Blog Posts in June 2008
DNS views work with EEM
Simple extensions to exec-mode CLI
Is Internet Melting Down?
A while ago I’ve read a post about the potential Internet meltdown by Michael Morris. He provided an amazingly accurate analysis of the facts … and ended with a wrong conclusion. To understand the whole issue, please thoroughly read his text in its entirety before proceeding.
Back? OK. As I said, his analysis was great, but the conclusions were wrong. Regardless of whether we use IPv4 (and advertise smaller and smaller prefixes) or IPv6, the problem is the same: everyone wants to have chunks of non-aggregatable provider-independent public address space (so you can freely move between Service Providers) and everyone advertises these PI prefixes to multiple service providers (because multihoming is so cheap these days). Even networks that are not multihomed today use their own PI address space and private AS numbers to connect to a single ISP, so they could get multi-homed in a second if they feel like it.
The growth of the Internet routing tables thus has nothing to do with the prefix sizes and version of IP, but with the requirements of the end-customers to have immediate capability to switch service providers at will. As long as this trend persists (and I cannot see it stopping, as Internet is considered a commodity these days), the routing tables will grow, regardless of whether we use IPv4 or IPv6 or CLNS or something not invented yet.
For more details watch Upcoming Internet Challenges and Surviving the Internet Default Free Zone webinars.
… updated on Monday, December 28, 2020 10:32 UTC
Using EIGRP in MPLS VPN Networks
We described EIGRP-in-VRF in MPLS and VPN Architectures, Volume II. A few details have changed in the meantime; you have to configure the following features to get EIGRP running within MPLS/VPN environment:
- The autonomous-system command within the VRF address family is mandatory, even if the VRF AS number matches the EIGRP process number.
- The default BGP-to-EIGRP redistribution metric has to be configured, otherwise remote EIGRP routes will not be redistributed even though they have EIGRP metric encoded in extended BGP communities.
- Things work best if you disable auto-summary on PE-routers.
Static DHCP assignment for clients without client-id
A while ago I've installed Fedora Linux on one of my workstations and spent enormous amount of time trying to give it a static IP address from the Cisco IOS DHCP server. I though I was the only one dumb enough to have this problem, so I didn’t document my solution, but then one of the readers made a comment to the Assigning server IP addresses with DHCP post describing almost identical symptoms:
I have a hp2300n and I want to make a static IP assignment with DHCP. Nothing is working: hardware-address, client-identifier, no prepend, 01 prepend, 00 prepend.
In my case, the Fedora DHCP client did not send any DHCP client-ID in the DHCPREQUEST message. One would think that the IOS DHCP server would use the MAC address as the client-ID, but that's not the case. You have to configure the hardware-address parameter in the host DHCP pool to match the MAC address of the DHCP client with the pool and the static IP address:
ip dhcp pool fedi
host 192.168.200.206 255.255.255.240
hardware-address 000f.fe83.bca9
dns-server 208.67.220.220 208.67.222.222
This article is part of You've asked for it series.
PPP default route
You do not need the route "ip route 0.0.0.0 0.0.0.0 Dialer0 10 track 100" and the tracking if you configure "ppp ipcp route default" on the dialer interface. Works the same way... :-)You might be wondering why Cisco's engineers decided to pollute IOS with yet another feature. The problem they had was the way PPP over Frame Relay is implemented: it uses virtual interfaces and although you have a very static connection, you cannot bind a static interface name to it. A dynamic interface (with potentially changing name) is cloned from the virtual template every time the PPP-over-Frame-relay session is started. Obviously you cannot configure a static default route pointing to it in advance, so you need yet another feature to do it (I'll not even try to figure out how to create non-default static routes pointing to cloned interface).
Display locally originated BGP routes
Displaying the BGP routes originated in the local AS is simple: you just filter the BGP table with a regular expression matching an empty AS path. Displaying routes originated by the local router is tougher. You could use the fact that the local routes have the weight set to 32768:
PE-A#show ip bgp quote-regexp "^$" | inc Network|32768
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.1/32 0.0.0.0 0 32768 i
This would work if you don’t play with BGP weights in network statements. If you’ve changed the weights, you should filter the routes based on the BGP next-hop: locally originated routes have the next-hop 0.0.0.0 and all other routes should have a non-zero BGP next-hop. To filter BGP routes based on the next-hop you have to:
- Define an access-list that matches desired next-hop (0.0.0.0)
- Define a route-map that uses the access-list to match IP next hop.
