatory to use class class-default/drop" at the end of every policy map?
As it turns out, the default action for any class (unless you specify otherwise) is drop. By default, packets not matched by any traffic class are therefore dropped (unless you specify a different action in the class-default), similar to well-known ip access-list behavior.
Use access-lists to filter IP packets with IP options
Don't use copy commands in EEM applets
Moral of the story: use write memory :)