- Display BGP routes matched by a route-map.
A sample configuration and show command printout is included below:
ip access-list standard AllZeros
permit 0.0.0.0
!
route-map NextHopSelf permit 10
match ip next-hop AllZeros
PE-A#show ip bgp route-map NextHopSelf | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.1/32 0.0.0.0 0 32768 i
To make this command simpler to use, define an alias: alias exec mybgp show ip bgp route-map NextHopSelf | begin Network.
Control Plane Protection logging does not work on transit subinterface
IOS auto-upgrade
Web 2.0 presentation
Display BGP routes originated in the local AS
The easiest way to display BGP routes originating in the local autonomous system is to use the regular expression ^$ (empty AS-path) in the show ip bgp regexp command, for example:
PE-A#show ip bgp regexp ^$
BGP table version is 10, local router ID is 10.0.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.1/32 0.0.0.0 0 32768 i
r>i10.0.1.2/32 10.0.1.2 0 100 0 i
If you want to apply a show filter to the printout of this command, you have to use the quote-regexp variant; otherwise the rest of the line is interpreted as regular expression. To skip the header explaining the BGP status code (we know them by heart by now, don’t we?), use …
PE-A#show ip bgp quote-regexp "^$" | begin Network
Network Next Hop Metric LocPrf Weight Path
*> 10.0.1.1/32 0.0.0.0 0 32768 i
r>i10.0.1.2/32 10.0.1.2 0 100 0 i
… and end with the eye candy – define this command as an alias: alias exec localbgp show ip bgp quote-regexp "^$" | begin Network.
Fix the "do" command
Configuring lines and terminals
- If you want to configure a permanent line characteristic (for example, international), you should do so in the VTY configuration (see also how the VTY configurations are merged);
- If you want a temporary change in the characteristic of your current line (VTY or console), use terminal characteristic to enable it or terminal no characteristic to disable it.
Use the explicit "address-family ipv4" in BGP configurations
To change the format of the BGP configuration, configure the IPv4 address family with the address-family ipv4 unicast router configuration command (the neighbor statements and other configuration settings pertinent to IPv4 configuration are automatically moved into the new address family) or manually activate a BGP neighbor for IPv4 route exchange with the neighbor activate router configuration command.
CEF and MLS
Harold Arley Morales has asked an interesting question:
What's the difference between Cisco Express Forwarding and Cisco MLS? Is Cisco's implementation of MLS standardized?
CEF is a routing table lookup mechanism. Instead of doing a lookup in the main IP routing table (displayed with the show ip route), the router does a lookup in a fully computed non-recursive version of the IP routing table (Forwarding Information Base - FIB) with layer-2 next-hop information attached to it (adjacency table).
MLS is a caching mechanism (similar to Netflow) that offloads layer-3 processing from the routing component into layer-2 ASICs that cannot perform full-blown layer-3 switching. When the layer-2 engine detects a single IP packet traversing multiple VLANs, the MLS populates the cache with the flow details and the subsequent packets belonging to the same flow (same source/destination IP addresses and port numbers ...) are switched without going through all the layer-3 mechanisms (for example, access lists). The Multilayer Switching Overview document gives you additional details.
The MLS uses a proprietary protocol (MLSP) through which the layer-2 switches identify routers.
This article is part of You've asked for it series.
Update 2008-12-08: Ofer Granit sent me the following information: according to Troubleshooting IP Multilayer Switching document, Supervisor Engine 2 and Supervisor Engine 720 no longer use MLS but rely exclusively on CEF to perform layer-3 forwarding.
Another Way to Generate SNMP Trap on High CPU Load
When testing the ERM functionality that together with an EEM applet generates SNMP traps whenever the CPU load exceeds predefined thresholds, I started to wonder what the snmp-server enable traps cpu threshold command does.
After lenghty conversation with uncle Google and Cisco documentation, I found that there's another way to detect and report high CPU load in Cisco IOS: the CPU threshold notification introduced in IOS release 12.3T.
Programming language consistency
Generate SNMP trap on high CPU load
How could I configure the EEM to send an SNMP trap when the cpu load (interval=30sec) is higher than 30%?My first solution was to enable resource policy traps with the snmp-server enable traps resource-policy, but this feature was introduced in 12.4(15)T and I am not sure everyone is willing to run the latest-and-greatest IOS code. Furthermore, it looks like the traps are sent only for resource policies defined through the ERM MIB; I was not able to generate a trap from a manually configured resource policy. Obviously it was time for another EEM applet.